全文获取类型
收费全文 | 11603篇 |
免费 | 1792篇 |
国内免费 | 732篇 |
专业分类
电工技术 | 309篇 |
综合类 | 746篇 |
化学工业 | 4758篇 |
金属工艺 | 1387篇 |
机械仪表 | 606篇 |
建筑科学 | 421篇 |
矿业工程 | 139篇 |
能源动力 | 192篇 |
轻工业 | 580篇 |
水利工程 | 467篇 |
石油天然气 | 171篇 |
武器工业 | 105篇 |
无线电 | 875篇 |
一般工业技术 | 1822篇 |
冶金工业 | 416篇 |
原子能技术 | 42篇 |
自动化技术 | 1091篇 |
出版年
2024年 | 43篇 |
2023年 | 199篇 |
2022年 | 269篇 |
2021年 | 472篇 |
2020年 | 439篇 |
2019年 | 422篇 |
2018年 | 453篇 |
2017年 | 538篇 |
2016年 | 572篇 |
2015年 | 567篇 |
2014年 | 716篇 |
2013年 | 815篇 |
2012年 | 811篇 |
2011年 | 835篇 |
2010年 | 666篇 |
2009年 | 681篇 |
2008年 | 636篇 |
2007年 | 726篇 |
2006年 | 694篇 |
2005年 | 508篇 |
2004年 | 495篇 |
2003年 | 417篇 |
2002年 | 343篇 |
2001年 | 249篇 |
2000年 | 217篇 |
1999年 | 201篇 |
1998年 | 185篇 |
1997年 | 170篇 |
1996年 | 99篇 |
1995年 | 115篇 |
1994年 | 87篇 |
1993年 | 74篇 |
1992年 | 86篇 |
1991年 | 71篇 |
1990年 | 47篇 |
1989年 | 40篇 |
1988年 | 21篇 |
1987年 | 23篇 |
1986年 | 14篇 |
1985年 | 24篇 |
1984年 | 20篇 |
1983年 | 12篇 |
1982年 | 33篇 |
1981年 | 7篇 |
1980年 | 1篇 |
1979年 | 6篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1975年 | 4篇 |
1973年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 123 毫秒
31.
32.
The high-temperature crystallization of poly(p-phenylene terephthalamide) (PPTA) from dilute organic solutions was achieved through the introduction of a non-solvent, or precipitating agent, at the desired crystallization temperature. The morphology and crystal structure were examined for crystals produced from PPTA polymer with two different molecular weights (Mw = 46000 and 3430 g mol−1), using transmission electron microscopy. For the high-molecular-weight polymer, ribbon-like crystals were produced, while the low-molecular-weight polymer yielded small needles or platelets. In both cases, electron diffraction showed that the Northolt allomorph was obtained. For the high-molecular-weight polymer, the molecular axes were parallel to the ribbon axes in a chain-extended type structure. A hypothesis for the orientation of the low-molecular-weight PPTA in the small platelets, is also given. 相似文献
33.
Wu Ting M. Badaye T. Morishita N. Koshizuka S. Tanaka 《Journal of Superconductivity》1996,9(6):637-645
Recently, superconducting Nd1Ba2Cu3Oy (Ndl23) thin films with high superconducting transition temperature (T
c) have been successfully fabricated at our institute employing the standard laser ablation method. In this paper, we report the results of surface characterization of the Nd123 thin films using an ultrahigh vacuum scanning tunneling microscope/spectroscopy (UHV-STM/STS) and an atomic force microscope (AFM) system operated in air. Clear spiral pattern is observed on the surfaces of Nd123 thin films by STM and AFM, suggesting that films are formed by two-dimensional island growth mode. Contour plots of the spirals show that the step heights of the spirals are not always the integer or half-integer number of thec-axis parameter of the structure. This implies that the surface natural termination layer of the films may not be unique. This result is supported byI-V STS measurements. The surface morphology of the Nd123 thin films is compared with that of thec-axis-oriented Y1Ba2Cu3Sy thin films. Surface atomic images of the as-prepared Nd123 thin films are obtained employing both STM and AFM. STS measurements show that most of the surfaces are semiconductive. The results of STS measurements together with the fact that we are able to see the surface atomic images using scanning probe microscopes suggest that exposure to air does not cause serious degradation to the as-prepared surfaces of Nd123 thin films. 相似文献
34.
A mini spray dryer has been used to investigate morphological changes that occur to milk particles during the spray drying process. We have found that the mini spray dryer is ideal for such investigations, because phenomena such as skin and vacuole formation in particles can be analyzed without the added complication of particle agglomeration, which only occurs in much larger spray dryers where particle number concentrations are higher. We have confirmed observations made by various researchers that the bulk density of spray-dried milk powder is greatly affected by the drying temperature, due to the strong influence of the latter on the porosity of the particles. In addition, we have attempted to explain observations made by various workers that fat accumulates preferentially at the surface of a particle during drying by postulating that fluid fat is transported towards the surface, via a network of cracks and pores, by the development of a vacuole overpressure which is also responsible for the inflation of the particle. Finally, we have shown that milk powders can be spray dried a second time, by reconstitution with water, with no change to the thermodynamic characteristics of the resultant powder. Thus, milk concentrates for spray drying research can be prepared from already-spray-dried milk powders rather than using the more arduous evaporation method to concentrate unprocessed milk. 相似文献
35.
Thermal history and solute precipitation behavior of suspended solution droplets of sodium chloride (NaCl), magnesium sulphate (MgSO4), and zirconium hydroxychloride (ZrO(OH)Cl) evaporating at atmospheric and reduced pressures are studied. Experimental measurements on the variation of droplet diameter, solution concentration, and temperature during the evaporation period are presented and discussed. The results of solute precipitation behavior in solution droplets observed under an optical microscope are displayed and discussed. Results indicate that reducing the pressure (∼ 33 kPa) results in a change in the solution droplet evaporation rate, but the thermal histories of a particular solution droplet are similar at the atmospheric and reduced pressures. At atmospheric and reduced pressures used in this study, the d2 law for solution droplets is valid at early stages of the evaporation and before the solute precipitation initiates. Drying of MgSO4 and ZrO(OH)Cl solution droplets results in the formation of spherical particles, whereas drying of spherical NaCl solution droplets results in the formation of cubic particles. 相似文献
36.
Identification of gaps in mangrove forests with airborne LIDAR 总被引:2,自引:0,他引:2
Mangrove forests change frequently due to disturbances from tropical storms, frost, lightning, and insects. It has been suggested that the death and regeneration of trees in small gaps due to lightning may play a critical role in mangrove forest turnover; however, the large-scale quantification of spatial pattern and areas of gaps is lacking for investigating this issue. Airborne light detection and ranging (LIDAR) technology provides an effective way for identifying gaps by remotely obtaining direct measurements of ground and canopy elevations. A method based on an alternative sequential filter and black top-hat mathematical morphological transformation was developed to extract gap features. Comparison of identified gap polygons with raw LIDAR measurements and field surveys shows that the proposed method successfully extracted gap features in mangrove forests in Everglades National Park. There are 400–500 lightning gaps per square kilometer in mangrove forests at the study sites. The distribution of gap sizes follows an exponential form and the area of gaps with sizes larger than 100 m2 account for 55–61% of the total area of gaps. The area of gaps in the mangrove forest in Everglades National Park is about 4–5% of the total forest area and the average gap formation rate is about 0.3% of the total forest area per year, indicating that lightning gaps play an important role in mangrove forest dynamics. 相似文献
37.
38.
39.
通用的自然语言词法分析机制 总被引:4,自引:1,他引:3
本文提出了一种通用的自然语言词法分析技术,其处理机构不仅可以独立于具体自然语言,而且又具有适合于未来智能计算机直接处理的规则形式和实现过程.文中还详细讨论了独立于自然语言的词法分析技术,以及我们在IMT/EC系统中实现的词法分析机制的工作原理. 相似文献
40.