全文获取类型
收费全文 | 13487篇 |
免费 | 986篇 |
国内免费 | 761篇 |
专业分类
电工技术 | 221篇 |
综合类 | 987篇 |
化学工业 | 2320篇 |
金属工艺 | 1057篇 |
机械仪表 | 466篇 |
建筑科学 | 628篇 |
矿业工程 | 688篇 |
能源动力 | 524篇 |
轻工业 | 682篇 |
水利工程 | 205篇 |
石油天然气 | 4359篇 |
武器工业 | 167篇 |
无线电 | 404篇 |
一般工业技术 | 1030篇 |
冶金工业 | 513篇 |
原子能技术 | 134篇 |
自动化技术 | 849篇 |
出版年
2024年 | 40篇 |
2023年 | 160篇 |
2022年 | 323篇 |
2021年 | 382篇 |
2020年 | 399篇 |
2019年 | 309篇 |
2018年 | 303篇 |
2017年 | 378篇 |
2016年 | 403篇 |
2015年 | 379篇 |
2014年 | 635篇 |
2013年 | 904篇 |
2012年 | 810篇 |
2011年 | 901篇 |
2010年 | 691篇 |
2009年 | 722篇 |
2008年 | 621篇 |
2007年 | 756篇 |
2006年 | 832篇 |
2005年 | 734篇 |
2004年 | 617篇 |
2003年 | 586篇 |
2002年 | 543篇 |
2001年 | 479篇 |
2000年 | 403篇 |
1999年 | 350篇 |
1998年 | 280篇 |
1997年 | 231篇 |
1996年 | 211篇 |
1995年 | 183篇 |
1994年 | 127篇 |
1993年 | 87篇 |
1992年 | 66篇 |
1991年 | 66篇 |
1990年 | 83篇 |
1989年 | 57篇 |
1988年 | 29篇 |
1987年 | 24篇 |
1986年 | 25篇 |
1985年 | 17篇 |
1984年 | 16篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1977年 | 3篇 |
1964年 | 5篇 |
1957年 | 3篇 |
1956年 | 3篇 |
1951年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The changes of the molecular weight and particle size with time during the oxidative polymerization of 2,6‐dimethylphenol in an aqueous medium were studied. At the beginning of the oxidative polymerization, the oligomers with the hydrophilic phenoxy anion at the end of oligomer chains are formed rapidly in the aqueous medium. When the molecular weight of the oligomer reaches up to a critical value, the oligomer precipitates out from the water, resulting in the formation of the original particle (or domain). With the increase of the molecular weight, the concentration of the phenoxy anion and the surface charge density of the original particles decrease; therefore, the repulsion force between original particles weaken and the stability of particles in water decreases, resulting in the coagulation of the original particle and the formation of the primary particle. With the further progression of the polymerization, the primary particles coagulate and final particles are formed. A three‐stage mechanism of the particle formation is proposed, that is, the particle nucleation, first coagulation, and second coagulation. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3649–3653, 2007 相似文献
992.
Albert Rimola Piero Ugliengo Mariona Sodupe 《International journal of molecular sciences》2009,10(3):746-760
The condensation (polymerization by water elimination) of molecular building blocks to yield the first active biopolymers (e.g. of amino acids to form peptides) during primitive Earth is an intriguing question that nowadays still remains open since these processes are thermodynamically disfavoured in highly dilute water solutions. In the present contribution, formation and hydrolysis of glycine oligopeptides occurring on a cluster model of sanidine feldspar (001) surface have been simulated by quantum mechanical methods. Results indicate that the catalytic interplay between Lewis and Brønsted sites both present at the sanidine surface, in cooperation with the London forces acting between the biomolecules and the inorganic surface, plays a crucial role to: i) favour the condensation of glycine to yield oligopeptides as reaction products; ii) inhibit the hydrolysis of the newly formed oligopeptides. Both facts suggest that mineral surfaces may have helped in catalyzing, stabilizing and protecting from hydration the oligopeptides formed in the prebiotic era. 相似文献
993.
Classification of weld flaws with imbalanced class data 总被引:1,自引:0,他引:1
This paper presents research results of our investigation of the imbalanced data problem in the classification of different types of weld flaws, a multi-class classification problem. The one-against-all scheme is adopted to carry out multi-class classification and three algorithms including minimum distance, nearest neighbors, and fuzzy nearest neighbors are employed as the classifiers. The effectiveness of 22 data preprocessing methods for dealing with imbalanced data is evaluated in terms of eight evaluation criteria to determine whether any method would emerge to dominate the others. The test results indicate that: (1) nearest neighbor classifiers outperform the minimum distance classifier; (2) some data preprocessing methods do not improve any criterion and they vary from one classifier to another; (3) the combination of using the AHC_KM data preprocessing method with the 1-NN classifier is the best because they together produce the best performance in six of eight evaluation criteria; and (4) the most difficult weld flaw type to recognize is crack. 相似文献
994.
995.
天然气成藏过程的地球化学示踪研究现状 总被引:1,自引:1,他引:0
程付启 《油气地质与采收率》2008,15(6):14-18
天然气成藏过程的地球化学示踪研究,即利用天然气残留的地球化学信息,反演其生成、运移、聚集和保存的动态过程,以确定其来源、资源前景和分布规律,并提供有利的勘探目标.通过数十年的研究,中外地球化学家提出了多种类型的示踪指标,并对各类指标在天然气生成、运移、聚集及保存等成藏过程中的行为有了一定的认识:天然气烷烃组分、稳定同位素、轻烃及稀有气体是天然气源对比、重塑运聚成藏过程的有效指标,它们共同构成了天然气成藏示踪的指标体系;初步揭示了天然气组分组成、同位素组成等指标在不同阶段成藏的动力学变化规律,成为利用指标重塑天然气成藏过程的依据.在总结已有成果的基础上,结合目前研究中与地质背景结合少、机理与定量化研究程度低、混源成藏及散失过程研究薄弱等问题,认为天然气成藏过程的地球化学示踪应加强地质条件分析、物理模拟与数理模型建立、混源成藏及散失过程示踪方面的研究. 相似文献
996.
The physical system studied is a brittle elastic film bonded to an elastic substrate with different elastic properties; a residual tensile stress is presumed to exist in the film. The focus of the study is the influence of the mismatch in elastic properties on patterns of crack formation in the film. The stress intensity factor and crack driving force for growth of a periodic array of cracks in the direction normal to the interface under two-dimensional conditions are determined for any crack depth and any mismatch in elastic parameters. It is found that, even for a relatively stiff film material, the stress intensity factor of each crack as a function of crack depth exhibits a local maximum. The driving force for crack extension in the direction parallel to the interface is then determined on the basis of these two-dimensional results, and the equilibrium spacing of crack arrays is estimated for given residual stress. The results of the calculations are used as a basis for qualitative arguments to explain the crack patterns which have been observed in GaN films on Si substrates. 相似文献
997.
Effects of inductively coupled plasma conditions on the etch properties of GaN and ohmic contact formations 总被引:1,自引:0,他引:1
Hyeon-Soo Kim Yong-Hyuk Lee Geun-Young Yeom Jae-Won Lee Tae-Il Kim 《Materials Science and Engineering: B》1997,50(1-3):82-87
In this study, n-GaN was etched using inductively coupled Cl2/H2 plasmas and the effects of plasma conditions on the etch properties, surface composition and ohmic contact formation were investigated as a function of gas composition using OES (optical emission spectroscopy), SEM (scanning electron microscope), XPS (X-ray photoelectron spectroscopy), AES (Auger electron spectroscopy) and TLM (transmission line method). The addition of hydrogen to Cl2 plasma decreased GaN etch rate and changed the surface composition from Ga-rich to N-rich. Etched profiles were near vertical with a smooth sidewall, however, the pure Cl2 case showed the most anisotropic etch profile. Specific contact resistivity was increased with increasing hydrogen percent in Cl2/H2, however, most of contact resistivities of the contacts fabricated on the GaN etched with Cl2/H2 (≤75% H2) were less than those fabricated on the non-etched GaN. 相似文献
998.
Scale formation on the Ni-base superalloys IN 939 and IN 738 LC 总被引:1,自引:0,他引:1
The scales formed on the superalloys IN 939 and IN 738 LC at 700, 900, and 1100°C in air have been investigated by optical, microprobe analysis and x-ray diffraction measurements. Both alloys form very similar scales. The main components are an outer TiO2 layer, an intermediate Cr2O3 layer with dissolved Ti, and an inner layer of (Ti, Nb, Ta)O2 with rutile structure. Beneath the scale an internal corrosion zone is formed that contained Al2O3 directly beneath the external scale and TiN further into the substrate. 相似文献
999.
1000.
分别对AurdCAD和Word中几种图形格式及生成与调用方法进行了说明,给出了在Windows环境下,通过Word、AutoCAD的数据交换,利用中文Word对AutoCAD中的工程图纸进行输出的方法。 相似文献