全文获取类型
收费全文 | 6654篇 |
免费 | 606篇 |
国内免费 | 895篇 |
专业分类
化学 | 6156篇 |
晶体学 | 22篇 |
力学 | 363篇 |
综合类 | 47篇 |
数学 | 335篇 |
物理学 | 1232篇 |
出版年
2024年 | 18篇 |
2023年 | 76篇 |
2022年 | 170篇 |
2021年 | 198篇 |
2020年 | 305篇 |
2019年 | 206篇 |
2018年 | 198篇 |
2017年 | 240篇 |
2016年 | 299篇 |
2015年 | 228篇 |
2014年 | 292篇 |
2013年 | 504篇 |
2012年 | 317篇 |
2011年 | 335篇 |
2010年 | 309篇 |
2009年 | 404篇 |
2008年 | 349篇 |
2007年 | 358篇 |
2006年 | 374篇 |
2005年 | 345篇 |
2004年 | 345篇 |
2003年 | 285篇 |
2002年 | 213篇 |
2001年 | 183篇 |
2000年 | 196篇 |
1999年 | 164篇 |
1998年 | 162篇 |
1997年 | 138篇 |
1996年 | 126篇 |
1995年 | 122篇 |
1994年 | 105篇 |
1993年 | 91篇 |
1992年 | 83篇 |
1991年 | 82篇 |
1990年 | 63篇 |
1989年 | 46篇 |
1988年 | 46篇 |
1987年 | 28篇 |
1986年 | 23篇 |
1985年 | 18篇 |
1984年 | 19篇 |
1983年 | 6篇 |
1982年 | 21篇 |
1981年 | 15篇 |
1980年 | 18篇 |
1979年 | 18篇 |
1978年 | 9篇 |
1973年 | 1篇 |
1966年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有8155条查询结果,搜索用时 15 毫秒
981.
Y. Kim M. Ree T. Chang C. S. Ha T. L. Nunes J. S. Lin 《Journal of Polymer Science.Polymer Physics》1995,33(14):2075-2082
Homogeneous precursor/precursor solutions with various compositions were obtained with appreciably high solid contents in N-methyl-2-pyrrolidone from soluble poly(amic diethyl ester) precursors of rodlike poly(p-phenylene biphenyltetracarboximide) (BPDA-PDA) and flexible poly(4,4′-oxydiphenylene biphenyltetracarboximide) (BPDA-ODA), which are hydrolytically more stable as well as more soluble than the corresponding poly(amic acid)s being equilibrated with the constituent monomers. Both optical microscopic and light scattering measurements showed that the dried precursor blend films and resultant polyimide composite films were optically transparent, regardless of compositions and process conditions. The composite films showed a single Tg behavior. However, for the composite of 30 wt % BPDA-PDA dispersed in the matrix of 70 wt % BPDA-ODA, a smectic crystalline-like aggregation of the BPDA-PDA component was detected on wide-angle x-ray diffraction patterns, indicative of microscopic phase separation between the two components. This phase separation was not detected on the optical microscopy, light scattering, and dynamic mechanical thermal analysis because of their resolution limits: Optical microscopy has a resolution of submicrometers, whereas dynamic mechanical thermal analysis and light scattering have a resolution of ca. 50 Å. Therefore, it is speculated that in the composite films BPDA-PDA and BPDA-ODA polyimide molecules have demixed on the scale of a few nanometers. The mean long periodicity, which was estimated from the small-angle x-ray scattering pattern, varied from 134 to 170 Å as the content of BPDA-ODA component increased. In addition, mechanical properties of the composite films were characterized. ©1995 John Wiley & Sons, Inc. 相似文献
982.
Hyperbranched polyurethane as novel solid-solid phase change material for thermal energy storage 总被引:1,自引:0,他引:1
A series of novel hyperbranched polyurethane copolymer (HB-PUPCM) using hyperbranched polyester as chain extender was prepared via a two-step process. The phase transition behaviors and morphology of the HB-PUPCM films were investigated using differential scanning calorimetry (DSC), dynamic mechanical thermal analysis (DMA), thermo-gravimetric analysis (TGA), wide-angle X-ray diffraction (WAXD), polarizing optical microscopy (POM) and tapping-mode atomic force microscopy (AFM). HB-PUPCM was proven to a good polymeric solid-solid phase change heat storage material. 相似文献
983.
984.
Review of proton conductors for hydrogen separation 总被引:1,自引:0,他引:1
There is a global push to develop a range of hydrogen technologies for timely adoption of the hydrogen economy. This is critical in view of the depleting oil reserves and looming transport fuel shortage, global warming, and increasing pollution. Molecular hydrogen (H2) can be generated by a number of renewable and fossil-fuel-based resources. However, given the high cost of H2 generation by renewable energy at this stage, fossil or carbon fuels are likely to meet the short- to medium-term demand for hydrogen. In view of this, effective technologies are required for the separation of H2 from a gas feed (by-products of coal or bio-mass gasification plants, or gases from fossil fuel partial oxidation or reforming) consisting mainly of H2 and CO2 with small quantities of other gases such as CH4, CO, H2O, and traces of sulphur compounds. Several technologies are under development for hydrogen separation. One such technology is based on ion transport membranes, which conduct protons or both protons and electrons. Although these materials have been considered for other applications, such as gas sensors, fuel cells and water electrolysis, the interest in their use as gas separation membranes has developed only recently. In this paper, various classes of proton-conducting materials have been reviewed with specific emphasis on their potential use as H2 separation membranes in the industrial processes of coal gasification, natural gas reforming, methanol reforming and the water–gas shift (WGS) reaction. Key material requirements for their use in these applications have been discussed. 相似文献
985.
Starting with the extended homogeneous balance method and a variable separation approach, a general variable separation solution of the Broer—Kaup system is derived. In addition to the usual localized coherent soliton excitations like dromions, lumps, rings, breathers, instantons, oscillating soliton excitations, peakon and fractal localized solutions, some new types of localized excitations, such as compacton and folded excitations, are obtained by introducing appropriate lower-dimensional piecewise smooth functions and multiple-valued functions, and some interesting novel features of these structures are revealed. 相似文献
986.
S. Sircar 《Adsorption》1996,2(4):323-326
The capital and energy costs of production of oxygen enriched air by a rapid pressure swing adsorption (RPSA) process can be reduced by decoupling the air drying and the air separation duties of the process. Integration of the oxygen-RPSA process with an enhanced combustion application system allows thermal swing adsorption drying of air feed to the RPSA process. The air separation process then can be run using an ad(de)sorption pressure envelope of 2:1 atmospheres, which significantly reduces the cost and energy of operation of the air compressor. 相似文献
987.
The performance of a novel nanoporous carbon membrane for separation of hydrogen-hydrocarbon gas mixtures is described. The membrane selectively adsorbs hydrocarbons from hydrogen at the high pressure side and the adsorbed molecules then diffuse along the pore walls to the low pressure side. Pressure levels at thigh gh and low pressure sides of the membrane and the type and flow rate of the sweep gas at the low pressure side of the membrane were varied. The effects of these variables on the hydrogen recovery and hydrocarbon rejection by the membrane were investigated. 相似文献
988.
The preparation and characterization of novel chromogenic materials has developed extremely rapid in the last years. Among them, thermotropic and thermochromic polymer gel networks have met with growing interest, because of their advanced properties. These novel polymer gels exhibit pronounced changes in transparency and/or color intensity in a practically relevant temperature range. For many future technical applications it is an essential condition, that the volume of the used gel materials is independent of temperature. A current overview of this field is given. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
989.
We present electric and magnetic properties of polycrystalline Pr0.5−δCa0.2+δSr0.3MnO3, for δ between −0.04 and 0.04, where the hole concentration is n=0.5+δ. In this series, we study the effects of moving n away from 0.5 on both, the phase diagram and phase separated state. We found that the low temperature ferromagnetic fraction XFM continuously decreases when n increases. As a result, the samples with n<0.5 (large XFM) exhibit metallic resistivity at low T while for n>0.5 the insulating state predominates. We construct a detailed T–n phase diagram showing the asymmetries around half-doping. 相似文献
990.