全文获取类型
收费全文 | 492568篇 |
免费 | 5485篇 |
国内免费 | 2025篇 |
专业分类
电工技术 | 8891篇 |
综合类 | 691篇 |
化学工业 | 70713篇 |
金属工艺 | 18830篇 |
机械仪表 | 14569篇 |
建筑科学 | 11665篇 |
矿业工程 | 2292篇 |
能源动力 | 13056篇 |
轻工业 | 40384篇 |
水利工程 | 4865篇 |
石油天然气 | 8383篇 |
武器工业 | 56篇 |
无线电 | 61998篇 |
一般工业技术 | 97915篇 |
冶金工业 | 94268篇 |
原子能技术 | 11407篇 |
自动化技术 | 40095篇 |
出版年
2021年 | 3988篇 |
2020年 | 2971篇 |
2019年 | 3696篇 |
2018年 | 6281篇 |
2017年 | 6376篇 |
2016年 | 6718篇 |
2015年 | 4351篇 |
2014年 | 7621篇 |
2013年 | 21553篇 |
2012年 | 12165篇 |
2011年 | 16841篇 |
2010年 | 13243篇 |
2009年 | 15263篇 |
2008年 | 15965篇 |
2007年 | 15971篇 |
2006年 | 13932篇 |
2005年 | 12707篇 |
2004年 | 12477篇 |
2003年 | 12391篇 |
2002年 | 11815篇 |
2001年 | 12303篇 |
2000年 | 11397篇 |
1999年 | 12188篇 |
1998年 | 31042篇 |
1997年 | 21646篇 |
1996年 | 16579篇 |
1995年 | 12545篇 |
1994年 | 11103篇 |
1993年 | 11148篇 |
1992年 | 7904篇 |
1991年 | 7579篇 |
1990年 | 7352篇 |
1989年 | 7166篇 |
1988年 | 6781篇 |
1987年 | 5943篇 |
1986年 | 5858篇 |
1985年 | 6454篇 |
1984年 | 5949篇 |
1983年 | 5607篇 |
1982年 | 5274篇 |
1981年 | 5170篇 |
1980年 | 5107篇 |
1979年 | 4771篇 |
1978年 | 4645篇 |
1977年 | 5398篇 |
1976年 | 7289篇 |
1975年 | 3921篇 |
1974年 | 3846篇 |
1973年 | 3818篇 |
1972年 | 3287篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
P. Cofta 《BT Technology Journal》2006,24(2):214-218
Trust is the key element of commerce, both for traditional models and — more recently — for eCommerce. Unlike its traditional
counterpart, eCommerce relies on trust being created and maintained with the help of digital devices and services. Convergence
brings significant changes to the way people interact through digital media by offering ubiquitous and seamless communication.
This has a significant impact on the way trust is established and maintained, with consequent impact upon eCommerce. This
paper identifies a number of the most important challenges to trust in eCommerce as well as trust-related aspects of key enablers
of eCommerce. From this perspective, the paper studies an impact of convergence on trust within eCommerce to identify several
positive relationships. 相似文献
12.
Intensive use of the benchmark simulation model No. 1 (BSM1), a protocol for objective comparison of the effectiveness of control strategies in biological nitrogen removal activated sludge plants, has also revealed a number of limitations. Preliminary definitions of the long-term benchmark simulation model No. 1 (BSM1_LT) and the benchmark simulation model No. 2 (BSM2) have been made to extend BSM1 for evaluation of process monitoring methods and plant-wide control strategies, respectively. Influent-related disturbances for BSM1_LT/BSM2 are to be generated with a model, and this paper provides a general overview of the modelling methods used. Typical influent dynamic phenomena generated with the BSM1_LT/BSM2 influent disturbance model, including diurnal, weekend, seasonal and holiday effects, as well as rainfall, are illustrated with simulation results. As a result of the work described in this paper, a proposed influent model/file has been released to the benchmark developers for evaluation purposes. Pending this evaluation, a final BSM1_LT/BSM2 influent disturbance model definition is foreseen. Preliminary simulations with dynamic influent data generated by the influent disturbance model indicate that default BSM1 activated sludge plant control strategies will need extensions for BSM1_LT/BSM2 to efficiently handle 1 year of influent dynamics. 相似文献
13.
14.
Improved homogenization of Ni in sintered steels through the use of Cr-containing prealloyed powders
The homogenization of Ni in powder metal (PM) steel compacts is usually difficult even after high-temperature sintering at
1250°C. An earlier study by the authors demonstrated that this problem can be alleviated through the addition of 0.5 wt pct
Cr in the form of stainless steel powders. To further improve the microstructure and mechanical properties of Ni-containing
PM steels and to understand the mechanisms, an attempt was made in this study using the Fe-3Cr-0.5Mo prealloyed powder as
the base material. The results showed that the distribution of the Ni additives was significantly improved. As a result, the
tensile strength of the Fe-3Cr-0.5Mo-4Ni-0.5C compact sintered at 1250°C reached 1323 MPa. The elongation was higher than
1 pct. These sinter-hardened properties, which were attained using a slow furnace cooling rate, were comparable to those of
the sinter-hardened alloys reported in the literature using accelerated cooling and were equivalent to those of the best quenched-and-tempered
alloys registered in the Metal Powder Industries Federation (MPIF) standards. These improvements were attributed to the positive
effect of Cr addition on alloy homogenization due to the reduction of the repelling effect between Ni and C, as was demonstrated
through the thermodynamic analysis using the Thermo-Calc program. 相似文献
15.
K.P. Pipe R.J. Ram 《Photonics Technology Letters, IEEE》2003,15(4):504-506
By measuring the total energy flow from an optical device, we can develop new design strategies for thermal stabilization. Here we present a comprehensive model for heat exchange between a semiconductor laser diode and its environment that includes the mechanisms of conduction, convection, and radiation. We perform quantitative measurements of these processes for several devices, deriving parameters such as a laser's heat transfer coefficient, and then demonstrate the feasibility of thermal probing for the nondestructive wafer-scale characterization of optical devices. 相似文献
16.
Note on B-splines, wavelet scaling functions, and Gabor frames 总被引:3,自引:0,他引:3
Grochenig K. Janssen A.J.E.M. Kaiblinger N. Pfander G.E. 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2003,49(12):3318-3320
Let g be a continuous, compactly supported function on such that the integer translates of g constitute a partition of unity. We show that the Gabor system (g,a,b), with window g and time-shift and frequency-shift parameters a,b>0 has no lower frame bound larger than 0 if b=2,3,... and a>0. In particular, (g,a,b) is not a Gabor frame if g is a continuous, compactly supported wavelet scaling function and if b=2,3,... and a>0. We give an example for our result for the case that g=B/sub 1/, the triangle function supported by [-1,1], by showing pictures of the canonical dual corresponding to (g,a,b) where ab=1/4 and b crosses the lines N=2,3,. 相似文献
17.
Supported metal catalysts, particularly noble metals supported on SiO2, have attracted considerable attention due to the importance of the silica–metal interface in heterogeneous catalysis and in electronic device fabrication. Several important issues, e.g., the stability of the metal–oxide interface at working temperatures and pressures, are not well-understood. In this review, the present status of our understanding of the metal–silica interface is reviewed. Recent results of model studies in our laboratories on Pd/SiO2/Mo(1 1 2) using LEED, AES and STM are reported. In this work, epitaxial, ultrathin, well-ordered SiO2 films were grown on a Mo(1 1 2) substrate to circumvent complications that frequently arise from the silica–silicon interface present in silica thin films grown on silicon. 相似文献
18.
The In-site of Ba2In2O5 with Brownmillerite structure was partially substituted for Ce4+ ions in order to examine the doping effect on the order-disorder transition. Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.1, 0.2, 0.3, 0.5, 1.0, and 1.5) were prepared by solid state reaction. X-Ray diffraction analyses of these powder samples demonstrated that Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.1 and 0.2) possesses Brownmillerite structure. With increasing content of Ce4+ ion the crystal system of Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.3, 0.5, and 1.0) changed to cubic perovskite structure above the order-disorder transition temperature of Ba2In2O5. Arrhenius plots of the electrical conductivities of Ba2In2 – x
Ce
x
O5 + x/2 (x = 0.2, 0.3, and 1.0) exhibited no discontinuity. These compounds had high transference numbers of oxide ion above 973 k. 相似文献
19.
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
20.
L. Gao P. Hrter Ch. Linsmeier J. Gstttner R. Emling D. Schmitt-Landsiedel 《Materials Science in Semiconductor Processing》2004,7(4-6):331
Deposition of Ag films by direct liquid injection-metal organic chemical vapor deposition (DLI-MOCVD) was chosen because this preparation method allows precise control of precursor flow and prevents early decomposition of the precursor as compared to the bubbler-delivery. Silver(I)-2,2-dimethyl-6,6,7,7,8,8,8-heptafluoro-3,5-octanedionato-triethylphosphine [Ag(fod)(PEt3)] as the precursor for Ag CVD was studied, which is liquid at 30 °C. Ag films were grown on different substrates of SiO2/Si and TiN/Si. Argon and nitrogen/hydrogen carrier gas was used in a cold wall reactor at a pressure of 50–500 Pa with deposition temperature ranging between 220 °C and 350 °C. Ag films deposited on a TiN/Si diffusion barrier layer have favorable properties over films deposited on SiO2/Si substrate. At lower temperature (220 °C), film growth is essentially reaction-limited on SiO2 substrate. Significant dependence of the surface morphology on the deposition conditions exists in our experiments. According to XPS analysis pure Ag films are deposited by DLI-MOCVD at 250 °C by using argon as carrier gas. 相似文献