全文获取类型
收费全文 | 27739篇 |
免费 | 2144篇 |
国内免费 | 1351篇 |
专业分类
电工技术 | 890篇 |
综合类 | 872篇 |
化学工业 | 5426篇 |
金属工艺 | 2710篇 |
机械仪表 | 5374篇 |
建筑科学 | 377篇 |
矿业工程 | 201篇 |
能源动力 | 496篇 |
轻工业 | 1203篇 |
水利工程 | 59篇 |
石油天然气 | 280篇 |
武器工业 | 106篇 |
无线电 | 4517篇 |
一般工业技术 | 6557篇 |
冶金工业 | 481篇 |
原子能技术 | 874篇 |
自动化技术 | 811篇 |
出版年
2024年 | 62篇 |
2023年 | 392篇 |
2022年 | 586篇 |
2021年 | 730篇 |
2020年 | 726篇 |
2019年 | 752篇 |
2018年 | 743篇 |
2017年 | 907篇 |
2016年 | 911篇 |
2015年 | 953篇 |
2014年 | 1211篇 |
2013年 | 1672篇 |
2012年 | 1538篇 |
2011年 | 2118篇 |
2010年 | 1538篇 |
2009年 | 1590篇 |
2008年 | 1684篇 |
2007年 | 1574篇 |
2006年 | 1441篇 |
2005年 | 1270篇 |
2004年 | 1100篇 |
2003年 | 999篇 |
2002年 | 936篇 |
2001年 | 626篇 |
2000年 | 620篇 |
1999年 | 575篇 |
1998年 | 500篇 |
1997年 | 512篇 |
1996年 | 422篇 |
1995年 | 360篇 |
1994年 | 296篇 |
1993年 | 261篇 |
1992年 | 234篇 |
1991年 | 232篇 |
1990年 | 203篇 |
1989年 | 180篇 |
1988年 | 164篇 |
1987年 | 104篇 |
1986年 | 92篇 |
1985年 | 118篇 |
1984年 | 104篇 |
1983年 | 77篇 |
1982年 | 67篇 |
1981年 | 14篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1978年 | 3篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1975年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
The dynamic mechanical relaxation of non-crystalline poly(aryl ether-ether-ketone) PEEK and the one irradiated with electron beam were studied. The three distinct γ, β, α′ relaxation maxima were observed in unirradiated PEEK from low to high temperature. It was revealed from the study on the irradiation effects that three different molecular processes are overlapped in γ relaxation peak, i.e., molecular motion of water bound to main chain (peak temperature; at ?100°C), local motion of main chain (at ?80°C), and local mode of the aligned and/or oriented moiety (at ?40°C). The β relaxation connected with the glass transition occurred at 150°C and it shifted to higher temperature by irradiation. The α′ relaxation which can be attributed to rearrangement of molecular chain due to crystallization was observed in unirradiated PEEK ~ 180°C and its magnitude decreased with the increase in irradiation dose. This effect indicates the formation of structures inhibiting crystallization such as crosslinking and/or short branching during irradiation. A new relaxation, β′, appeared in the temperature range of 40° to 100°C by irradiation and its magnitude increased with dose. This relaxation was attributed to rearrangement of molecular chain from loosened packing around chain ends, which were introduced into the non-crystalline region by chain scission under irradiation, to more rigid molecular packing, From these observations, we proposed that deterioration in mechanical properties of non-crystalline PEEK by high energy electron beam was brought about not only by chain scission but structural changes such as crosslinking and/or branching in the main chain. 相似文献
72.
To study the effect of the molecular architecture of a copolymer on its miscibility with corresponding homopolymers a series of block copolymers of styrene and isoprene with diblock, triblock and four-arm star architectures have been prepared and the morphologies of the blends of the copolymers and polyisoprene with different molecular weights have been examined by electron microscopy. The results show that miscibility varies in the sequence diblock>triblock>four-arm star copolymers. This sequence is in the opposite direction to the variation of the architectural complexity of the block copolymers, i.e. the more complex is molecular architecture, the greater is conformation restriction in microdomain formation and the less is solubility of homopolymer in corresponding domains. 相似文献
73.
Several polymer‐layered silicate (clay) nanocomposites (PLSNs) were analyzed by transmission electron microscopy (TEM) and wide‐angle X‐ray diffraction (XRD) in an effort to characterize the nanoscale dispersion of the layered silicate. The PLSNs investigated included thermoset (cyanate esters) and thermoplastic polymers (polystyrene, nylon 6, and polypropylene‐g‐maleic anhydride). The results of this study reveal that the overall nanoscale dispersion of the clay in the polymer is best described by TEM, especially when mixed morphologies are present. XRD is useful for the measurement of d‐spacings in intercalated systems but cannot always observe low clay loadings (<5%) or be used as a method to identify an exfoliated nanocomposite where no XRD peaks are present (constituting a negative result). Most importantly, the study showed that XRD is not a stand‐alone technique, and it should be used in conjunction with TEM. Our studies suggest that new definitions, or a clarification of existing definitions, are needed to properly describe the diversity of PLSN nanostructures seen in various materials. Published 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 1329–1338, 2003 相似文献
74.
CHEN KeFan DENG JianHua ZHAO Fei CHENG GuoAn &ZHENG RuiTing Key Laboratory of Beam Technology Material Modification of Ministry of Education College of Nuclear Science Technology Beijing Normal University Beijing China 《中国科学:信息科学(英文版)》2010,(3)
The structures and field emission properties of multi-walled carbon nanotube arrays implanted with Zn+ by MEVVA ion implanter have been investigated.The results revealed that Zn+implantation induced structural damage and that the top of carbon nanotubes with multi-layered graphite structure were transformed into carbon nanowires with amorphous structure.Meanwhile,C:Zn solid solution was synthesized after Zn+ implantation.The turn-on field and threshold field were 0.80 and 1.31 V/μm,respectively for original... 相似文献
75.
研究了两种含有给电子体分别为对硝基苯胺、乙二醇二甲醚的Ziegler-Natta催化剂催化乙烯与1-丁烯高压的共聚反应性能,主要考察了反应条件对聚合活性、聚合物物理性质的影响,同时采用红外光谱法测定了聚合物的支化度.研究表明,给电子体为乙二醇二甲醚的催化剂催化活性高于对硝基苯胺.反应条件如反应压力、温度、铝钛比对聚合活性、丁烯的插入率以及聚合物性质都有明显影响.高的反应压力有利于1-丁烯插入和聚合物堆密度的提高;反应温度在50℃时的聚合活性最大,聚合物的支化度较高,聚合物熔点较低;铝钛比为500时聚合活性高于低铝钛比的聚合活性. 相似文献
76.
采用不同的方法测定膨胀石墨对不同油质的饱和吸附量,来探讨膨胀石墨的结构与饱和吸附量的关系.由扫描电镜可知,膨胀石墨具有较大的比表面积,其孔结构是以中、大孔为主;蠕虫石墨的表面结构与其内部结构不同,其内部为典型的片层结构.通过观察石墨内部孔结构特征变化,可以分析膨胀石墨对油的去除效率以及影响因素. 相似文献
77.
Atsushi Tanaka 《Carbon》2004,42(3):591-597
The present study confirmed that highly crystalline nanofibers with controlled structure may be prepared over Fe and Fe-Ni alloy catalysts. The degree of graphitization of various carbon nanofibers (CNFs) was analyzed by using C(0 0 2) peaks from the XRD profiles. The C(0 0 2) peaks of CNFs over Fe catalyst shifted to higher angle and became narrower as the preparation temperature increased from 560 to 620 °C. Tubular CNFs prepared at temperature higher than 630 °C showed lower 2θ angles compared to those of platelet fibers. CNFs prepared over Fe-Ni catalysts tended to resemble those prepared over Fe catalysts. The degree of graphitization of platelet CNFs resembled natural graphite, while d0 0 2 of the tubular CNFs showed values below the 3.39 Å reported as a theoretical minimum for a cylindrical alignment. Lc0 0 2 of platelet and tubular CNFs increased by heat treatment at 2000 and 2800 °C though d0 0 2 changed little. A transverse section of platelet and tubular CNFs had a hexagonal shape, not a round shape. The hexagonal column allows AB stacking of hexagonal planes that can give perfect hexagonal alignment. 相似文献
78.
79.
利用传感技术为监测手段开发一套基于3G传输网络的道路气象预警系统,使道路管理部门精确掌握气象资料和天气预报,并根据各路段的地形及气候特点,结合路段交通状况,制定各自的应急预案,以保证道路的行车安全和顺畅。此系统对道路沿线气象状况进行监测、分析并采取有效的应急预案,对保障道路过往车辆和人身财产安全有着积极作用。 相似文献
80.
前文报道了外加磁场对吸附在硅胶表面上的芳砜光解反应的影响,本文报道外加磁场影响光化学反应的另一个例子——电子转移光敏化1,2-二苯乙烯的异构化。 近年来电子转移光敏化反应引起了有机光化学家们的极大注意。一些异构化、重排、裂解、氧化反应都可以通过电子转移机理敏化。电子转移光敏化的机理可以用(1)描述。图中A和D分别代表电子的受体和给体。 相似文献