首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   260796篇
  免费   3233篇
  国内免费   823篇
电工技术   5138篇
综合类   332篇
化学工业   37617篇
金属工艺   10258篇
机械仪表   8500篇
建筑科学   6126篇
矿业工程   1076篇
能源动力   6415篇
轻工业   22953篇
水利工程   2567篇
石油天然气   3910篇
武器工业   68篇
无线电   34754篇
一般工业技术   50139篇
冶金工业   46550篇
原子能技术   5418篇
自动化技术   23031篇
  2021年   2134篇
  2019年   2050篇
  2018年   3490篇
  2017年   3557篇
  2016年   3766篇
  2015年   2356篇
  2014年   4079篇
  2013年   11334篇
  2012年   6604篇
  2011年   8927篇
  2010年   7016篇
  2009年   7889篇
  2008年   8708篇
  2007年   8643篇
  2006年   7821篇
  2005年   7159篇
  2004年   6901篇
  2003年   6742篇
  2002年   6381篇
  2001年   6477篇
  2000年   6173篇
  1999年   6258篇
  1998年   14239篇
  1997年   10271篇
  1996年   8111篇
  1995年   6411篇
  1994年   5715篇
  1993年   5583篇
  1992年   4553篇
  1991年   4212篇
  1990年   4049篇
  1989年   3779篇
  1988年   3625篇
  1987年   3171篇
  1986年   3065篇
  1985年   3611篇
  1984年   3382篇
  1983年   3031篇
  1982年   2850篇
  1981年   2952篇
  1980年   2761篇
  1979年   2649篇
  1978年   2499篇
  1977年   2945篇
  1976年   3554篇
  1975年   2316篇
  1974年   2312篇
  1973年   2318篇
  1972年   1849篇
  1971年   1745篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The engineering and management of human safety is an important societal objective that includes extensive efforts by governments, both legislative and administrative, to enhance the health and safety of the public. Although the achievement of safety goals depend primarily on individuals and organizations responsible for safety, much support is drawn from expertise in diverse scientific and engineering disciplines. The activities range from structural safety (dams, tunnels, bridges to tall buildings) to safe operation of hazardous industrial installations (energy generation facilities, LNG terminals, petrochemical plants) to transportation systems (airline, rail, car safety) to technologies designed to minimize adverse impacts on the environment. All these activities are crucially concerned with risk: with the likelihood and the probable effects of various measures on life and health. We have developed a unified rationale and a clear basis for effective strategic management of risk across diverse sectors. Safety is an important objective in society but it is not the only one. The allocation of society's resources devoted to safety must be continually appraised in light of competing needs, because there is a limit on the resources that can be expended to extend life. The paper presents the Life Quality Index (LQI) as a tool for the assessment of risk reduction initiatives that would support the public interest and enhance safety and quality of life. The paper provides an intuitive reformulation of the LQI as equivalent to a valid utility function that is consistent with the principles of rational decision analysis. The LQI is further refined to consider the issues of discounting of life years, competing background risks, and population age and mortality distribution. The LQI is applied to quantify the societal willingness-to-pay, which is an acceptable level of public expenditure in exchange for a reduction in the risk of death that results in improved life-quality.  相似文献   
992.
Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 4, pp. 16–17, April, 1992.  相似文献   
993.
994.
995.
Griffin  R.A. Sampson  D.D. 《Electronics letters》1992,28(13):1202-1203
It is shown that the unique coherence properties of mode-locked lasers can be exploited to provide new opportunities for high-speed data transmission. Using passive Mach-Zehnder optical delay stages, very high bit rate optical data streams constructed from asynchronously combined transmitters can be demultiplexed directly in the optical domain, without imposing restrictions on the source wavelength. Potential implementation and performance of the technique are discussed.<>  相似文献   
996.
Nascent high-molecular-weight (UHMW) polyethylene (PE) samples of different origins show a rather high crystallinity of about 70–75% and contain both a major portion of orthorhombic extended chain crystallites and a minor portion of triclinic crystallites. The triclinic content is greater the higher the molecular weight of the sample and the higher the activity of the used catalyser. A melting / recooling treatment results in a reduction of crystallinity by about 15–25% and disappearance of the triclinic phase. Further, an irreversible conversion of nascent orthorhombic extended chain crystallites to orthorhombic folded chain crystallites of increased lateral dimensions and crystalline perfections takes place during the melting / recooling treatment. The results are compared to those obtained for lower-molecular-weight PE samples and for high-strength / modulus PE fibers of different origins.  相似文献   
997.
In this paper, we report on the characteristics of sedimentary breccias derived from, and deposited adjacent to, the Cretaceous Maiella carbonate platform in central Italy. Previous work has interpreted the northern platform margin to have been inherited from Early Jurassic rifting, and to have remained tectonically passive throughout the Cretaceous. However, our field studies show that the platform margin was segmented by north‐south oriented normal faults in the Late Cretaceous. Thick breccias composed of lithoclastic rudstones and floatstones were derived from the collapse of the shelf margin, and were deposited as rock avalanche and debris flow deposits in hangingwall depocentres. Four breccia units (thicknesses up to 250m, 130m, 60m and 20m) were mapped and studied in detail. Clasts consist of shallow‐water bioclastic limestones; textures vary within each breccia unit from rudstones in the bottom two‐thirds to floatstones above. Vuggy, late stage dissolution porosity is present throughout each unit, and probably developed in a meteoric environment after Pliocene thrusting and exhumation. Much earlier diagenesis, associated with a Late Cretaceous shallow‐marine to meteoric environment, generated the mouldic and intergranular porosity observed in the rudstones. The skeletal floatstones display biomouldic porosity which has likewise been preserved since the Late Cretaceous. Bulk porosity volumes have been estimated for three of the breccias at the time of expected hydrocarbon charge (Miocene‐Pliocene): they range between 72 × 106 and 42 × 106 m3. Oil seepage from these breccias can be observed at the present day in outcrops near to the study area, indicating that migration and charge are still active. The significance of the breccias as potential reservoirs is that high‐porosity platform margin facies clasts were transported into a depositional environment which would otherwise have been occupied by low‐porosity slope facies. This extends the platform margin reservoir fairway beyond the defined platform edge.  相似文献   
998.
A system has been developed for tracking the motion of objects in two dimensions in real-time. The system consists of a conventional CCD camera linked to a transputer-based frame grabber and an array of nine transputers. A parallel moments algorithm is used to extract the co-ordinates of the object's centre of gravity and orientation at field rate, i.e. 60 Hz. Since the position data are made available in real-time—with a small time delay—the system has the potential for inclusion in a feedback loop. Results are presented for tracking the trajectory of a chocolate bar diverted by an air jet. The potential of the system for higher sampling rates—up to 200 Hz—is discussed.  相似文献   
999.
A method for the calculation of the current distribution, resistance, and inductance matrices for a system of coupled superconducting transmission lines having finite rectangular cross-section is presented. These calculations allow accurate characterization of both high-Tc and low-T c superconducting strip transmission lines. For a single stripline geometry with finite ground planes, the current distribution, resistance, inductance, and kinetic inductance are calculated as functions of the penetration depth for various film thicknesses. These calculations are then used to determine the penetration depth for Nb, NbN, and YBa2Cu3O7-x superconducting thin films from the measured temperature dependence of the resonant frequency of a stripline resonator. The calculations are also used to convert measured temperature dependence of the quality factor to the intrinsic surface resistance as a function of temperature for an Nb stripline resonator  相似文献   
1000.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号