全文获取类型
收费全文 | 30129篇 |
免费 | 3836篇 |
国内免费 | 2229篇 |
专业分类
电工技术 | 2178篇 |
技术理论 | 2篇 |
综合类 | 3630篇 |
化学工业 | 4268篇 |
金属工艺 | 573篇 |
机械仪表 | 1238篇 |
建筑科学 | 2926篇 |
矿业工程 | 1502篇 |
能源动力 | 944篇 |
轻工业 | 3451篇 |
水利工程 | 1908篇 |
石油天然气 | 1726篇 |
武器工业 | 328篇 |
无线电 | 2360篇 |
一般工业技术 | 2481篇 |
冶金工业 | 1036篇 |
原子能技术 | 147篇 |
自动化技术 | 5496篇 |
出版年
2024年 | 170篇 |
2023年 | 453篇 |
2022年 | 951篇 |
2021年 | 1047篇 |
2020年 | 1152篇 |
2019年 | 995篇 |
2018年 | 1004篇 |
2017年 | 1152篇 |
2016年 | 1344篇 |
2015年 | 1294篇 |
2014年 | 1978篇 |
2013年 | 2063篇 |
2012年 | 2361篇 |
2011年 | 2470篇 |
2010年 | 1840篇 |
2009年 | 1738篇 |
2008年 | 1645篇 |
2007年 | 2002篇 |
2006年 | 1724篇 |
2005年 | 1485篇 |
2004年 | 1227篇 |
2003年 | 1040篇 |
2002年 | 806篇 |
2001年 | 718篇 |
2000年 | 572篇 |
1999年 | 549篇 |
1998年 | 440篇 |
1997年 | 353篇 |
1996年 | 290篇 |
1995年 | 259篇 |
1994年 | 232篇 |
1993年 | 181篇 |
1992年 | 111篇 |
1991年 | 122篇 |
1990年 | 86篇 |
1989年 | 63篇 |
1988年 | 60篇 |
1987年 | 31篇 |
1986年 | 37篇 |
1985年 | 24篇 |
1984年 | 29篇 |
1983年 | 26篇 |
1982年 | 28篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 5篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1964年 | 3篇 |
1958年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
102.
The equations for predicting the number-average molecular weight are derived on the basis of the three stage polymerization model (TSPM) in this paper. By applying the equations, a plotting approach is proposed to determine the apparent initiator efficiency defined as f[(αtd+1)/2] and the constant of chain transfer to monomer, where f is the initiator efficiency and αtd denotes the fraction of the termination rate constants by disproportionation. Using the approach to plot the experimental data in the literature, it is found that the chain transfer to monomer can be neglected for both methylmethacrylate (MMA) and styrene (St) polymerizations, but it can exert a significant effect on ethylmethacrylate (EMA) polymerization. In addition, the apparent initiator efficiency is found to be independent of reaction temperature and initiator concentration at each stage. The values of f[(αtd+1)/2] at gel effect stage are slightly reduced as compared with that at low conversion stage for MMA and EMA polymerizations. However, it decreases significantly at gel effect stage for St polymerization. Using the equations derived and the apparent initiator efficiencies obtained from TSPM plots, the number-average molecular weights at different conversions can be predicted. Comparisons show that the agreement between predictions and experimental data is satisfactory. 相似文献
103.
Two series of polypropylenes with different molecular weight distribution and tacticity characteristics were injection molded into flexural test specimens by varying cylinder temperature and the effects of the molecular weight distribution and tacticity on the structure and properties of the moldings were studied. Measured propertied were flexural modulus, flexural strength, heat distortion temperature, Izod impact strength, and mold shrinkage and structures studied were crystallinity, the thickness of skin layer, a*‐axis‐oriented component fraction and crystalline orientation functions. The relations between the structures and properties were also studied. It was found that the molecular weight distribution and tacticity characteristics affect the properties mainly through the molecular orientation and crystallinity, respectively. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 2142–2156, 2002 相似文献
104.
C. Ranganathaiah 《应用聚合物科学杂志》2002,86(13):3336-3345
I measured positron lifetime in natural polymer–cotton fibers as a function of isochronal annealing temperature in the range 27–290°C. The variations in the positron results indicated structural changes occurring in the cotton fibers and determined the glass‐transition temperature as 80°C. Activation energies were measured separately for the crystalline and amorphous regions, indicating the versatility of the technique. These values were close to the O? H bond dissociation energy, suggesting O? H bond dissociation, the most probable process occurring under thermal treatment. As an extension of the positron results, the molecular weight of the cotton fibers was determined to be 1,200,000 based on free volume, which was within the range suggested for cotton. There seemed to be an indication that crosslinking changed the spiral structure of cotton fibers to the network type. However, this needs to be validated by other measurements. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3336–3345, 2002 相似文献
105.
106.
自聚焦棒耦合性能的分析 总被引:1,自引:0,他引:1
本文采用光线光学的原理分析了自聚焦棒(GRIN)的光学性能,在折射率分布为抛物线近似的条件下,推导出较为简单的GRIN中的光线轨迹方程,并实验测量了GRIN的耦合效率,从而由理论和实验两方面讨论GRIN的聚焦特性。 相似文献
107.
He Ping 《电子科学学刊(英文版)》1994,11(4):296-302
The minimum squared Euclidean distance(MSED) of binary multi-h phase codes is presented. The signal segregation degree(SSD) has been put forward to determine MSED of multi-h phase codes. In order to maximize MSED, SSD should be as large as possible. The necessary and sufficient conditions of maximizing
SSD are derived. Finally, SSD and the exact formulae for MSED of binary 2-h phase codes are also presented. 相似文献
108.
本文提出光束轨迹方程的一般解,导出解析解的存在条件,推广了文献[1]的光线传播理论,文中以一特定梯度折射率棒为例,讨论了高斯光束在棒中的传播特性。 相似文献
109.
谢意红 《宝石和宝石学杂志》2002,4(4):28-31
市场上出现的紫色、浅紫色、浅黄色、粉红色、酒黄色、黄褐色、红色人造立方氧化锆均为亚金刚光泽, 火彩明显; 密度为5.99~6.34 g/cm3, 相对硬度为8~9; 除粉红色者表现出非均质性和弱多色性且内含大量定向排列的白色脱溶物外, 其余的为均质性, 内部洁净; 不同颜色的立方氧化锆的吸收光谱、紫外荧光以及滤色镜下的变色情况均存在差异; 浅紫色立方氧化锆的吸收光谱特征表明它是Nd致色. 能谱分析结果表明: 彩色立方氧化锆的常见元素有Zr, Y, P, Hf, Ca等; Er, Ce, Fe分别是粉红色、红色、酒黄色立方氧化锆的致色元素; 当Y2O3的摩尔分数大于或近于7%时, 立方氧化锆为立方面心结构且内部洁净; 当Y2O3的摩尔分数小于5%时, 立方氧化锆表现为非均质性且内含大量定向分布的白色脱溶物. 用拉尔森改写的格拉斯顿-代尔公式计算其近似折射率为2.12~2.19. 相似文献
110.
基于供应链管理的供应商选择问题初探 总被引:56,自引:3,他引:53
首先讨论了供应链环境下选择供应商的意义,然后建立了供应商选择的评价指标体系,重点讨论了应用TOPSIS 方法对供应商选择的模型及算法,最后用一个简单的案例进行了说明。 相似文献