全文获取类型
收费全文 | 399篇 |
免费 | 35篇 |
国内免费 | 68篇 |
专业分类
化学 | 246篇 |
力学 | 169篇 |
数学 | 6篇 |
物理学 | 81篇 |
出版年
2022年 | 4篇 |
2021年 | 10篇 |
2020年 | 13篇 |
2019年 | 12篇 |
2018年 | 7篇 |
2017年 | 14篇 |
2016年 | 15篇 |
2015年 | 14篇 |
2014年 | 19篇 |
2013年 | 34篇 |
2012年 | 17篇 |
2011年 | 29篇 |
2010年 | 14篇 |
2009年 | 14篇 |
2008年 | 10篇 |
2007年 | 16篇 |
2006年 | 21篇 |
2005年 | 17篇 |
2004年 | 25篇 |
2003年 | 15篇 |
2002年 | 20篇 |
2001年 | 15篇 |
2000年 | 14篇 |
1999年 | 8篇 |
1998年 | 15篇 |
1997年 | 13篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1994年 | 9篇 |
1993年 | 10篇 |
1992年 | 20篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 11篇 |
1988年 | 8篇 |
1981年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有502条查询结果,搜索用时 31 毫秒
491.
492.
Carlos Elvira Beln Levenfeld Blanca Vzquez Julio San Romn 《Journal of polymer science. Part A, Polymer chemistry》1996,34(13):2783-2789
Two tertiary amines with a chemical structure rather similar to dimethyl-4-toluidine have been prepared and tested as activators for the free radical polymerization of methyl methacrylate. 4-Dimethylaminobenzyl alcohol, DMOH, was synthesized by reduction of the corresponding benzaldehyde. 4-Dimethylaminobenzyl methacrylate, DMMO, was synthesized by condensation of methacryloyl chloride with DMOH in the presence of triethylamine as catalyst. Kinetic studies of the bulk polymerization of methyl methacrylate initiated by the redox system BPO–amine have been carried out by differential scanning calorimetry at different temperatures in the interval 30–40°C. An increase of the overall rate constant, k, with increasing temperature was observed for all redox systems. The system BPO/DMT gave the highest values of k. The polymerizations catalyzed by DMOH and DMMO respectively gave lower values of the overall Arrhenius activation than that obtained with DMT. DMMO may participate in the polymerization not only as activator but also as an acrylic derivative which can be incorporated into the polymeric growing chains during the propagation step of the free radical polymerization. © 1996 John Wiley & Sons, Inc. 相似文献
493.
Abstract. From practical and theoretical standpoints, the initiation of combustion in gaseous media due to the shock waves focusing
process at various reflectors is a subject of much current interest. The complex gas flowfield coupled with chemical kinetics
provides a wide spectrum of possible regimes of combustion, such as fast flames, deflagration, detonation etc. Shock wave
reflection at concave surfaces or wedges causes converging of the flow and produces local zones with extremely high pressures
and temperatures. The present work deals with the initiation of detonation due to shock waves focusing at parabolic and wedge
reflectors. Particular attention has been given to the determination of the critical values of the incident shock wave (ISW)
Mach number, parameters of the combustible mixture, and geometrical sizes of reflector at which different combustion regimes
could be generated.
Received 30 August 1999 / Accepted 23 February 2000 相似文献
494.
495.
半导体桥起爆炸药的实验研究 总被引:9,自引:0,他引:9
介绍了用电爆炸半导体桥起爆炸药的实验 :被起爆装药是重结晶泰安 ,装药尺寸5 6mm 14mm ,密度 1 0~ 1 3g/cm3;装药壳体是直径 6.2mm的紫铜管 ,壁厚 0 .3mm ;采用快速放电的电容器放电装置 (CDU)作为半导体桥起爆炸药的能源。用这种新型半导体桥雷管起爆密度1 0g/cm3的泰安装药所需能量为 2 90mJ,雷管的外观尺寸是6 2mm 2 0mm ,作用时间t =3 2 7s,初始装药的到爆轰距离r=6 31mm。这种新型半导体桥雷管能可靠起爆密度为 1 64g/cm3的钝化 (含 5%石蜡 )泰安传爆药柱。 相似文献
496.
通过构造反向传播神经网络,对裂纹尖端的应力场进行模拟,进而实现对裂纹尖端应力场甬数的逼近。得到的网络具有较高的联想、记忆能力和相当的稳定性,并且可以快速、准确地得到带裂纹构件的裂纹尖端应力场,从而确定裂纹尖端的塑性区和分析裂纹的扩展。数值计算给出了LY12-CZ材料裂纹扩展方向的计算结果,与实验结果吻合较好,还给出了两相材料含界面裂纹在复合型载荷作用下的塑性区形状的变化情况,并对两相材料含界面裂纹在复合型载荷作用下裂纹的扩展方向进行了预测。 相似文献
497.
二过碘酸合铜(Ⅲ)钾引发丙烯腈在聚丙烯酰胺上接枝共聚合反应的研究 总被引:1,自引:0,他引:1
本文以二过碘酸合铜(Ⅲ)钾为氧化剂,聚丙烯酰胺(PAAM)自身为还原剂组成的氧化还原引发体系,在PAAM上引发丙烯腈接枝共聚合反应,获得了高接枝效率和高接枝百分比的产物,由红外光谱、扫描电镜及X射线衍射分析对接枝反应进行了表征,测定了各种条件对接枝参数的影响,并对引发机理进行了探讨。 相似文献
498.
疲劳短裂纹萌生及发展的细观过程和理论 总被引:21,自引:1,他引:20
本文综述了疲劳短裂纹研究的进展和现状,包括关于短裂纹的属性,短裂纹萌生和发展的细观过程以及描述短裂纹行为的若干模型。 相似文献
499.
采用高压电点火进行直接起爆,通过放电过程中电流的输出信号确定起爆能量,实验测定了C2H2-2.5O2气体和加入摩尔浓度为70%氩气的C2H2-2.5O2混合气体直接起爆的临界起爆能量,研究了高浓度氩气稀释对C2H2-2.5O2混合物临界起爆能量的影响。实验测得的混合物临界起爆能量实验值与Lee等人的表面积能量理论值基本吻合。研究表明:C2H2-2.5O2气体和加入摩尔浓度为70%氩气的C2H2-2.5O2混合气体的临界起爆能量均依赖于初始压力,并呈反相关指数关系;在相同实验条件下,高浓度氩气稀释极大提高了混合气体直接起爆的临界起爆能量。分析认为,由于临界起爆能量正比于诱导区长度的3次方,因此在相同初始压力下,高浓度氩气的稀释增加了C2H2-2.5O2混合气体爆轰诱导区长度,并最终导致其临界起爆能量的显著上升。 相似文献
500.
Yosuke ISHII Yuichiro KAI Takashi MORITA Hisayoshi AIKAWA Ryoichi NAKAMURA 《Physical Therapy Research》2020,23(2):160
Objective: Although elderly inpatients are known to experience decreased physical activity in the morning, falls occur frequently during this time. Gait variability is an evaluation of gait instability and a risk factor for falls. Gait initiation requires complex processes, and it is important to evaluate gait variability not only during steady-state gait but also during gait initiation. However, the effect of the diurnal pattern on variability in gait characteristics is still unknown. The aim of this study was to investigate the effect of the diurnal pattern on initiation and steady-state gait variability in elderly inpatients. Method: Thirty-seven elderly inpatients (28 women; mean age, 79.7 ± 9.5 years) who could walk without support were sampled in this study. The quantitative measure of gait variability was evaluated using the coefficient of variation (CV) based on four consecutive stride durations determined using triaxial accelerometers. Gait characteristics were evaluated during initiation and steady-state gait and defined as initiation CV and steady-state CV, respectively. This measurement was performed at two time points, morning and daytime. Results: There was no significant difference between initiation and steady-state gait characteristics in the daytime condition. However, in the morning condition, the initiation CV was higher than the steady-state CV. Furthermore, the initiation CV was higher in the morning than during daytime (p < 0.01). Conclusion: Our study revealed that the variability of initiation gait is higher in the morning. It may be important to assess the risk of falls, including initiation gait, in the morning. 相似文献