全文获取类型
收费全文 | 485109篇 |
免费 | 5352篇 |
国内免费 | 1188篇 |
专业分类
电工技术 | 8550篇 |
综合类 | 415篇 |
化学工业 | 77358篇 |
金属工艺 | 19338篇 |
机械仪表 | 14518篇 |
建筑科学 | 11598篇 |
矿业工程 | 2947篇 |
能源动力 | 11018篇 |
轻工业 | 45999篇 |
水利工程 | 5228篇 |
石油天然气 | 10763篇 |
武器工业 | 44篇 |
无线电 | 51519篇 |
一般工业技术 | 92886篇 |
冶金工业 | 90136篇 |
原子能技术 | 11838篇 |
自动化技术 | 37494篇 |
出版年
2021年 | 3858篇 |
2019年 | 3609篇 |
2018年 | 6457篇 |
2017年 | 6357篇 |
2016年 | 6807篇 |
2015年 | 4322篇 |
2014年 | 7408篇 |
2013年 | 20633篇 |
2012年 | 11826篇 |
2011年 | 16081篇 |
2010年 | 12921篇 |
2009年 | 14612篇 |
2008年 | 15176篇 |
2007年 | 15121篇 |
2006年 | 13407篇 |
2005年 | 12214篇 |
2004年 | 11509篇 |
2003年 | 11077篇 |
2002年 | 11222篇 |
2001年 | 10919篇 |
2000年 | 10577篇 |
1999年 | 10537篇 |
1998年 | 25286篇 |
1997年 | 18447篇 |
1996年 | 14322篇 |
1995年 | 10905篇 |
1994年 | 9900篇 |
1993年 | 9624篇 |
1992年 | 7505篇 |
1991年 | 7183篇 |
1990年 | 7212篇 |
1989年 | 7120篇 |
1988年 | 6940篇 |
1987年 | 5963篇 |
1986年 | 6019篇 |
1985年 | 6945篇 |
1984年 | 6631篇 |
1983年 | 6041篇 |
1982年 | 5651篇 |
1981年 | 5861篇 |
1980年 | 5665篇 |
1979年 | 5496篇 |
1978年 | 5564篇 |
1977年 | 6336篇 |
1976年 | 8168篇 |
1975年 | 5039篇 |
1974年 | 4785篇 |
1973年 | 4926篇 |
1972年 | 4197篇 |
1971年 | 3882篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
It was found that vanadyl porphyrin complexes synthesized from petroleum metal porphyrin concentrates stimulated epoxidation during the olefin oxygenation process. The yields of obtained oxiranes turned out to be 38–75%, depending on the olefin structure. An epoxidation mechanism that suggests the formation of a protonated dioxygen adduct as an intermediate during oxygenation of olefins in the presence of vanadyl porphyrin complexes was proposed. An analogy is drawn between the epoxide formation reaction upon the catalytic oxygenation of olefins and the Prilezhaev reaction. 相似文献
12.
The basic methods of verifying continuous automatic belt weighers are described. A comparative analysis of these methods on the basis of experimental studies is made and ways of implementing the results in industry are recommended. 相似文献
13.
The proliferation of both online and bricks and mortar outlet stores underscores the observation that secondary markets are readily accessible to retailers of short-life-cycle products. These secondary markets provide recourse channels for retailers to sell excess inventory of out-of-favor items at reduced prices when overstocking occurs in a primary market. We study the problem of determining when a retailer should terminate its primary selling season by selling remaining inventory on a secondary market. The retailer has a single opportunity to procure prior to a primary selling season consisting of multiple periods. Demand in each period is random, but correlated. At the end of each period, any remaining inventory incurs a holding cost. Then, based upon the current level of inventory and the cumulative demand-to-date, the retailer decides either to terminate the primary selling season by selling all or part of the remaining inventory on a secondary market, or to extend the current primary selling season by another period. We develop structural properties of the optimal policy for determining when to terminate the primary selling season, and we develop corresponding implications for procurement. 相似文献
14.
Rheological properties of MR fluids under large step strain shear are presented in this paper. The experiments were carried out using a rheometer with parallel-plate geometry. Under the large step strain shear, MR fluids behave as nonlinear viscoelastic properties, where the stress relaxation modulus, G(t, γ), shows a decreasing trend with step strain. The experimental results indicate that G(t, γ) obeys time-strain separability. Thus, a mathematical form based on finite exponential serials is proposed to predict MR behavior. In this model, G(t, γ) is represented as the product of a linear stress relaxation, G(t), and the damping function, h(γ), i.e. G(t, γ)=G(t) h(γ). G(t) is simply represented as a three-parameter exponential serial and h(γ) has a sigmoidal form with two parameters. The parameters are identified by adopting an efficient optimization method proposed by Stango et al. The comparison between the experimental results and the model-predicted values indicates that this mathematical model can accurately predict MR behavior. 相似文献
15.
We propose a standardization procedure that provides a convenient, quantitative and reproducible laboratory-based method for measuring the state of polarization (SOP) fluctuations produced by polarization varying devices. This method is based on the SOP distributions generated by commercial polarization scramblers. We show that these devices generate distributions of the maximum change of the SOP (in a given sample time) that follow Rayleigh statistics, which scale linearly with scrambling frequency and the sample time. We use this procedure to measure the SOP fluctuations in a short length of coiled fiber subject to mechanical perturbations. 相似文献
16.
V. G. Deibuk 《Semiconductors》2003,37(10):1151-1155
The miscibility gaps and the critical temperatures of spinodal decomposition of ternary semiconducting Ga-In-Sb, Ga-In-P, and In-As-Sb systems are calculated by taking into account the deformation energy and the effect of plastic relaxation caused by the misfit dislocations. It is shown that taking into consideration elastic energy narrows the ranges of spinodal decomposition and lowers its critical temperature. The introduction of the phenomenological parameter into Matthews-Blakeslee formula makes it possible to reach a satisfactory agreement between theoretically calculated values of critical thickness of epitaxial films and the experimental data. 相似文献
17.
The application of barrier discharges at atmospheric pressure in air expands on the market of plasma technology, because it is an ecological and cost‐effective alternative to other processes of surface treatment. These plasmas usually consist of a multitude of spatially and temporally localized filaments, whose distribution should be as even as possible for homogeneous treatment. This holds especially for the plasma treatment of sensitive goods such as wool or other textiles. In equipment for continuous pass of material the barrier arrangements often consist of a system cylinder – cylinder or cylinder – plane, whereby the gap width changes locally. Space distribution and intensity of filaments has been investigated by means of short‐time photography and spatially resolved measurement of current distribution and energy distribution derived from it. The local dependency found can be explained by means of a capacitive equivalent circuit. 相似文献
18.
The tensile elastic modulus (E), yield stress (σY) and microhardness (MH) of neat and binary and ternary blends of glassy semicrystalline ethylene–vinyl alcohol copolymer (EVOH), a glassy amorphous polyamide and a semicrystalline nylon‐containing ionomer covering a broad range of properties were examined. The tests were carried out on dry and water‐equilibrated samples to produce stiffer and softer materials, respectively. From the results, more accurate linear correlations were found to describe adequately the microhardness, modulus and yield stress of these strongly self‐associated polymers through hydrogen bonding. Copyright © 2003 Society of Chemical Industry 相似文献
19.
Ciofi C. Crupi F. Pace C. Scandurra G. 《IEEE transactions on instrumentation and measurement》2003,52(5):1533-1536
Low-frequency noise measurements represent an interesting investigation technique for the characterization of the quality and reliability of microelectronic materials and devices. Performing meaningful noise measurements at low and very low (f<1 Hz) frequencies, however, may be quite challenging, particularly because of the many sources of interference that superimpose on the noise signal. For this reason, packaged samples are preferred because they allow accurate shielding from the external environment, and because keeping the sample in close proximity to the low-noise biasing system and amplifier reduces microphonic and electromagnetic disturbances. Notwithstanding this, the possibility of performing low-frequency noise measurements at wafer level would be quite interesting, both because of the ease of obtaining wafer-level samples from industries with respect to packaged samples, and because this would avoid possible packaging-process induced device degradation. The purpose of this work is to demonstrate that it is, in fact, possible to design and build a dedicated probe system for performing high-sensitivity, low-frequency noise measurements on metal-oxide-semiconductor devices at wafer level. 相似文献
20.