全文获取类型
收费全文 | 420758篇 |
免费 | 3105篇 |
国内免费 | 921篇 |
专业分类
化学 | 200716篇 |
晶体学 | 6690篇 |
力学 | 22483篇 |
综合类 | 13篇 |
数学 | 61183篇 |
物理学 | 133699篇 |
出版年
2020年 | 3148篇 |
2019年 | 3745篇 |
2018年 | 11863篇 |
2017年 | 12478篇 |
2016年 | 10533篇 |
2015年 | 4523篇 |
2014年 | 6898篇 |
2013年 | 15163篇 |
2012年 | 14966篇 |
2011年 | 23444篇 |
2010年 | 16024篇 |
2009年 | 16304篇 |
2008年 | 21313篇 |
2007年 | 23957篇 |
2006年 | 11912篇 |
2005年 | 15301篇 |
2004年 | 12303篇 |
2003年 | 11663篇 |
2002年 | 9943篇 |
2001年 | 10029篇 |
2000年 | 7677篇 |
1999年 | 5685篇 |
1998年 | 4805篇 |
1997年 | 4748篇 |
1996年 | 4596篇 |
1995年 | 4132篇 |
1994年 | 4186篇 |
1993年 | 4014篇 |
1992年 | 4542篇 |
1991年 | 4581篇 |
1990年 | 4440篇 |
1989年 | 4446篇 |
1988年 | 4367篇 |
1987年 | 4386篇 |
1986年 | 4138篇 |
1985年 | 5359篇 |
1984年 | 5465篇 |
1983年 | 4678篇 |
1982年 | 4749篇 |
1981年 | 4568篇 |
1980年 | 4347篇 |
1979年 | 4824篇 |
1978年 | 4838篇 |
1977年 | 4885篇 |
1976年 | 4959篇 |
1975年 | 4693篇 |
1974年 | 4603篇 |
1973年 | 4879篇 |
1972年 | 3498篇 |
1971年 | 3073篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Mechanistic Aspects of a Highly Active Dinuclear Zinc Catalyst for the Co‐polymerization of Epoxides and CO2 下载免费PDF全文
Stefan Kissling Peter T. Altenbuchner Dr. Maximilian W. Lehenmeier Dr. Eberhardt Herdtweck Dr. Peter Deglmann Dr. Uwe B. Seemann Prof. Dr. Bernhard Rieger 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(22):8148-8157
The dinuclear zinc complex reported by us is to date the most active zinc catalyst for the co‐polymerization of cyclohexene oxide (CHO) and carbon dioxide. However, co‐polymerization experiments with propylene oxide (PO) and CO2 revealed surprisingly low conversions. Within this work, we focused on clarification of this behavior through experimental results and quantum chemical studies. The combination of both results indicated the formation of an energetically highly stable intermediate in the presence of propylene oxide and carbon dioxide. A similar species in the case of cyclohexene oxide/CO2 co‐polymerization was not stable enough to deactivate the catalyst due to steric repulsion. 相似文献
32.
M. Kroon E. Andreasson E. Persson Jutemar V. Petersson L. Persson M. Dorn P.A.T. Olsson 《Experimental Mechanics》2018,58(1):75-86
Injection-moulding is one of the most common manufacturing processes used for polymers. In many applications, the mechanical properties of the product is of great importance. Injection-moulding of thin-walled polymer products tends to leave the polymer structure in a state where the mechanical properties are anisotropic, due to alignment of polymer chains along the melt flow direction. The anisotropic elastic-viscoplastic properties of low-density polyethylene, that has undergone an injection-moulding process, are therefore examined in the present work. Test specimens were punched out from injection-moulded plates and tested in uniaxial tension. Three in-plane material directions were investigated. Because of the small thickness of the plates, only the in-plane properties could be determined. Tensile tests with both monotonic and cyclic loading were performed, and the local strains on the surface of the test specimens were measured using image analysis. True stress vs. true strain diagrams were constructed, and the material response was evaluated using an elastic-viscoplasticity law. The components of the anisotropic compliance matrix were determined together with the direction-specific plastic hardening parameters. 相似文献
33.
Indermeet Kohli Taylor L. Braunberger Amanda F. Nahhas Fatima N. Mirza Mohsen Mokhtari Alexis B. Lyons Nikiforos Kollias Eduardo Ruvolo Henry W. Lim Iltefat H. Hamzavi 《Photochemistry and photobiology》2020,96(1):208-214
Human skin is exposed to visible light (VL; 400–700 nm) and long-wavelength ultraviolet A1 (UVA1) radiation (370–400 nm) after the application of organic broad-spectrum sunscreens. The biologic effects of these wavelengths have been demonstrated; however, a dose–response has not been investigated. Ten subjects with Fitzpatrick skin phototype IV-VI were enrolled. Subjects were irradiated with 2 light sources (80–480 J cm−2): one comprising VL with less than 0.5% UVA1 (VL+UVA1) and the other pure VL. Skin responses were evaluated for 2 weeks using clinical and spectroscopic assessments. 4-mm punch biopsies were obtained from nonirradiated skin and sites irradiated with 480 J cm−2 of VL+UVA1 and pure VL 24 h after irradiation. Clinical and spectroscopic assessments demonstrated a robust response at VL+UVA1 sites compared with pure VL. Histology findings demonstrated a statistically significant increase in the marker of inflammation (P < 0.05) and proliferation (P < 0.05) at the irradiated sites compared with nonirradiated control. Threshold doses of VL+UVA1 resulting in biologic responses were calculated. Results indicate that approximately 2 h of sun exposure, which equates to VL+UVA1 dose (~400 J cm−2), is capable of inducing inflammation, immediate erythema and delayed tanning. These findings reinforce the need of photoprotection beyond the UV range. 相似文献
34.
35.
36.
37.
38.
39.
P. A. Drokin 《Journal of Applied Mechanics and Technical Physics》2018,59(3):531-541
A model of the explosive fragmentation of a thin ring is developed which takes into account the statistical dispersion of the relative fracture deformation along the length of the ring. A formula is proposed for calculating the velocity of the boundary of the region near a plastic rupture in which the plastic flow of the ring material ceases. Methods for the numerical and analytical calculation of the average number of fragments of the ring are developed. The calculation results are compared with available experimental data. 相似文献
40.
P. W. J. Glover 《Transport in Porous Media》2018,124(1):31-56
In this paper, we have significantly modified an existing model for calculating the zeta potential and streaming potential coefficient of porous media and tested it with a large, recently published, high-quality experimental dataset. The newly modified model does not require the imposition of a zeta potential offset but derives its high salinity zeta potential behaviour from Stern plane saturation considerations. The newly modified model has been implemented as a function of temperature, salinity, pH, and rock microstructure both for facies-specific aggregations of the new data and for individual samples. Since the experimental data include measurements on samples of both detrital and authigenic overgrowth sandstones, it was possible to model and test the effect of widely varying microstructural properties while keeping lithology constant. The results show that the theoretical model represents the experimental data very well when applied to model data for a particular lithofacies over the whole salinity, from 10?5 to 6.3 mol/dm3, and extremely well when modelling individual samples and taking individual sample microstructure into account. The new model reproduces and explains the extreme sensitivity of zeta and streaming potential coefficient to pore fluid pH. The low salinity control of streaming potential coefficient by rock microstructure is described well by the modified model. The model also behaves at high salinities, showing that the constant zeta potential observed at high salinities arises from the development of a maximum charge density in the diffuse layer as it is compressed to the thickness of one hydrated metal ion. 相似文献