全文获取类型
收费全文 | 176777篇 |
免费 | 2296篇 |
国内免费 | 704篇 |
专业分类
电工技术 | 3290篇 |
综合类 | 101篇 |
化学工业 | 27892篇 |
金属工艺 | 7774篇 |
机械仪表 | 5119篇 |
建筑科学 | 4413篇 |
矿业工程 | 875篇 |
能源动力 | 4677篇 |
轻工业 | 16379篇 |
水利工程 | 1705篇 |
石油天然气 | 3126篇 |
武器工业 | 2篇 |
无线电 | 19848篇 |
一般工业技术 | 33331篇 |
冶金工业 | 33109篇 |
原子能技术 | 4291篇 |
自动化技术 | 13845篇 |
出版年
2021年 | 1313篇 |
2019年 | 1247篇 |
2018年 | 2096篇 |
2017年 | 2079篇 |
2016年 | 2188篇 |
2015年 | 1573篇 |
2014年 | 2720篇 |
2013年 | 7783篇 |
2012年 | 4553篇 |
2011年 | 6364篇 |
2010年 | 5037篇 |
2009年 | 5882篇 |
2008年 | 5862篇 |
2007年 | 5841篇 |
2006年 | 5011篇 |
2005年 | 4683篇 |
2004年 | 4474篇 |
2003年 | 4144篇 |
2002年 | 4101篇 |
2001年 | 4126篇 |
2000年 | 3922篇 |
1999年 | 4054篇 |
1998年 | 10381篇 |
1997年 | 7392篇 |
1996年 | 5654篇 |
1995年 | 4281篇 |
1994年 | 3617篇 |
1993年 | 3565篇 |
1992年 | 2606篇 |
1991年 | 2537篇 |
1990年 | 2417篇 |
1989年 | 2436篇 |
1988年 | 2376篇 |
1987年 | 2127篇 |
1986年 | 2067篇 |
1985年 | 2370篇 |
1984年 | 2185篇 |
1983年 | 2012篇 |
1982年 | 1880篇 |
1981年 | 1944篇 |
1980年 | 1796篇 |
1979年 | 1816篇 |
1978年 | 1776篇 |
1977年 | 2109篇 |
1976年 | 2687篇 |
1975年 | 1558篇 |
1974年 | 1549篇 |
1973年 | 1609篇 |
1972年 | 1350篇 |
1971年 | 1262篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
961.
The glass‐transition temperature as a function of curing conversion for a modified diallylbisphenol A/diaminodiphenylsulfone/bismaleimide (BMI) resin was investigated at different temperature regimes and modeled using a modified Di Benedetto equation. Although the relationship between the glass‐transition temperature and conversion of the BMI system conforms to the Di Benedetto equation for α < 0.6 and at lower cure temperatures, at higher cure temperatures the results deviated significantly from the equation; thus, it was an inadequate model for the system. Fourier transform IR analysis showed that the major crosslinking reactions did not occur during cure for the modified BMI at and below 150°C. However, as the cure temperature was increased, the crosslinking reactions responsible for 3‐dimensional network structures became more dominant. At 190°C the C? N? Cstretch vibration of the uncured maleimide ring converted into succinimide rings in the curing process. Simultaneously, a decrease was observed for the absorbance bands of ? C? Hbending (maleimide). The higher cure temperatures induced a significantly faster initial crosslinking rate and also resulted in a shorter period of time after which further crosslinking was retarded, because the increase in the crosslinks also physically slowed further crosslinking activity. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 227–235, 2002 相似文献
962.
Wax crystallization can cause serious damage to petroleum flow because as the viscosity increases, there is organic deposition in the pipe lines, which causes oil production reduction. Ethylene–vinyl acetate copolymers (EVA) are used as an alternative to prevent such problems. The influence of EVA copolymers on the properties of wax synthetic systems, composed of a solvent mixture and paraffin and petroleum asphalt residue (PAR), was evaluated in this work. The performance of EVA as a wax inhibitor depends on the molecular weight and vinyl acetate content of the copolymer and on the presence of the asphalt fraction. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1337–1348, 2002 相似文献
963.
Zhihong Ye Boroyevich D. Jae-Young Choi Lee F.C. 《Power Electronics, IEEE Transactions on》2002,17(5):609-615
One unique feature in parallel three-phase converters is a potential zero-sequence circulating current. To avoid the circulating current, most present technology uses an isolation approach, such as transformers or separate power supplies. This paper proposes a parallel system where individual converters connect both AC and DC sides directly without additional passive components to reduce size and cost of the overall parallel system. In this case, the control of the circulating current becomes an important objective in the converter design. This paper: (1) develops an averaged model of the parallel converters based on a phase-leg averaging technique; (2) a zero-sequence model is then developed to predict the dynamics of the zero-sequence current; (3) based on the zero-sequence model, this paper introduces a new control variable, which is associated with space-vector modulation; (4) a strong zero-sequence current control loop is designed to suppress the circulating current; and (5) simulation and experimental results validate the developed model and the proposed control scheme. 相似文献
964.
Statistical characterization of urban spatial radio channels 总被引:3,自引:0,他引:3
Toeltsch M. Laurila J. Kalliola K. Molisch A.F. Vainikainen P. Bonek E. 《Selected Areas in Communications, IEEE Journal on》2002,20(3):539-549
We present a statistical analysis of wideband three-dimensional channel measurements at base station locations in an urban environment. Plots of the received energy over azimuth, elevation, and delay planes suggest that the incident waves group to clusters in most measured transmitter positions. A super-resolution algorithm (Unitary ESPRIT) allows one to resolve individual multipath components in such clusters and hence enables a detailed statistical analysis of the propagation properties. The origins of clusters-sometimes even individual multipath components-such as street apertures, large buildings, roof edges, or building corners can be localized on the city map. Street guided propagation dominates most of the scenarios (78%-97% of the total received power), while quasi-line-of-sight over-the-rooftop components are weak(3%-13% of the total received power). For this measurement campaign, in 90% of the cases, 75% of the total received power is concentrated in the two strongest clusters, but only 55% in the strongest one. Our analysis yields an exponential decay of power with 8.9 dB/μs, and a standard deviation of the log-normally distributed deviations from the exponential of 9.0 dB. The power of cross-polarized components is 8 dB below copolarized ones on average (vertical transmission) 相似文献
965.
Laney D.C. Maggio G.M. Lehmann F. Larson L. 《Selected Areas in Communications, IEEE Journal on》2002,20(9):1692-1700
Pseudochaotic time hopping (PCTH) is a previously proposed encoding/modulation scheme for ultra-wideband (UWB) impulse radio. PCTH exploits concepts from symbolic dynamics to generate aperiodic spreading sequences, resulting in a noise-like spectrum. We present a multiple-access technique suitable for the PCTH scheme. In particular, we provide an analytical expression of the bit-error rate performance as a function of the number of users and validate it by simulation. 相似文献
966.
A new model for simulating temporal fluctuations in the power emitted by a semiconductor laser is described. Light in the cavity is assumed to circulate in the form of traveling photon packets, in which the photon number fluctuates due to the processes of spontaneous emission, stimulated emission, absorption, scattering, and reflection. The dipole dephasing time T plays a critical role in modeling the interaction of the photon packets and gain medium. The Monte Carlo method is used to simulate the temporal behavior of a continuously pumped Fabry-Perot laser. The laser output power is found to exhibit periodic fluctuations at the cavity transit time frequency (longitudinal mode beat frequency). The amplitude of these fluctuations, as well as the relaxation oscillation, which occurs at a much lower frequency, is strongly influenced by the magnitude of T . The results of these simulations are related to the temporal behavior expected from a conventional FP laser 相似文献
967.
Evolution of the capillary network in a reactive powder concrete during hydration process 总被引:7,自引:0,他引:7
V. Morin F. Cohen-Tenoudji A. Feylessoufi P. Richard 《Cement and Concrete Research》2002,32(12):1907-1914
Ultrasonic waves in echographic mode, combined with autogenous shrinkage measurements, were used to study the evolution of the capillary network of reactive powder concrete (RPC) from the time after the mixing. Two characteristic porous classes have been identified: the first, between 10 and 20 nm, begins when the material reaches its solid hyperstatic state, and the second about 1 or 2 nm. The first class is associated with the porous space between the C-S-H hydrate clusters and the second with the internal porosity of the hydrate. The evolution of the active capillary radius as a function of the degree of hydration allows us to understand the strong interaction between the capillary network size and the chemical activity given by the dissipated calorimetric power curve. Indeed, the maximum point of the chemical activity marks the transition of the first class of pores to the second one. Finally, measurements of electrical conductivity through RPC samples show that after the maximum of the dissipated power, the curve of this electrical conductivity presents the same evolution as the capillary radius. As the electrical conductivity clearly depends on the evolution of the capillary network, the similarity between the results confirms our analysis in pore classes. 相似文献
968.
969.
I. Z. Babievskaya K. S. Gavrichev N. F. Drobot O. A. Noskova N. A. Ovchinnikova A. E. Shubina V. A. Krenev 《Inorganic Materials》2007,43(3):292-295
The water adsorption and retention behaviors of vermiculite, silica gel, activated carbon, and a zeolite are analyzed in relation to their pore structures, and their effects on the dynamics of heat generation in Fe-C-NaCl-H2O-O2-water sorbent systems are investigated. 相似文献
970.
F. Fetting 《化学,工程师,技术》1992,64(3):288-288