全文获取类型
收费全文 | 19007篇 |
免费 | 954篇 |
国内免费 | 137篇 |
专业分类
电工技术 | 326篇 |
综合类 | 34篇 |
化学工业 | 4175篇 |
金属工艺 | 593篇 |
机械仪表 | 635篇 |
建筑科学 | 369篇 |
矿业工程 | 47篇 |
能源动力 | 1371篇 |
轻工业 | 1164篇 |
水利工程 | 148篇 |
石油天然气 | 84篇 |
武器工业 | 1篇 |
无线电 | 2375篇 |
一般工业技术 | 4364篇 |
冶金工业 | 1441篇 |
原子能技术 | 175篇 |
自动化技术 | 2796篇 |
出版年
2024年 | 76篇 |
2023年 | 417篇 |
2022年 | 963篇 |
2021年 | 1162篇 |
2020年 | 914篇 |
2019年 | 947篇 |
2018年 | 1226篇 |
2017年 | 966篇 |
2016年 | 929篇 |
2015年 | 611篇 |
2014年 | 848篇 |
2013年 | 1529篇 |
2012年 | 899篇 |
2011年 | 1073篇 |
2010年 | 864篇 |
2009年 | 813篇 |
2008年 | 713篇 |
2007年 | 577篇 |
2006年 | 475篇 |
2005年 | 364篇 |
2004年 | 273篇 |
2003年 | 241篇 |
2002年 | 193篇 |
2001年 | 178篇 |
2000年 | 171篇 |
1999年 | 169篇 |
1998年 | 290篇 |
1997年 | 239篇 |
1996年 | 221篇 |
1995年 | 168篇 |
1994年 | 157篇 |
1993年 | 151篇 |
1992年 | 104篇 |
1991年 | 131篇 |
1990年 | 99篇 |
1989年 | 96篇 |
1988年 | 79篇 |
1987年 | 88篇 |
1986年 | 75篇 |
1985年 | 85篇 |
1984年 | 73篇 |
1983年 | 71篇 |
1982年 | 60篇 |
1981年 | 71篇 |
1980年 | 48篇 |
1979年 | 31篇 |
1978年 | 27篇 |
1977年 | 26篇 |
1976年 | 35篇 |
1971年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
131.
Md. A. Mohiddon R. Kumar P. Goel K. L.Yadav 《Dielectrics and Electrical Insulation, IEEE Transactions on》2007,14(1):204-211
The aim of this study is to determine the effect of Nb5+ doping on PZT (65/35) based bulk materials in their structure, micro structure and electrical properties. The Nb content was chosen 0-9 mole%. These materials were prepared by conventional mixed oxide method. X ray diffraction studies suggest the compound to be of rhombohedral perovskite phase. Excess addition of Nb result in pyrochlore and fluorite phase develops in PZT (65/35) sample. Detailed studies of dielectric constant as a function of temperature (40degC to 500degC) and frequency (100 Hz to 1 MHz) suggest that the compounds undergo diffuse type of phase transition. Maximum dielectric constant and resistivity were found to be strongly dependent on doping and measuring frequencies. Using complex impedance analysis micro structural parameters such as bulk and grain boundary resistance, bulk charge carrier concentration and relaxation time were determined 相似文献
132.
133.
134.
Power semiconductor devices find wide application in modern power electronic converters. Protection of these devices against overload/short circuit conditions is of paramount importance. Present day protection topologies employing different circuits have invariably one main drawback in that the fault current reaches the set value before action is initiated to trip the system. This poses a severe stress on the device. Hence an adequate safety margin has to be necessarily provided to prevent excessive device stresses and care has to be taken to see that the device is operated well within its safe operating areas. The present paper proposes a method wherein the slope or rate of rise of the fault current is detected and once the slope exceeds the set reference, action is initiated to trip the system much before the fault current reaches dangerous levels. The method provides a fast means of detection of overload and short circuit currents and can be conveniently adopted for the protection of devices in power transistor/IGBT based inverters against short circuited load conditions or shoot through faults. The possible reduction of stresses in the power devices are also highlighted 相似文献
135.
136.
A new finite element crack growth algorithm has been developed to simulate dynamic fracture. In this algorithm, pseudo elements with very high initial density are placed below the crack plane and the density is reduced to zero in a gradual manner as the crack passes the element. A number of linear elastic and elasto-viscoplastic problems have been carried out to test the new algorithm. The results are compared with some of the existing crack growth models. 相似文献
137.
138.
OBJECTIVE: To compare skinfold thickness measurements with bioelectrical impedance analysis (BIA) as a measure of body fat for use in a survey of children (the National Study of Health and Growth). DESIGN: Part cross-sectional, part repeated measurement study. SETTING: A junior school in Bath. SUBJECTS: 42 boys and 33 girls aged from 9 to 11 years. INTERVENTIONS: Measurements of BIA, height, weight, and triceps, biceps, subscapular and suprailiac skinfolds. RESULTS: All measurements were highly repeatable with intraclass correlation coefficients > 0.90. The level of agreement between estimates of percentage of body fat derived from prediction equations based on impedance or skinfold measurements respectively was poor: the mean difference (impedance estimate minus skinfold estimate) was 4.67% (95% range -3.47 to 12.82) for boys and 7.81% (95% range 1.27 to 14.34) for girls. The two estimates were found to correlate highly (r = 0.83 for boys and r = 0.81 for girls) because weight, used to convert estimates of fat-free mass derived from impedance to fat mass, was highly correlated with impedance and moderately highly correlated with skinfold thicknesses. The correlations of resistance (R) and (H)2/R with skinfold thicknesses were very low. There was a moderate correlation of R and H2/R with log(weight-for-height index), but lower than that of log(weight-for-height index) with each of the skinfolds. CONCLUSIONS: As currently available equations for converting impedance-based estimates of total body water to fat mass are not fully developed for use in children of varying ages, estimates of body fat calculated from skinfold thickness measurements remain preferable in epidemiological studies of children's health and growth. 相似文献
139.
Poly(vinylbenzyltrimethylammonium chloride)‐graft‐cotton cellulose, an anion‐exchange matrix, was synthesized by a mutual radiation‐induced grafting technique with a 60Co γ‐radiation source. The grafted matrix was characterized by grafting yield estimation, elemental analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy. The grafting yield decreased with the increase in the dose rate. However, the grafting yield and nitrogen content of grafted samples increased almost linearly with an increase in the total irradiation dose. To evaluate the performance of the grafted anion‐exchange matrix, the protein adsorption and elution behavior were investigated in a continuous column process under various experimental conditions, with bovine serum albumin used as a model protein. The binding and elution behavior of the anion‐exchange matrix depended on different experimental parameters, such as the grafting yield, ionic strength, pH of the medium, and amount of protein loaded. From a breakthrough curve, the equilibrium binding capacity and elution percentage of the grafted anion‐exchange matrix were estimated to be 40 mg/g and 94%, respectively. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5512–5521, 2006 相似文献
140.
A series of short fibre reinforced styrene butadiene rubber (SBR) composites were prepared by incorporating sisal fibres of
different lengths and different concentrations. The vulcanization behaviour and the properties of the vulcanizates were measured
by the relevant ASTM procedures. The processing characteristics such as green strength and mill shrinkage were improved by
the addition of fibres. The properties like tensile strength, tear strength, moduli at 10% and 20% elongations were found
to be maximum for composites containing longitudinally oriented fibres of length 6 mm. Mixes containing 35 phr loading of
fibres and bonding agent showed better mechanical performance than others. The failure behaviour of tensile and tear specimens
have been analysed by scanning electron microscopy techniques.
Paper presented at the poster session of MRSI AGM VI, Kharagpur, 1995 相似文献