全文获取类型
收费全文 | 245377篇 |
免费 | 3366篇 |
国内免费 | 1248篇 |
专业分类
电工技术 | 4917篇 |
综合类 | 264篇 |
化学工业 | 35951篇 |
金属工艺 | 10934篇 |
机械仪表 | 7102篇 |
建筑科学 | 6280篇 |
矿业工程 | 481篇 |
能源动力 | 6339篇 |
轻工业 | 24358篇 |
水利工程 | 1838篇 |
石油天然气 | 1338篇 |
武器工业 | 3篇 |
无线电 | 31245篇 |
一般工业技术 | 45353篇 |
冶金工业 | 50496篇 |
原子能技术 | 3935篇 |
自动化技术 | 19157篇 |
出版年
2021年 | 1604篇 |
2019年 | 1438篇 |
2018年 | 2352篇 |
2017年 | 2254篇 |
2016年 | 2481篇 |
2015年 | 1930篇 |
2014年 | 3213篇 |
2013年 | 10879篇 |
2012年 | 5525篇 |
2011年 | 7892篇 |
2010年 | 6350篇 |
2009年 | 7052篇 |
2008年 | 7647篇 |
2007年 | 7746篇 |
2006年 | 7112篇 |
2005年 | 6591篇 |
2004年 | 6328篇 |
2003年 | 6339篇 |
2002年 | 6316篇 |
2001年 | 6408篇 |
2000年 | 5844篇 |
1999年 | 6460篇 |
1998年 | 16724篇 |
1997年 | 11690篇 |
1996年 | 9025篇 |
1995年 | 6736篇 |
1994年 | 5977篇 |
1993年 | 5778篇 |
1992年 | 4083篇 |
1991年 | 4029篇 |
1990年 | 3623篇 |
1989年 | 3537篇 |
1988年 | 3467篇 |
1987年 | 2933篇 |
1986年 | 2866篇 |
1985年 | 3438篇 |
1984年 | 3062篇 |
1983年 | 2757篇 |
1982年 | 2565篇 |
1981年 | 2579篇 |
1980年 | 2444篇 |
1979年 | 2286篇 |
1978年 | 2247篇 |
1977年 | 2820篇 |
1976年 | 4201篇 |
1975年 | 1907篇 |
1974年 | 1798篇 |
1973年 | 1787篇 |
1972年 | 1467篇 |
1971年 | 1319篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
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
82.
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
83.
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
84.
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
85.
86.
87.
88.
Dr. Vlastimil Jirasko Dr. Nils-Alexander Lakomek Dr. Susanne Penzel Dr. Marie-Laure Fogeron Prof. Dr. Ralf Bartenschlager Prof. Dr. Beat H. Meier Dr. Anja Böckmann 《Chembiochem : a European journal of chemical biology》2020,21(10):1453-1460
Proton-detected 100 kHz magic-angle-spinning (MAS) solid-state NMR is an emerging analysis method for proteins with only hundreds of microgram quantities, and thus allows structural investigation of eukaryotic membrane proteins. This is the case for the cell-free synthesized hepatitis C virus (HCV) nonstructural membrane protein 4B (NS4B). We demonstrate NS4B sample optimization using fast reconstitution schemes that enable lipid-environment screening directly by NMR. 2D spectra and relaxation properties guide the choice of the best sample preparation to record 2D 1H-detected 1H,15N and 3D 1H,13C,15N correlation experiments with linewidths and sensitivity suitable to initiate sequential assignments. Amino-acid-selectively labeled NS4B can be readily obtained using cell-free synthesis, opening the door to combinatorial labeling approaches which should enable structural studies. 相似文献
89.
90.
A new constitutive equation of thermoelasticity for crystals is presented based on the interatomic potential and solid mechanics at finite temperature. Using the new constitutive equation, the calculations for crystal copper and graphene are carried out under different loading paths at different temperatures. The calculated results are in good agreement with those of the previous thermoelasticity constitutive equation based on quantum mechanics, which clearly indicates that our new constitutive equation of thermoelasticity is correct. A lot of comparisons also show that the present theory is more concise and efficient than the previous thermal stress theory in the practical application. 相似文献