全文获取类型
收费全文 | 61120篇 |
免费 | 2979篇 |
国内免费 | 2179篇 |
专业分类
电工技术 | 2274篇 |
技术理论 | 1篇 |
综合类 | 1659篇 |
化学工业 | 9044篇 |
金属工艺 | 4190篇 |
机械仪表 | 2447篇 |
建筑科学 | 2474篇 |
矿业工程 | 608篇 |
能源动力 | 1961篇 |
轻工业 | 3937篇 |
水利工程 | 824篇 |
石油天然气 | 2250篇 |
武器工业 | 236篇 |
无线电 | 6470篇 |
一般工业技术 | 13237篇 |
冶金工业 | 7868篇 |
原子能技术 | 1033篇 |
自动化技术 | 5765篇 |
出版年
2023年 | 528篇 |
2022年 | 1050篇 |
2021年 | 1300篇 |
2020年 | 1048篇 |
2019年 | 1026篇 |
2018年 | 1347篇 |
2017年 | 1389篇 |
2016年 | 1429篇 |
2015年 | 1421篇 |
2014年 | 1842篇 |
2013年 | 3636篇 |
2012年 | 2754篇 |
2011年 | 3278篇 |
2010年 | 2752篇 |
2009年 | 2933篇 |
2008年 | 2697篇 |
2007年 | 2814篇 |
2006年 | 2425篇 |
2005年 | 1978篇 |
2004年 | 1740篇 |
2003年 | 1655篇 |
2002年 | 1809篇 |
2001年 | 1781篇 |
2000年 | 1364篇 |
1999年 | 1371篇 |
1998年 | 2349篇 |
1997年 | 1861篇 |
1996年 | 1615篇 |
1995年 | 1126篇 |
1994年 | 865篇 |
1993年 | 883篇 |
1992年 | 699篇 |
1991年 | 626篇 |
1990年 | 567篇 |
1989年 | 517篇 |
1988年 | 390篇 |
1987年 | 421篇 |
1986年 | 378篇 |
1985年 | 391篇 |
1984年 | 314篇 |
1983年 | 309篇 |
1982年 | 306篇 |
1981年 | 297篇 |
1980年 | 352篇 |
1979年 | 342篇 |
1978年 | 294篇 |
1977年 | 415篇 |
1976年 | 629篇 |
1975年 | 308篇 |
1973年 | 311篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Using the three-dimensional Monte Carlo model, the effect of monovacancies on the diffusion exchange between the steps on the (111) surface of a diamond-like crystal during sublimation was investigated. The critical terrace width Lcr (the distance from the step edge to the nearest stable vacancy islands) was determined as a function of the relationship between the energy of adatom formation at the smooth terrace En and the energy of adatom desorption from the terrace Ed with retention of the sum of these energies Esub=En+Ed, which characterizes the sublimation of a substance. A well-pronounced maximum exists in the dependences Lcr(En) obtained. The formula is suggested which well describes the dependences Lcr(En) for various values of Esub. The extrapolation of the dependence Lcr(En) to the value Esub=4.2 eV, which is characteristic of Si, permits us to compare the model results with the experimental data of other authors. The explanation of a threefold increase in the terrace width, which is observed experimentally at about 1500 K, is suggested. 相似文献
992.
Urteaga M. Scott D. Krishnan S. Yun Wei Dahlstrom M. Griffith Z. Parthasarathy N. Rodwell M.J.W. 《Solid-State Circuits, IEEE Journal of》2003,38(9):1451-1456
We report tuned amplifiers designed for the 140-220-GHz frequency band. The amplifiers were designed in a transferred-substrate InP-based heterojunction bipolar transistor technology that enables efficient scaling of the parasitic collector-base junction capacitance. A single-stage amplifier exhibited 6.3-dB small-signal gain at 175 GHz. Three-stage amplifiers were subsequently fabricated with one design demonstrating 12.0-dB gain at 170 GHz and a second design exhibiting 8.5-dB gain at 195 GHz. 相似文献
993.
A high power, mid-infrared (/spl lambda/=3.8 /spl mu/m) lead-salt vertical-cavity surface-emitting laser (VCSEL) operating at 325 K is demonstrated. The laser emission is generated in a half-wavelength microcavity consisting of seven pairs of PbSe/PbSrSe multiple-quantum-wells embedded in two mirrors and optically pumped with 5 ns laser pulses. 相似文献
994.
In this paper, we present an implementation of a thermal modelling method applied to a multichip module used as a power converter. Analytical functions of thermal impedances, with original formulations for the mutuals are defined. They are derived from 3D thermal simulations and experimental validations with direct chips temperature measurements. Finally, simulations are performed in order to improve the capability of our model to assess, with fast computation, the thermal constraints applied on the multichip module in a real operating condition. 相似文献
995.
D. A. Vinokurov S. A. Zorina V. A. Kapitonov A. V. Murashova D. N. Nikolaev A. L. Stankevich M. A. Khomylev V. V. Shamakhov A. Yu. Leshko A. V. Lyutetskii T. A. Nalyot N. A. Pikhtin S. O. Slipchenko Z. N. Sokolova N. V. Fetisova I. S. Tarasov 《Semiconductors》2005,39(3):370-373
Asymmetric separate-confinement laser heterostructures with ultrawide waveguides based on AlGaAs/GaAs/InGaAs solid solutions, with an emission wavelength of ~1080 nm, are grown by MOCVD. The optical and electrical properties of mesa-stripe lasers with a stripe width of ~100 μm are studied. Lasers based on asymmetric heterostructures with ultrawide (>1 μm) waveguides demonstrate lasing in the fundamental transverse mode with an internal optical loss of as low as 0.34 cm?1. In laser diodes with a cavity length of more than 3 mm, the thermal resistance is reduced to 2°C/W, and the characteristic temperature T 0= 10°C is obtained in the range 0–100°C. A record-breaking wallplug efficiency of 74% and an output optical power of 16 W are reached in CW mode. Mean-time-between-failures testing for 1000 h at 65°C with an operation power of 3–4 W results in the power decreasing by 3–7%. 相似文献
996.
We investigate the achievable rate of receive antenna selection MIMO systems in the presence of mutual coupling and spatial correlation. For that, we assume the antenna array to consist of dipole antennas placed side-by-side in a linear pattern and in a very limited physical space. In a first step, we will assume perfect channel state information at the receiver side only and a negligible training overhead compared with the payload. We will demonstrate that in contrast to what might be expected based on results for cases without mutual coupling, MIMO receive antenna selection can achieve higher data rates than the system using all antennas provided that the total number of receive antennas is larger than a critical value that we will further discuss. We then propose an optimal antenna selection processing that ensures rate maximization regardless of the number of antennas used. In a later step, we will address the impact of training overhead on the system achievable rate when the training overhead is considerable. We will show that such a rate is reduced dramatically due to the large amount of training overhead arising from the presence of mutual coupling. To overcome this problem, we will thus propose a novel channel estimation method, which reduces the training overhead greatly and improves the system achievable rate performance. 相似文献
997.
998.
Effects of grain source and enzyme additive on site and extent of nutrient digestion in dairy cows 总被引:4,自引:0,他引:4
Four lactating, cannulated Holstein cows were used in a 4 x 4 Latin square design to investigate the effects of grain source and fibrolytic enzyme supplementation on ruminal fermentation, nutrient digestion in the rumen and in the intestine, and milk production. A 2 x 2 factorial arrangement was used; two grains (barley and hull-less barley) were combined with and without enzyme. The enzyme supplement (Pro-Mote; Biovance Technologies Inc., Omaha, NE) contained primarily cellulase and xylanase activities and was applied daily to the total mixed diet. Dry matter intake was not affected by diet, but starch intake was greatest when hull-less barley was fed. Starch from hull-less barley was more digestible in the rumen and in the total tract than was starch from barley, but opposite results occurred for fiber digestion, indicating that hull-less barley depressed fiber digestion. As a result, cows fed the hull-less barley diets tended to produce more milk with a higher milk lactose content than did cows fed the barley diets. Enzyme supplementation had minimal effects on ruminal digestion but increased nutrient digestibility in the total tract and the proportion of microbial N in nonammonia N. Consequently, cows fed diets supplemented with enzyme had a higher milk protein content and tended to produce more 4% fat-corrected milk than did control cows. These results indicate that the use of hull-less barley rather than barley increased the digestible energy intake of dairy cows, resulting in higher milk production. The use of a fibrolytic enzyme mixture enhanced feed digestibility and milk production. 相似文献
999.
Santos EP Freitas ZM Souza KR Garcia S Vergnanini A 《International journal of cosmetic science》1999,21(1):1-5
The aim of this work was to evaluate the correlation between in vitro and in vivo determinations of sun protection factors (SPF) of sunscreen lotions containing the synthetic sunscreen octyl methoxycinnamate. Three sunscreen lotions containing either 2, 4.5 or 7.5% octyl methoxycinnamate were prepared. The in vitro SPFs of these sunscreen lotions were determined according to the spectrophotometric method of Mansur et al. The in vivo SPF values were determined according to the Food and Drug Administration (FDA) method. The results indicate that there was a good correlation between the in vitro and in vivo determinations for the sunscreen lotions examined. 相似文献
1000.
Z. Marinković O. Pronić-Rančić V. Marković 《Journal of Communications Technology and Electronics》2014,59(11):1303-1309
A new method for accurate determination of noise parameters of microwave transistors for various bias conditions is proposed in this paper. The proposed model consists of a transistor empirical noise model (modification of Pospieszalski’s noise model) and two artificial neural networks. With the aim to avoid extraction of the empirical model parameters for each bias point, an artificial neural network is used to introduce bias-dependence of the equivalent circuit parameters. Accuracy of such bias-dependent model is further improved by using an additional neural network aimed to correct the noise parameters’ values. The proposed modeling approach is exemplified by modelling of a MESFET device in packaged form. The noise parameters obtained by the simulation agree well with the measured data. 相似文献