全文获取类型
收费全文 | 1650篇 |
免费 | 87篇 |
国内免费 | 256篇 |
专业分类
化学 | 1581篇 |
晶体学 | 4篇 |
力学 | 131篇 |
综合类 | 14篇 |
数学 | 11篇 |
物理学 | 252篇 |
出版年
2024年 | 1篇 |
2023年 | 6篇 |
2022年 | 17篇 |
2021年 | 28篇 |
2020年 | 64篇 |
2019年 | 47篇 |
2018年 | 29篇 |
2017年 | 98篇 |
2016年 | 82篇 |
2015年 | 82篇 |
2014年 | 59篇 |
2013年 | 196篇 |
2012年 | 80篇 |
2011年 | 72篇 |
2010年 | 72篇 |
2009年 | 64篇 |
2008年 | 88篇 |
2007年 | 87篇 |
2006年 | 83篇 |
2005年 | 80篇 |
2004年 | 89篇 |
2003年 | 74篇 |
2002年 | 98篇 |
2001年 | 62篇 |
2000年 | 54篇 |
1999年 | 37篇 |
1998年 | 33篇 |
1997年 | 26篇 |
1996年 | 27篇 |
1995年 | 34篇 |
1994年 | 19篇 |
1993年 | 28篇 |
1992年 | 18篇 |
1991年 | 10篇 |
1990年 | 7篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 5篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
排序方式: 共有1993条查询结果,搜索用时 218 毫秒
81.
Graft copolymer of natural rubber and poly(methyl methacrylate) (NR‐g‐PMMA) was prepared using semi‐batch emulsion polymerization technique via bipolar redox initiation system. It was found that the grafted PMMA increased with the increase of methyl methacrylate (MMA) concentration used in the graft copolymerization. The NR‐g‐PMMA was later used to prepare thermoplastic vulcanizates (TPVs) by blending with PMMA through dynamic vulcanization technique. Conventional vulcanization (CV) and efficient sulphur vulcanization (EV) systems were studied. It was found that the CV system provided polymer melt with lower shear stress and viscosity at a given shear rate. This causes ease of processability of the TPVs via extrusion and injection molding processes. Furthermore, the TPVs with the CV system showed higher ultimate tensile strength and elongation. The results correspond to the morphological properties of the TPVs. That is, finer dispersion of the small vulcanized rubber particles were observed in the PMMA matrix. Various blend ratios of the NR‐g‐PMMA/PMMA blends using various types of NR‐g‐PMMA (i.e. prepared using various percentage molar ratios of NR and MMA) were later studied via dynamic vulcanization by a conventional sulphur vulcanization system. It was found that increasing the level of PMMA caused increasing trend of the tensile strength and hardness properties but decreasing level of elongation properties. Increasing level of the grafted PMMA in NR molecules showed the same trend of mechanical properties as in the case of increasing concentration of PMMA used as a blend component. From morphological studies, two phase morphologies were observed with a continuous PMMA phase and dispersed elastomeric phase. It was also found that more finely dispersed elastomeric phase was obtained with increasing the grafted PMMA in the NR molecules. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
82.
磺化丁基橡胶离聚体在混合溶剂中的聚集行为 总被引:1,自引:0,他引:1
用顺磁共振(ESR)谱和粘度法考察了磺化丁基橡胶离聚体在二甲苯/正己醇混合溶剂中的聚集行为.ESR谱表明离聚体的离子基团发生聚集,其聚集程度受极性共溶剂(正己醇)的影响.粘度考察表明,在不同的浓度时离子基团有不同的聚集形式,低浓度时以分子内聚集为主,高浓度时以分子间聚集为主.离聚体的聚集度(DA)与浓度(c)的关系可用经验式DA=Aexp(kc2)表示,其中A、k为常数.k值反映聚集度受浓度影响的程度,醇含量增大时k值减小,这是因为醇对离子的溶剂化作用导致离子聚集的倾向减小. 相似文献
83.
Aiming to develop a high performance fiber reinforced rubber of SBR, a special technique using electron beam (EB) irradiation-induced graft-polymerization was applied to ultra-high molecular-weight polyethylene (UHMWPE) fibers. Although UHMWPE is chemically inert, N-vinyl formamide (NVF) could be graft-polymerized onto the UHMWPE fiber surface with this special technique. A maximum grafting percentage of 23.6% was achieved. The composite of SBR and grafted UHMWPE fibers with maximum grafting indicated a linear increase in the initial modulus and strength with the fiber content. At the fiber content of 10%, the initial modulus was improved about five times with respect to that of the pure SBR, while the strength was done about twice. At this moment, only a small reduction could be observed in the strain compared with that of pure SBR. The fiber reinforced rubber with a good performance was obtained in the system of SBR and grafted UHMWPE fibers. 相似文献
84.
Radio frequency plasma polymerization of vinylidene fluoride was used to modify the surface properties of nitrile rubber. The chemistry and frictional properties of the plasma films were characterized. FTIR transmission spectra and EDX analysis of plasma polymer films deposited on NaCl windows showed that the degree of fluorination of the plasma polymers increased as plasma power was increased from 25 to 50 W, and then decreased monotonically at higher powers. An estimation of the actual F/C ratio from EDX data indicated that the plasma polymer films contained approximately one fluorine atom for every 2–5 carbon atoms. Sliding friction tests on a Delrin countersurface showed that the coefficient of friction of the plasma treated rubbers was lower than untreated rubber, but slighly higher than rubber coated with silicone oil. Repetitive sliding friction testing showed that silicone oil treated samples had a longer lubricating lifetime than plasma treated samples. However, cyclic friction tests conducted with nitrile rubber o-rings yielded similar frictional behavior and lubricating lifetimes for silicone oil and plasma treatments. There was no correlation between chemical composition and the frictional and wear properties of the plasma films. Environmental scanning electron micrographs showed that the plasma films were brittle and tended to crack and flake off during wear testing. 相似文献
85.
Study on microstructure and mechanical properties relationship of short fibers/rubber foam composites 总被引:1,自引:0,他引:1
Research on short fibers/rubber foam composites is rarely found in the literature. In this paper, microcellular rubber foams unfilled (MF), strengthened by pretreated short fibers (MFPS) and untreated short fibers (MFUS) are prepared, respectively. The microstructure and mechanical properties of the three composites have been studied via scanning electron microscope (SEM) and mechanical testing, respectively. The SEM results show that both pretreated and untreated short fibers disperse uniformly in the composites and in bidimensional orientation. Moreover, the pretreated short fibers have much better adhesion with the rubber matrix than untreated ones. The experimental results also indicate that the introduction of short fibers is mainly responsible for the great enhancement of most mechanical properties of the microcellular rubber foams, and the good interfacial adhesion of the short fibers with the matrix contributes to the more extensive improvement in the mechanical properties. It is also found that the reinforcement effect of short fibers to compressive modulus strongly depends on the density of microcellular rubber foams, the orientation of short fiber and the deformation ratio. The compressive modulus of microcellular rubber foams at the normalized density less than 0.70 and beyond 0.70 is predicted by the modified Simple Blending Model and the Halpin-Kerner Model, respectively. The theoretically predicted values are in good accordance with the experimental results. 相似文献
86.
87.
New double negative and positive temperature coefficients of conductive EPDM rubber TiC ceramic composites 总被引:1,自引:0,他引:1
Farid El-Tantawy 《European Polymer Journal》2002,38(3):567-577
In this paper, we have successfully prepared ethylene-propylene-diene monomer (EPDM)/TiC composites as thermistors, with new double negative and positive temperature coefficients of conductivity (NTCC/PTCC). EPDM composites loaded with 50 phr HAF carbon black and different concentrations of TiC were prepared. This study focuses on the effect of TiC content on the vulcanization process, the network structure and the electrical and thermal properties of EPDM/TiC composites. The effect of TiC on the network structure was evaluated e.g. the curing process, the characteristic time constant during vulcanization, the volume fraction of rubber, gel fraction, interparticle distance between conductive particles, the extent of TiC reinforcement in the rubber matrix and molecular weight between cross-linking through experimental and affine-phantom models. The effects of TiC content on the percolation theory, electrical conductivity, conducting mechanism of conductivity, conducting hysteresis and I-V characteristics were also studied, as well as its TiC on the (NTCC/PTCC), thermoelectric power, dielectric constant and thermal conductivity. Stability and reproducibility of the thermal cycles for heating element applications was tested. Specific heat and the amount of heat transfer by radiation and convection as a function of TiC content was calculated using both the calorimetric technique and a theoretical model. It was proved that TiC improves the network structure, electrical and thermal properties of EPDM composites for practical applications. 相似文献
88.
Yanzhen Guo Qian Yang Wei Yan Bing Li Tian Li 《International journal of environmental analytical chemistry》2013,93(10):1001-1012
Acetochlor is an important herbicide for gramineous weeds and some small seed broadleaf weeds. Controlled-release formulations of herbicide are highly desirable not only for attaining the most effective utilisation of the weed control, but also for reducing environmental pollution. Acetochlor was incorporated in poly (butyl methacrylate-diacetone acrylamide) based formulation to obtain controlled release properties. The acetochlor nanocapsules were characterised by size distribution, infrared spectroscopy (IR) and field emission scanning electron microscopy (FESEM), and factors related to loading efficiency, swelling behaviour of the formulation were investigated. For this controlled-release formulation, the loading efficiency could reach about 50% (w/w). n, the diffusion parameter was indicative of the transport mechanism, and the values for ‘n’ were in the range of 0.28–0.61, which indicated that the release of acetochlor was diffusion-controlled. The time taken for 50% of the active ingredient to be released into water, t50, was also calculated for the comparison of formulations in different conditions. The formulation with higher temperature and more diacetone acrylamide had lower value of t50, which means a quicker release of the active ingredient. This study highlighted some pieces of evidence that improved herbicide incorporation and slower release were linked to potential interactions between the herbicide and the polymer. 相似文献
89.
New 3,3‐diphenylpropoxyphthalonitrile (5) was obtained from 3,3‐diphenylpropanol (3) and 4‐nitrophthalonitrile (4) with K2CO3 in DMF at 50 °C. The novel cobalt(II) phthalocyanine complexes, tetrakis‐[2‐(1,4‐dioxa‐8‐azaspiro[4.5]dec‐8‐yl)ethoxy] phthalocyaninato cobalt(II) (2) and tetrakis‐(3,3‐diphenylpropoxy)phthalocyaninato cobalt(II) (6) were prepared by the reaction of the phthalonitrile derivatives 1 and 5 with CoCl2 by microwave irradiation in 2‐(dimethylamino)ethanol for at 175 °C, 350 W for 7 and 10 min, respectively. These new cobalt(II)phthalocyanine complexes were characterized by spectroscopic methods (IR, UV–visible and mass spectroscopy) as well as elemental analysis. Complexes 2 and 6 are employed as catalyst for the oxidation of cyclohexene using tert‐butyl hydroperoxide (TBHP), m‐chloroperoxybenzoic acid (m‐CPBA), aerobic oxygen and hydrogen peroxide (H2O2) as oxidant. It is observed that both complexes can selectively oxidize cyclohexene to give 2‐cyclohexene‐1‐ol as major product, and 2‐cyclohexen‐1‐one and cyclohexene oxide as minor products. TBHP was found to be the best oxidant since minimal destruction of the catalyst, higher selectivity and conversion were observed in the products. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
90.
Precipitation temperature-total polymer concentration diagrams for toluene solutions of two styrene-acrylonitrile copolymers different in chemical composition and their mixtures were determined and then triangular phase diagrams of this system were constructed from these diagrams. It is speculated from the triangular phase diagrams and experimentally shown that the copolymer may be effectively fractionated by chemical composition in this system. 相似文献