全文获取类型
收费全文 | 14089篇 |
免费 | 735篇 |
国内免费 | 42篇 |
专业分类
电工技术 | 249篇 |
综合类 | 68篇 |
化学工业 | 2772篇 |
金属工艺 | 327篇 |
机械仪表 | 370篇 |
建筑科学 | 418篇 |
矿业工程 | 53篇 |
能源动力 | 461篇 |
轻工业 | 2050篇 |
水利工程 | 148篇 |
石油天然气 | 80篇 |
无线电 | 1180篇 |
一般工业技术 | 2094篇 |
冶金工业 | 2799篇 |
原子能技术 | 103篇 |
自动化技术 | 1694篇 |
出版年
2023年 | 110篇 |
2022年 | 274篇 |
2021年 | 459篇 |
2020年 | 317篇 |
2019年 | 398篇 |
2018年 | 440篇 |
2017年 | 437篇 |
2016年 | 410篇 |
2015年 | 295篇 |
2014年 | 431篇 |
2013年 | 858篇 |
2012年 | 631篇 |
2011年 | 749篇 |
2010年 | 529篇 |
2009年 | 571篇 |
2008年 | 556篇 |
2007年 | 500篇 |
2006年 | 439篇 |
2005年 | 323篇 |
2004年 | 315篇 |
2003年 | 344篇 |
2002年 | 357篇 |
2001年 | 309篇 |
2000年 | 230篇 |
1999年 | 244篇 |
1998年 | 732篇 |
1997年 | 505篇 |
1996年 | 354篇 |
1995年 | 275篇 |
1994年 | 237篇 |
1993年 | 239篇 |
1992年 | 133篇 |
1991年 | 100篇 |
1990年 | 131篇 |
1989年 | 118篇 |
1988年 | 108篇 |
1987年 | 91篇 |
1986年 | 89篇 |
1985年 | 103篇 |
1984年 | 84篇 |
1983年 | 65篇 |
1982年 | 59篇 |
1981年 | 55篇 |
1980年 | 61篇 |
1979年 | 69篇 |
1978年 | 61篇 |
1977年 | 85篇 |
1976年 | 184篇 |
1975年 | 59篇 |
1974年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
61.
Cigarette smoking continues to be the single, most preventable cause of death and disability in the United States. For individuals who have cancer, continuing to smoke negatively impacts their treatment, survival, and risk for second primary tumors. This review of behavioral and pharmacological approaches to smoking cessation focuses on the recent comprehensive review of cessation interventions by the Agency for Health Care Policy and Research (AHCPR), as well as on new developments in the field. An intervention model is outlined that provides oncologists with a brief and easily implemented method of systematically treating patients who smoke. By assessing patient smoking status, advising smoking patients to quit, and proactively assisting their patients in quitting, oncologists can significantly influence patient health and fulfill their professional and ethical responsibility to address this life-threatening behavior. 相似文献
62.
J. K. Richard Weber† Collin D. Anderson Dennis R. Merkley Paul C. Nordine† 《Journal of the American Ceramic Society》1995,78(3):577-582
Solidification of aluminum oxide from undercooled melts was investigated in containerless experiments. Specimens were levitated in a gas jet, stabilized with an acoustic positioning device, and melted with cw CO2 laser beams. Cooling curves were obtained by optical pyrometry when the laser intensity was reduced. The materials examined were high-purity Verneuil sapphire, 99.5% polycrystalline alumina, and oxide materials recovered from the effluent of an aluminum-fueled rocket motor. The degree of undercooling, the apparent temperature behavior during the thermal arrest on solidification, and the structure of the materials formed were different in argon and oxygen atmospheres. Undercooling of the sapphire and alumina materials was 360 ± 10 K in an oxygen atmosphere and approximately 450 K in argon. Melting and solidification of high-purity sapphire resulted in a dendritic and porous polycrystalline material in oxygen. Dense, larger crystals were obtained in argon. Products formed from 99.5% alumina were discolored and the cores were white, indicating impurity segregation effects. More reproducible behavior was observed for the sapphire and 99.5% alumina than for the tungstencontaminated rocket motor effluent materials. 相似文献
63.
G. M. Gusev X. Kleber U. Gennser D. K. Maude J. C. Portal D. I. Lubyshev P. Basmaji M. de P. A. Silva J. C. Rossi Yu. V. Nastaushev M. R. Baklanov 《Solid-state electronics》1996,40(1-8):441-446
Electron scattering by a single barrier is predicted to reveal singularities as the magnetic field is changed, because the number of electron collisions with the barrier dramatically increases as chaotic orbits around the barrier are changed into periodic orbits. To test this experimentally we have measured the magnetoresistance of AlGaAs/GaAs heterostructures with a two-dimensional electron gas and a lateral lattice containing a macroscopic number of oval-shaped antidots fabricated using electron lithography. Reproducible fluctuations in the magnetoresistance are observed at low field, which are due to the oscillations of the number of electron collisions with the antidots. The number of collisions N before the electron escapes from the antidot has been calculated as a function of B in an electric field. The position of the maxima in N(B) obtained from calculations and experiment are in reasonable agreement. 相似文献
64.
65.
Pertussis in Massachusetts, 1981-1991: incidence, serologic diagnosis, and vaccine effectiveness 总被引:1,自引:0,他引:1
CD Marchant AM Loughlin SM Lett CW Todd LH Wetterlow R Bicchieri S Higham P Etkind E Silva GR Siber 《Canadian Metallurgical Quarterly》1994,169(6):1297-1305
Massachusetts provides diphtheria-tetanus toxoid-pertussis (DTP) vaccine, and since 1980 has monitored pertussis with a statewide diagnostic service. The incidence of bacteriologically confirmed pertussis was 104.5 per 100,000 person-years in 1-month-old infants and declined progressively thereafter. Infants < 6 months old experienced disproportionate morbidity: 44% of bacteriologically confirmed pertussis, 64% of hospitalizations, and 71% of hospital days. Most children with pertussis had received < 3 DTP doses during childhood, whereas 87% of adolescents with pertussis had received > or = 4 doses. Serodiagnosis by single serum anti-pertussis toxin antibody ELISA increased the incidence of confirmed pertussis in persons 11-19 years old from 3.0 to 12.9 per 100,000 and in persons > or = 20 years old from 0.16 to 0.56 per 100,000. Bacteriologic methods underestimate pertussis incidence, but a single serum anti-pertussis toxin antibody ELISA is a practical method for population-based diagnosis in adolescents and adults. 相似文献
66.
The MSMIE protocol [SBC89] allows processors in a distributed system to communicate via shared memory. It was designed to meet the reliability and efficiency needs of applications such as nuclear safety systems. We present a formal model of the MSMIE protocol expressed in the notation CCS. Desirable properties of the protocol are expressed in the modal mu-calculus, an expressive modal logic. We show that the protocol lacks an important liveness property. In actual operation, additional operating constraints are checked to avoid potential problems. We present a modified protocol and show that it possesses the liveness property even without checking operating constraints. We also show how parts of the analysis were automated with the Concurrency Workbench. 相似文献
67.
In 3 experiments, participants memorized 8 examples, each exemplifying a different rule. Participants were asked to extend these rules to new examples. They practiced applications of the rules to examples over a period of 4 days (Experiment 1) or 5 days (Experiments 2 and 3). Although these rules were bidirectional, an asymmetry gradually built up such that participants became more facile in using the rules in the practiced direction. Participants also showed an advantage when the initial study example was repeated or when test examples were repeated. It is argued that skill acquisition involves development of a complex set of strategies based on use of rules and retrieval of examples. Four overlapping stages of skill acquisition are described. 相似文献
68.
D.L Orphal C.E Anderson Jr. R.R Franzen J.D Walker P.N Schneidewind M.E Majerus 《International Journal of Impact Engineering》1993,14(1-4):551-560
Calculations of steel target penetration by L/D ≤ 1 tungsten and tungsten alloy projectiles have been extended to L/D = 1/32 over the velocity range 1.5 to 5 km/s. The ratio of crater to projectile diameter tends to 1 as L/D decreases over this entire velocity range. For impact velocities of 1.5 and 3 km/s, penetration depth normalized by projectile length, P/L, increases with decreasing projectile L/D up to a maximum value and then decreases for still lower L/D. Experiments at impact velocities of 2 and 3 km/s confirm these results. For 5 km/s impact velocity, the calculations show P/L increasing with decreasing projectile L/D over the entire range 1/32 ≤ L/D ≤ 1. The projectile L/D for which the maximum P/L occurs appears to depend on the impact velocity. P/L generally scales with impact velocity as P/L vf(L/D) where f(L/D) ranges from 0 for a long rod to, we believe, 2 in the limit as projectile L/D approaches zero. The calculations show for 1/8 ≤ L/D ≤ 1/2, P/L v0.9; for L/D = 1/16, P/L v1.5; and for L/D = 1/32, the new results give P/L v1.9. 相似文献
69.
Machine ethics is concerned with how machines behave toward human users and other machines. It aims to create a machine that's guided by an acceptable ethical principle or set of principles in the decisions it makes about possible courses of action it could take. As ethics experts continue to progress toward consensus concerning the right way to behave in ethical dilemmas, the task for those working in machine ethics is to codify these insights. Eight articles in this special issue address the issues.This article is part of a special issue on Machine Ethics. 相似文献
70.
An adaptive 2D mesh refinement technique based on mortar spectral elements applied to diffuse-interface methods is presented. The refinement algorithm tracks the movement of the 2D diffuse-interface and subsequently refines the mesh locally at that interface, while coarsening the mesh in the rest of the computational domain, based on error estimators. Convergence of the method is validated using a Gaussian distribution problem and results are presented for a Cahn–Hilliard diffuse-interface model applied to capture the transient dynamics of polymer blends. 相似文献