全文获取类型
收费全文 | 262517篇 |
免费 | 4573篇 |
国内免费 | 1285篇 |
专业分类
电工技术 | 5095篇 |
综合类 | 282篇 |
化学工业 | 39827篇 |
金属工艺 | 11634篇 |
机械仪表 | 8268篇 |
建筑科学 | 6620篇 |
矿业工程 | 487篇 |
能源动力 | 7094篇 |
轻工业 | 25833篇 |
水利工程 | 1879篇 |
石油天然气 | 1350篇 |
武器工业 | 3篇 |
无线电 | 34281篇 |
一般工业技术 | 48892篇 |
冶金工业 | 51487篇 |
原子能技术 | 4104篇 |
自动化技术 | 21239篇 |
出版年
2021年 | 2220篇 |
2020年 | 1508篇 |
2019年 | 1901篇 |
2018年 | 2997篇 |
2017年 | 2857篇 |
2016年 | 3189篇 |
2015年 | 2482篇 |
2014年 | 4050篇 |
2013年 | 12084篇 |
2012年 | 6832篇 |
2011年 | 9456篇 |
2010年 | 7471篇 |
2009年 | 8193篇 |
2008年 | 8647篇 |
2007年 | 8464篇 |
2006年 | 7742篇 |
2005年 | 7114篇 |
2004年 | 6776篇 |
2003年 | 6776篇 |
2002年 | 6743篇 |
2001年 | 6798篇 |
2000年 | 6161篇 |
1999年 | 6780篇 |
1998年 | 17162篇 |
1997年 | 11951篇 |
1996年 | 9297篇 |
1995年 | 6907篇 |
1994年 | 6039篇 |
1993年 | 5852篇 |
1992年 | 4133篇 |
1991年 | 4078篇 |
1990年 | 3642篇 |
1989年 | 3577篇 |
1988年 | 3502篇 |
1987年 | 2963篇 |
1986年 | 2883篇 |
1985年 | 3446篇 |
1984年 | 3077篇 |
1983年 | 2764篇 |
1982年 | 2571篇 |
1981年 | 2589篇 |
1980年 | 2453篇 |
1979年 | 2282篇 |
1978年 | 2238篇 |
1977年 | 2830篇 |
1976年 | 4201篇 |
1975年 | 1913篇 |
1974年 | 1806篇 |
1973年 | 1790篇 |
1972年 | 1465篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
101.
The reported point centromeres of Scheffersomyces stipitis are retrotransposon long terminal repeats
Point centromeres, found in some ascomycete yeasts such Saccharomyces cerevisiae, are very different in structure from the centromeres of other eukaryotes. They are tiny and nonrepetitive and contain only two short conserved sequence motifs. Until recently, point centromeres were thought to have a single evolutionary origin, in the budding yeast family Saccharomycetaceae. Most yeasts outside this family have centromeres that are many kilobases in size. Some have centromeres consisting of a large inverted repeat sequence, others have centromeric clusters of retrotransposons, and a third group including Candida albicans has centromeres with no conserved sequence features. It was recently reported that Scheffersomyces stipitis has point centromeres with a strongly conserved 125-bp core sequence, which is unexpected because S. stipitis is only distantly related to the known point-centromere species. We show here that the 125-bp core sequence is actually part of the long terminal repeat (LTR) of the Ty5-like retrotransposon Tps5, which forms a cluster in the centromeric region of each S. stipitis chromosome. Thus, the LTR of a centromere-associated retrotransposon confers centromere-like mitotic stability when cloned into a plasmid. The centromeric regions of S. stipitis contain three types of Tps5 element (Tps5a, Tps5b, and Tps5c) and a noncoding nonautonomous large retrotransposon derivative. 相似文献
102.
Antti Voss Niko Hänninen Mohammad Pour-Ghaz Marko Vauhkonen Aku Seppänen 《Materials and Structures》2018,51(3):68
The development of visualizing tools to monitor unsaturated moisture flow in cement-based materials is of great importance, as most degradation processes in cement-based materials are connected to and take place in the presence moisture. This paper investigates the ability of electrical capacitance tomography (ECT) to image two-dimensional (2D) unsaturated moisture flow in cement-based materials. In ECT, the electrical permittivity distribution within an object is reconstructed based on measured capacitances between electrodes attached on the object’s surface. In a series of experiments, mortar specimens with and without discrete cracks were imaged with ECT during a 2D moisture ingress. The results show that ECT is able to monitor the evolution of the moisture flow, and to approximate the shape and position of the moisture front. These findings indicate that ECT is a viable method for monitoring and visualizing 2D unsaturated moisture flow in cement-based materials in the presence and absence of discrete cracks. 相似文献
103.
An agriglass composition containing different oxides acts as a slow release for macro and micro nutrients and was chosen to improve maize yield under most important abiotic stresses which affecting agriculture development; salinity and drought. A field experiment was performed in salt affected soil (EC =?7.5 dSm??1) by using different water deficit rates (I1 = 100, I2 = 85 and I3 = 70% of maize water requirements). Irrigation levels were located in main plots. Every main-plot divided into six sub-plots contained glassy fertilizer treatments [F1 = 55 kg fed?1 with 1/2 mm diameter of agriglass (fed. =?4200 m2), F2 = 55 kg fed?1 with 1 mm diameter, F3 = 80 kg fed?1 with 1/2 mm diameter, F4 = 80 kg fed?1 with 1 mm diameter, F5 = Recommendations of Ministry of Agriculture and F6 = control]. The experimental results demonstrated that, ears, straw, grains and biological yields increased with increasing both water and agriglass rates. Application of agriglass as a slow release fertilizer improved yield more than mineral fertilizer. Some growth parameters, water use efficiency (IWUE), macronutrients concentration and their relations were included. Other studies on residual effect of agriglass and the annual application rates to withstand salinity and drought stress by strategic crops are required. 相似文献
104.
Radiochemistry - Azathioprine, an antitumor agent, was labeled with 99mTc using stannous chloride dihydrate as a reducing agent. Factors such as the amounts of the reducing agent and substrate, pH,... 相似文献
105.
Estimation of earth-fill dam height and dimension of spillway are strongly depend on accurate evaluation of dam overtopping reliability. Ideally, whole random and effective variables on overtopping should be considered for dam overtopping reliability assessment. However, taking into account all affecting random variables as well as overtopping reliability assessment using algorithms such as Monte Carlo (MC) could be excessively time consuming and impossible in some cases. In this study, new approach has been introduced as conditional reliability method and it has been tried to reduce the simulations time significantly by using combination of rainfall threshold theory, SUFI (Sequential Uncertainty Fitting) and MC methods. According two defined indexes, the obtain results shown, the relative error of new approach in computation of Jamishan dam overtopping reliability is less than 0.23%. Also, time saving in new method against to regular MC method is more than 87%. So, the new introduced method has more efficiency with acceptable accuracy for assessment of dam overtopping reliability. In addition, among hydrological and hydraulic uncertainties, hydrological uncertainties have more effect on overtopping reliability. 相似文献
106.
107.
108.
109.
Park Sangwon Nam Hyunbin Na Youngsang Kim Hyoungseop Moon Younghoon Kang Namhyun 《Metals and Materials International》2020,26(5):641-649
Metals and Materials International - This study investigated the influence of the initial grain size on the plastic deformation and tunnel defects that occurred from friction stir welding of... 相似文献
110.
Mohammad Reza Gaeeni Morteza Sasani Ghamsari Abbas Majd Abadi Mohammad Hossein Majles Ara Wooje Han Hyung‐Ho Park 《Journal of the American Ceramic Society》2015,98(6):1818-1822
A facile sol–gel procedure has been developed for the synthesis of colloidal alumina nanocrystals. For the first time, optical characterization procedures were employed to study the quantum confinement effects in optical properties of the prepared Al2O3 sol. Accordingly, the hyperbolic band model was used to determine the optical band gap of colloidal alumina nanocrystals. X‐Ray diffraction pattern was used to study the crystallographic phase of the dried gel. Morphological characterization was performed using scanning electron microscopy (SEM). Inductively Coupled Plasma (ICP) emission spectroscopy was used to determination purity of the Al2O3 powder. High‐resolution TEM showed that the diameter of colloidal nanocrystals is about 10 nm. Photoluminescence spectroscopy demonstrated that quantum yields for colloidal nanocrystals are 68% with 300 nm excitation wavelength. The experimental observations confirm that highly stable alumina sol with strong UV emission was synthesized. The mentioned optical properties have not been reported before. 相似文献