全文获取类型
收费全文 | 5376篇 |
免费 | 907篇 |
国内免费 | 273篇 |
专业分类
化学 | 4159篇 |
晶体学 | 266篇 |
力学 | 52篇 |
综合类 | 49篇 |
数学 | 431篇 |
物理学 | 1599篇 |
出版年
2024年 | 1篇 |
2023年 | 14篇 |
2022年 | 34篇 |
2021年 | 48篇 |
2020年 | 30篇 |
2019年 | 116篇 |
2018年 | 138篇 |
2017年 | 177篇 |
2016年 | 181篇 |
2015年 | 345篇 |
2014年 | 355篇 |
2013年 | 536篇 |
2012年 | 433篇 |
2011年 | 369篇 |
2010年 | 347篇 |
2009年 | 359篇 |
2008年 | 321篇 |
2007年 | 314篇 |
2006年 | 288篇 |
2005年 | 262篇 |
2004年 | 256篇 |
2003年 | 314篇 |
2002年 | 218篇 |
2001年 | 231篇 |
2000年 | 215篇 |
1999年 | 146篇 |
1998年 | 94篇 |
1997年 | 92篇 |
1996年 | 60篇 |
1995年 | 47篇 |
1994年 | 35篇 |
1993年 | 34篇 |
1992年 | 26篇 |
1991年 | 25篇 |
1990年 | 14篇 |
1989年 | 23篇 |
1988年 | 15篇 |
1987年 | 12篇 |
1986年 | 3篇 |
1985年 | 5篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1979年 | 5篇 |
1978年 | 2篇 |
1977年 | 3篇 |
1975年 | 1篇 |
1974年 | 3篇 |
1973年 | 3篇 |
排序方式: 共有6556条查询结果,搜索用时 31 毫秒
41.
The kinetics of domain size equilibration were studied for asymmetric poly(ethylene‐alt‐propylene)‐b‐poly(dimethyl siloxane) (EPDMS) and polyisoprene‐b‐poly(dimethyl siloxane) (IDMS) block copolymers in the body‐centered cubic ordered phase. Small‐angle X‐ray scattering measurements of the principal peak position (q*) were made as a function of time after temperature jumps within the ordered state. The equilibration times were remarkably long, especially on cooling and for temperatures below 100 °C. For example, after a quench to 40 °C, q* for EPDMS had not fully equilibrated even after several weeks of annealing; IDMS required several days to equilibrate at the same temperature. In contrast, a lamella‐forming EPDMS sample was able to adjust q* within the timescale of the measurements (i.e., minutes) with both heating and cooling over the same temperature range. Measurements of tracer diffusion indicated that chain mobility was not the rate‐limiting step, although differences in mobility did account for the differences between EPDMS and IDMS. Rather, the limiting step was the required reduction in the number density of spheres on cooling; the disappearance of spheres, either by evaporation or by fusion, provided a large kinetic barrier. Lamellae, however, could adjust domain dimensions simply by local displacements of individual chains. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 715–724, 2003 相似文献
42.
43.
Nadya Dencheva Teresa G. Nunes M. Jovita Oliveira Zlatan Denchev 《Journal of Polymer Science.Polymer Physics》2005,43(24):3720-3733
The crystalline structure of polyamide‐12 (PA12) was studied by solid‐state 13C nuclear magnetic resonance (NMR) as well as by synchrotron wide‐ and small‐angle X‐ray scattering (WAXS and SAXS). Isotropic and oriented PA12 showed different NMR spectra ascribed to γ‐ and γ′‐crystalline modifications, respectively. On the basis of the position of the first diffraction peak, the isotropic γ‐form and the oriented γ′‐form were shown to be with hexagonal crystalline lattice at room temperature. When heated, the two PA12 polymorphs demonstrated different behaviors. Above 140 °C, the isotropic γ‐PA12 partially transformed into α‐modification. No such transition was observed with the oriented γ′‐PA12 phase even after annealing at temperatures close to melting. A γ′–γ transition was observed here only after isotropization by melting point. Various structural parameters were extracted from the WAXS and SAXS patterns and analyzed as a function of temperature and orientation: the degree of crystallinity, the d‐spacings, the Bragg's long spacings, the average thicknesses of the crystalline (lc) and amorphous (la) phases, and the linear crystallinity xcl within the lamellar stacks. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3720–3733, 2005 相似文献
44.
Thi Thi Nge Naruhito Hori Akio Takemura Hirokuni Ono Tsunehisa Kimura 《Journal of Polymer Science.Polymer Physics》2003,41(7):711-714
A high magnetic field of 5 T was used to fabricate a magnetically aligned, optically anisotropic, liquid‐crystalline chitin/poly(acrylic acid) composite. The aligned mesophase was fixed by photoinitiated free‐radical polymerization. From an examination of polarized optical micrographs and an X‐ray diffraction study, a high degree of orientation of 0.70 was observed for the composite with a higher liquid‐crystalline chitin concentration (10.70 wt %); the orientation was reduced with a decreased chitin concentration at a given acrylic acid concentration. The X‐ray data for the developed composite showed a uniplanar orientation for the chitin crystallites, with its molecular long axes perpendicular to the direction of the magnetic field. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 711–714, 2003 相似文献
45.
The central zinc(II) atom in the title complex is tetrahedrally coordinated by four nitrogen atoms derived from 4‐methyl‐5‐imidazolecarboxyaldehyde ligands with Zn? N in the range 2.007(3) to 2.026(4) Å. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
46.
由于痕量元素在煤中的含量低微、检测困难,加之其原子量一般较大,可能的反应途径多,使得相关的反应机理研究难度极大。本文结合作者的研究成果,介绍了煤燃烧过程中痕量元素化学反应动力学的国内外研究进展,包括痕量元素化学动力学机理的建立;相关的典型实验、计算模拟及其实验验证、动力学机理模型的简化;痕量元素反应动力学机理的完善和发展,包括采用简单碰撞理论、活化络合物理论(亦称过渡态理论,或绝对反应速率理论)对痕量元素化学反应动力学机理的修正;最后指出煤燃烧过程中痕量元素动力学研究的若干方向是: (1)痕量元素反应动力学模型数据库的建立;(2)煤燃烧过程中,主量元素和次量元素的动力学机理的完善; (3)各痕量元素之间动力学研究的开展; (4)实际燃烧过程中痕量元素动力学行为的研究。 相似文献
47.
对西安地区150例0~3岁儿童进行了六种微量元素:钙、锌、铜、铁、镁、硒的检测,发现微量元素缺乏可导致急慢性感染、免疫力低下、小儿营养不良、贫血、发育迟缓等症状。并对小儿营养不良、喂养不当进行矫正。其中铁、钙、锌同时缺乏占50%。 相似文献
48.
《Magnetic resonance in chemistry : MRC》2003,41(1):61-64
Palladium(II) dichloride reacts with 1,10‐bis(2‐pyrrolyl)‐2,5,9‐triaza‐1,9‐decadiene to give a [Pd(C15H20N5)]Cl complex in which the ligand is four‐coordinated, leaving one pyrrole group dangling. By using COSY, gHSQC, gHMBC connectivities and NOE experiments it has been concluded that one linkage isomer exists in DMSO solution, in spite of the fact that different sets of N atoms of potentially pentadentate ligand might be involved in coordination, and that the three chelate rings in the complex cation are arranged in a sequence: five‐membered, six‐membered, five‐membered which is different from that (5–5–6) found by x‐ray studies on the related [Ni(C15H20N5)]Cl compound. NMR studies allowed an unambiguous assignment of all 1H and 13C NMR resonances for the complex. Results of x‐ray structural analysis of [Pd(C15H20N5)](CH3COO)H2O supported the five‐membered, six‐membered, five‐membered ring sequence in the [Pd(C15H20N5)]+ complex cation and show an E (trans) orientation of the dangling pyrrole group with respect to the metal center. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
49.
μ子动量谱的精确测量是L3C宇宙线实验最重要的目标之一.事例重建的好坏是该测量的关键,而能量确定的衰变为双μ子的事例是检验重建程序的最好手段.该数据分析利用在2000年中获取的数据作为样本,筛选出了LEP在2000年4月、5月、8月和9月等Z0能量运行期间L3C宇宙线实验数据中记录的Z0→μ+μ-事例数据,得到通过重建程序后动量为45GeV的μ子动量分辨率为(5.4?7±0.25)%. 相似文献
50.