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   242413篇
  免费   2651篇
  国内免费   427篇
电工技术   4298篇
综合类   259篇
化学工业   39594篇
金属工艺   9387篇
机械仪表   7411篇
建筑科学   5483篇
矿业工程   1392篇
能源动力   5172篇
轻工业   24961篇
水利工程   2502篇
石油天然气   5098篇
武器工业   32篇
无线电   25094篇
一般工业技术   44884篇
冶金工业   45763篇
原子能技术   5950篇
自动化技术   18211篇
  2021年   2139篇
  2019年   2038篇
  2018年   3513篇
  2017年   3442篇
  2016年   3702篇
  2015年   2252篇
  2014年   3932篇
  2013年   10232篇
  2012年   6185篇
  2011年   8366篇
  2010年   6593篇
  2009年   7276篇
  2008年   7632篇
  2007年   7758篇
  2006年   6940篇
  2005年   6082篇
  2004年   5856篇
  2003年   5624篇
  2002年   5705篇
  2001年   5575篇
  2000年   5410篇
  1999年   5319篇
  1998年   12601篇
  1997年   8917篇
  1996年   6870篇
  1995年   5351篇
  1994年   4752篇
  1993年   4642篇
  1992年   3687篇
  1991年   3396篇
  1990年   3619篇
  1989年   3486篇
  1988年   3322篇
  1987年   2906篇
  1986年   2941篇
  1985年   3395篇
  1984年   3233篇
  1983年   2901篇
  1982年   2746篇
  1981年   2837篇
  1980年   2727篇
  1979年   2634篇
  1978年   2697篇
  1977年   2953篇
  1976年   3811篇
  1975年   2376篇
  1974年   2248篇
  1973年   2382篇
  1972年   1973篇
  1971年   1861篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
'Software defined radio' (SDR) is a technology that will appear in future generations of mobile phones, i.e. following the third-generation mobile phone technology that is currently being defined and developed. Early versions of 'pragmatic' SDR will allow the terminal to be reconfigured at any level of its protocol stack. Ultimately, the 'pure' SDR technology will allow a mobile phone or terminal to have its air interface software configured or reconfigured by other software (or software parameters) that have been downloaded to the terminal, e.g. over the air, or from a remote server via the Internet and one's personal computer (PC). A number of security issues arise with downloaded code that implements the air interface functions, and these may not be obvious simply from looking at the way PC software is updated on-line today. This paper starts with an outline of the code that allows a mobile phone to operate over a particular air interface. This sets the baseline for a discussion of the security issues surrounding the change of this code from one that is fixed and downloaded once only, to code that is reconfigurable during the life of a product.  相似文献   
162.
Single crystals of Ti-Al alloys containing 1.4, 2.9, 5, and 6.6 pct Al (by weight) were oriented for 〈a〉 slip on either basal or prism planes or loaded parallel along the c-axis to enforce a nonbasal deformation mode. Most of the tests were conducted in compression and at temperatures between 77 and 1000 K. Trace analysis of prepolished surfaces enabled identification of the twin or slip systems primarily responsible for deformation. Increasing the deformation temperature, Al content, or both, acted to inhibit secondary twin and slip systems, thereby increasing the tendency toward strain accommodation by a single slip system having the highest resolved stress. In the crystals oriented for basal slip, transitions from twinning to multiple slip and, finally, to basal slip occurred with increasing temperature in the lower-Al-content alloys, whereas for Ti-6.6 pct Al, only basal slip was observed at all temperatures tested. A comparison of the critically resolved shear stress (CRSS) values for basal and prism slip as a function of Al content shows that prism slip is favored at room temperature in pure Ti, but the stress to activate these two systems becomes essentially equal in the Ti-6.6 pct Al crystals over a wide range of temperatures. Compression tests on crystals oriented so that the load was applied parallel to the c-axis showed extensive twinning in lower Al concentrations and 〈c+a〉 slip at higher Al concentrations, with a mixture of 〈c+a〉 slip and twinning at intermediate compositions. A few tests also were conducted in tension, with the load applied parallel to the c-axis. In these cases, twinning was observed, and the resolved shear for plastic deformation by twinning was much lower that that for 〈c+a〉 slip observed in compression loading. This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following ASM committees: Materials Science and Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee.  相似文献   
163.
164.
We report here observational results demonstrating that a three-station network of properly distributed VLBI observatories can routinely determine UT1 with a formal standard error of ±0.05 ms of time, in an observing period of 24 h. We also report the results of a three-month series of daily observing sessions of only 1-h duration with a single interferometer, which produced estimates of UT1 with standard errors of ±0.1 ms. The UT1 values obtained from the 1-h observing sessions track smoothly between the points of the 24-h time series, and the combined time series shows that it is not unusual for UT1 to vary by 1-2 ms in periods of several days. Preliminary results of reprocessing the 24-h observing sessions in 2-h segments suggest that variations of 0.4 ms may occur on time scales of only 6-8 h.  相似文献   
165.
166.
Translated from Khimiya i Tekhnologiya Topliv i Masel, No. 2, pp. 14–15, February, 1989.  相似文献   
167.
The data of literature about the structure of cytoskeleton proteins in the normal and malignant cells are analyzed. The possibilities of using a specific polyclonal serum and a monoclonal antibody for a more precise ascertainment of the origin and diagnostic of tumours are discussed. The intermediate and active filaments may be used as a new class of histochemical markers. Laminin, the protein of basement membranes, probably, plays an important role in metastatic spreading.  相似文献   
168.
Aspects of a prototype binding of GKS to the C++ programming language are presented. The binding makes use of classes and derived classes to define GKS concepts such as segments and workstations. Operator overloading is used for some GKS Functions.  相似文献   
169.
The usefulness of selected PCR-protocols for the detection of Salmonella in 117 samples of animal origin (17 raw minced meat, 27 raw chicken meat, 8 raw sausages, and 25 egg samples, as well as 18 poultry faecal, and caecal swabs samples) and DNA-fingerprinting typing is shown. To establish an accurate PCR-procedure for Salmonella detection the following parameters were evaluated: two pre-PCR concentration procedures, centrifugation and immunomagnetic separation (IMS) using Dynabeads anti- Salmonella; the specificity and sensitivity of 10 sets of primers; and different conditions of the amplification reaction. In light of the results obtained from the use of PCR-based procedures alone or in combination with conventional methods, the following findings can be underlined: First, IMS is more efficient than centrifugation in the recovery of Salmonella. Second, the selected IMS/PCR-detection protocol is less time-consuming (45 h) than the IMS/culture procedure (90 h), and a good concordance between them was found when the Kappa coefficient was calculated (0·87). Third, PCR-ribotyping technique showed a very low discrimination power, being able to differentiate only three profiles. Fourth, RAPD technique using specific primers supports previous works in which it was proposed as a simple and useful tool for discriminating isolates between and within serotypes. Fifth, The efficiency, rapidity, and flexibility of the PCR-protocols applied were high, and they can be performed using two PCR-programs and the same basic equipment.  相似文献   
170.
The behavior of two series of concrete slabs exposed to sulfate-bearing soils was investigated by a numerical model called STADIUM. In addition to the diffusion of ions and moisture, the model also accounts for the effects of dissolution/precipitation reactions on the transport mechanisms. The simulations yielded by the model were compared to the actual degradation of the slabs after 8 years of exposure. The microstructural alterations of concrete resulting from the penetration of magnesium, chloride and sulfate ions were studied by backscatter mode scanning electron microscope observations and energy-dispersive X-ray analyses. The comparison of both series of data indicates that the model can reliably predict the various features of the microstructural alterations of concrete.
Résumé Le comportement de deux séries de dalles sur sol en béton exposées à des sols chimiquement agressifs a été étudié à l'aide d'un code de calcul numérique appelé STADIUM. Ce modèle permet de décrire le transport couplé de l'eau et des ions dans des matériaux poreux non-saturés en prenant en considération l'influence des réactions chimiques. Les résultats des simultations de la dégradation du béton après huit ans d'exposition à des ions chlore, sulfate et magnésium. Les observations ont été réalisées par microscopie électronique à balayage. Des analyses par dispersion des rayons X ont également été effectuées. Les données démontrent clairement que le modèle perment de prédire avec précision le comportement du béton soumis à différents types d'agression chimique.


Editorial Note Laval University (Canada) is a RILEM Titular Member. Prof. J. Marchand was awarded the 2000 Robert L'Hermite Medal. He is Editor in Chief for Concrete Science and Engineering and Associate Editor for Materials and Structures. He participates in RILEM TC 186-ISA ‘Internal Sulfate attack’.  相似文献   
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