全文获取类型
收费全文 | 13593篇 |
免费 | 1772篇 |
国内免费 | 1828篇 |
专业分类
化学 | 10925篇 |
晶体学 | 183篇 |
力学 | 619篇 |
综合类 | 110篇 |
数学 | 1547篇 |
物理学 | 3809篇 |
出版年
2024年 | 16篇 |
2023年 | 101篇 |
2022年 | 298篇 |
2021年 | 291篇 |
2020年 | 331篇 |
2019年 | 435篇 |
2018年 | 330篇 |
2017年 | 364篇 |
2016年 | 527篇 |
2015年 | 610篇 |
2014年 | 750篇 |
2013年 | 995篇 |
2012年 | 1165篇 |
2011年 | 1189篇 |
2010年 | 861篇 |
2009年 | 897篇 |
2008年 | 1030篇 |
2007年 | 881篇 |
2006年 | 794篇 |
2005年 | 772篇 |
2004年 | 692篇 |
2003年 | 541篇 |
2002年 | 599篇 |
2001年 | 467篇 |
2000年 | 383篇 |
1999年 | 240篇 |
1998年 | 213篇 |
1997年 | 173篇 |
1996年 | 160篇 |
1995年 | 124篇 |
1994年 | 127篇 |
1993年 | 99篇 |
1992年 | 80篇 |
1991年 | 74篇 |
1990年 | 74篇 |
1989年 | 60篇 |
1988年 | 50篇 |
1987年 | 41篇 |
1986年 | 44篇 |
1985年 | 47篇 |
1984年 | 34篇 |
1983年 | 36篇 |
1982年 | 23篇 |
1981年 | 24篇 |
1980年 | 14篇 |
1979年 | 14篇 |
1978年 | 13篇 |
1977年 | 19篇 |
1976年 | 21篇 |
1975年 | 18篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
A de Bruijn covering code is a q‐ary string S so that every q‐ary string is at most R symbol changes from some n‐word appearing consecutively in S. We introduce these codes and prove that they can have size close to the smallest possible covering code. The proof employs tools from field theory, probability, and linear algebra. Included is a table of the best known bounds on the lengths of small binary de Bruijn covering codes, up to R = 11 and n = 13, followed by several open questions in this area. © 2004 Wiley Periodicals, Inc. Random Struct. Alg., 2004 相似文献
92.
93.
Chongyi Wan Shiming Liu Rongqing Tan Jin Wu Jinwen Zhou Yan Lv Yanning Yu Hua Yang 《Optics & Laser Technology》2004,36(8):2389-649
A high average power, high-repetition-rate TEA CO2 laser employing printed-circuit-board preionizer is described. The power scaling of this preionization configuration has been demonstrated. The average output power reaches 3.6 kW at pulse repetition rate of 180 pps. Furthermore, the scaling-up ability of the rotating spark gap as high-voltage switch for high-average-power gas laser has been validated. 相似文献
94.
95.
用核黄素 蛋氨酸光照法和黄嘌呤氧化酶 细胞色素C还原法证实,在1.0×10-6~1.0×10-5mol/L范围内四羧基锰酞菁(TcPcMn)表现出良好的清除超氧阴离子自由基(O—·2)的活性;用黄嘌呤氧化酶 NBT还原法测算出的TcPcMn清除O—·2的二级反应速率常数为7.77×105mol-1·L·s-1,表明TcPcMn作为超氧化物歧化酶(SOD)模拟酶能很好的抑制O—·2的还原性.TcPcMn既能催化H2O2与4 氨基安替比林和酚的显色反应,也能催化邻苯二胺的聚合,表明TcPcMn具有过氧化物酶(POD)活性.用邻苯三酚自氧法得出,TcPcMn将O—·2的氧化性转化成了POD和H2O2的氧化性.因此只要牺牲一定的POD底物,TcPcMn就可以清除掉因歧化O—·2而产生的H2O2,继而能避免因发生Fenton反应而产生氧化性高于O—·2的羟自由基,从而彻底消除O—·2的氧化性,与SOD相比,这是TcPcMn的一个优势. 相似文献
96.
97.
The chemometric calibration performance was systematically investigated by two parameters (changing the chemical matrix as well as the signal-to-noise ratio) of the NIR (near-infrared) spectrum. Three different analytes (hexane, cyclohexane, toluene) were selected and heptane was used as a solvent. The degree of spectral difference significantly affected the calibration performance. The largest structural difference between the analyte and the solvent provided the best calibration result for a given signal-to-noise ratio. Additionally, the signal-to-noise ratio of the spectra also directly influenced the calibration performance. Overall, the spectral difference and signal-to-noise ratio were the major factors for governing the chemometric calibration performance, especially in the low-concentration range. 相似文献
98.
The electronic structure and magnetism of SmCo7-xZrx alloy are investigated using the spin-polarized MS-X. method. The results show that a few of electrons are transferred to the Sm(5d0) orbital due to orbital hybridization between Sm and Co atoms. The exchange interactions between 3d and 5d electrons are more important than the polarization effects of the conductive electrons, thus it is the main reason resulting in the long-range ferromagnetic order in SmCo7-xZrx. The Curie temperature of SmCo7-xZrx is generally lower than that of corresponding pure Co, which may be explained by the weaker average coupling strength between Co lattices due to some negative couplings mainly occurring of 2e site. The calculated results for the Sm5Co32Zr2 cluster may lead to a better understanding of why SmCo7-xZrx is stable phase. Since the spin-up DOS peak of d electrons at EF arises and the bonding of electrons at EF strengthens with increasing Zr concentration, which results in the internal energy of the system decrease, the stable ferromagnetic order forms in SmCo7-xZrx. 相似文献
99.
Mitsunori Oda Yosuke Fukuchi Satoshi Ito Nguyen Chung Thanh Shigeyasu Kuroda 《Tetrahedron letters》2007,48(52):9159-9162
Reactions of pyrrolidine with 2 equiv of aldehydes without any catalyst in a pressurized vessel at 140–200 °C yielded 1,3-disubstituted pyrroles. -Branched aldehydes gave fairly good yields of the corresponding products by this method, which provides a facile non-oxidative procedure for synthesizing 1,3-dialkylpyrroles from inexpensive pyrrolidine and aldehydes. 相似文献
100.
多阶游程存储是一种不改变光学系统而显著提高光存储容量和数据传输率的新方法.介绍了光致变色多阶游程存储原理和实验系统.提出了基于光致变色原理的多阶游程存储数学模型,该模型反映了记录符反射率与曝光功率、曝光时间之间的非线性关系,并在此基础上确定了光致变色多阶游程存储的写策略.基于650nm光致变色材料进行了4阶游程存储的动态实验.结果表明,实验中采用的650nm光致变色材料可用于多阶游程存储,采用的写策略能够有效地使记录信道线性化,利于采用适合线性系统的信号处理方法.
关键词:
多阶游程
光致变色
光存储
写策略 相似文献