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   3607篇
  免费   88篇
  国内免费   4篇
电工技术   46篇
综合类   23篇
化学工业   918篇
金属工艺   83篇
机械仪表   80篇
建筑科学   148篇
矿业工程   16篇
能源动力   50篇
轻工业   646篇
水利工程   7篇
石油天然气   12篇
无线电   195篇
一般工业技术   662篇
冶金工业   405篇
原子能技术   25篇
自动化技术   383篇
  2021年   51篇
  2020年   43篇
  2019年   44篇
  2018年   45篇
  2016年   68篇
  2015年   56篇
  2014年   70篇
  2013年   101篇
  2012年   110篇
  2011年   146篇
  2010年   117篇
  2009年   118篇
  2008年   118篇
  2007年   114篇
  2006年   107篇
  2005年   110篇
  2004年   97篇
  2003年   54篇
  2002年   74篇
  2001年   58篇
  2000年   69篇
  1999年   75篇
  1998年   126篇
  1997年   100篇
  1996年   68篇
  1995年   75篇
  1994年   69篇
  1993年   54篇
  1992年   64篇
  1991年   40篇
  1990年   36篇
  1989年   39篇
  1988年   36篇
  1987年   38篇
  1986年   46篇
  1985年   53篇
  1984年   60篇
  1983年   39篇
  1982年   39篇
  1980年   46篇
  1979年   41篇
  1978年   43篇
  1977年   47篇
  1976年   55篇
  1975年   50篇
  1974年   55篇
  1973年   44篇
  1971年   34篇
  1970年   39篇
  1967年   33篇
排序方式: 共有3699条查询结果,搜索用时 187 毫秒
71.
We analyze the influence of B2C e-commerce on IT structure and brand architecture based on conceptual considerations, a case study and an empirical survey. We first discuss the changes in transaction costs that are induced by B2C e-commerce. The effects of B2C e-commerce on IT structure and brand architecture, which are anticipated from a transaction cost perspective, are illustrated using a case study from the tourism industry. We conducted a quantitative study among 101 chief information officers (CIOs) and chief marketing officers (CMOs) from the most important consumer brand companies in Austria. The results of a correspondence analysis show that a high importance of e-commerce triggers a typical bundle of changes in IT structure and brand architecture, which point toward a stronger integration within and between these two structural elements. We therefore conclude that B2C e-commerce calls for a better coordination of those measures that impact IT structure and brand architecture.  相似文献   
72.
A combinatorial characterization is given for those one-rule Thue systems of the form {(w 1,w 2)} with 0 |w 2||ov(w 1)| that are Church-Rosser. Here ov(w 1) denotes the longest proper self-overlap ofw 1. Further, it is shown that a Thue system of this form is Church-Rosser if and only if there is an equivalent Thue system that is Church-Rosser.This research was supported in part by the National Science Foundation under Grant No. MCS83-14977. It was performed while the first author was visiting the Department of Mathematics of the University of California at Santa Barbara.  相似文献   
73.
We report measurements of the temperature, density, and concentration gradients in3He-4He mixtures, induced by a vertical heat flux. The flat horizontal cell included two superposed capacitors and the density was determined by means of the dielectric constant method. The experiments were carried out on mixtures with mole fractionsX 3=0.37, 0.15, and 0.05 at saturated vapor pressure, with special emphasis on the region near the superfluid transition. Our measurements under steady-state conditions give the conductivity , the thermal diffusion ratiok T , and the coefficient of thermal expansion. We describe the singular behavior of these quantities in the neighborhood ofT (X). In the superfluid phase, we test with fair success a relation by Khalatnikov between gradX/ gradT and several static properties. From the relaxation times needed to attain steady-state conditions, and in combination with measured static and transport properties, we obtain in the normal phase the mass diffusionD, which diverges strongly asT is approached. In the superfluid phase, we test successfully a scaled relation that results from the solution of Khalatnikov's hydrodynamic equations. From our data the dispersion relations for scattered light are calculated: o/q 2 in both the normal and the superfluid phases and 2/q 2 in the normal phase.Research supported by NSF grant DMR 8024056.  相似文献   
74.

Definition of the problem

Hospitals serve the general purpose to improve the health of patients. Health care professionals are a necessary means to achieve that end but have their own more specific professional ends. What is the due relationship between those kinds of ends with respect to the organisation of hospitals?

Arguments

Individual health care in hospitals is delivered by professionals who are dependent upon the hospital??s human, administrative and technical facilities. As organised social entities, hospitals use professionals as the means to an end we normally call health of patients. More precisely, the professional task consists in a specific, namely, therapeutic interaction, an asymmetric relationship between morally equals. Professionals have to explore, interpret and pursue patients?? interests according to their consent. From a moral perspective, this interaction should be kept free from distortion through external incentives which divert the professional??s intrinsic motivation. Although a professional responsibility, the individual professional would be overcharged with this organisational task.

Conclusion

The profession has a responsibility to protect and foster professional behaviour of its members including participation in organising hospitals and the individual professional should engage in organising the profession as a collective agent.  相似文献   
75.
The design of two custom sample holders with a spherical cavity for commercial vibrating sample magnetometer systems is described. For such cavities, the magnetization M[over ->] and the internal magnetic field H(i)[over ->] of a sample are both homogeneous. Consequently, the material parameter M(H(i)) of a sample can be determined even for liquids and powders with a high magnetic susceptibility.  相似文献   
76.
We describe a novel quadrotor Micro Air Vehicle (MAV) system that is designed to use computer vision algorithms within the flight control loop. The main contribution is a MAV system that is able to run both the vision-based flight control and stereo-vision-based obstacle detection parallelly on an embedded computer onboard the MAV. The system design features the integration of a powerful onboard computer and the synchronization of IMU-Vision measurements by hardware timestamping which allows tight integration of IMU measurements into the computer vision pipeline. We evaluate the accuracy of marker-based visual pose estimation for flight control and demonstrate marker-based autonomous flight including obstacle detection using stereo vision. We also show the benefits of our IMU-Vision synchronization for egomotion estimation in additional experiments where we use the synchronized measurements for pose estimation using the 2pt+gravity formulation of the PnP problem.  相似文献   
77.
Workers in physically demanding occupations require work breaks to recover from exertion. In a laboratory setting, we investigated the impact of ergometer cycling for 7 h in two conditions with an identical total break time but with two different activity-rest schedules. We hypothesised that more frequent but shorter breaks lead to less psychophysical strain and its effects than do less frequent but longer breaks, particularly for older workers. Twenty-nine participants representing three different age groups were tested in both conditions. Heart rate, perceived exertion/tension and feelings of fatigue were assessed and used as dependent variables. Results indicate no general activity-rest differences as well as no age-related differences of break effects under the condition of subjectively equal straining load. However, heart rate was found to be lower at some measurement points in the frequent-short-break condition and perceived exertion was lower in the infrequent-long-break condition. PRACTITIONER SUMMARY: Design of activity-rest schedules in physically demanding occupations is a key issue in the prevention of strain and hence of interest to ergonomists. Our study suggests that breaks during physically demanding work have the same effect if they are frequent and short or infrequent and long, regardless of age.  相似文献   
78.
It has been shown that stimulation of the uterus of mares by the daily taking of biopsies can result in the occurrence of oestrous symptoms. This is accompanied by some follicular growth and a progesterone content in the blood often higher than 1 ng/ml. The following observations suggest that this progesterone originates in the uterus and not in the ovaries: (1) no active corpora lutea appeared to be present in the ovaries after ovariectomy, (2) ovariectomized mares showed the same oestrous symptoms in similar experiments and even mating took place, (3) in the uterine biopsies the concentration of the compound referred to as "5.4", which is assumed to be easily convertible into progesterone, had already increased considerably in the second biopsy, (4) administration of stilboestrol reduced the rise of the progesterone level in the uterine biopsies as well as in the blood. The absence of oestradiol-17 beta in the ovarian follicles and the fact that ovariectomized mares also come into heat suggest that oestrogens cannot be held responsible for the oestrous symptoms in these mares. Our experiments demonstrate that the uterus can be involved in sexual behaviour and the formation of steroids.  相似文献   
79.
The initiation and growth of porous oxide on Ta was investigated in mixed H2SO4/HF electrolytes. Under selected potentiodynamic anodic oxidation conditions the formation of nearly uniform porous Ta2O5 layer was observed. The porous Ta2O5 layers consist of self-organized pore arrays with single pore diameters ranging from 2 to 10 nm. The morphology and the thickness of the layer depend strongly on the applied potential, the scan rate and on HF presence. The composition of the porous oxide layer is Ta2O5.  相似文献   
80.
This paper describes a new sheet metal forming process for the production of sheet components for prototypes and small lot sizes. The generation of the shape is based on kinematics and is implemented by means of a new forming machine consisting of two industrial robots. Compared to conventional sheet metal forming machines, this newly developed forming process offers a high geometrical form flexibility, and comparatively small deformation forces enable high deformation degrees. The principle of the procedure is based on flexible shaping by means of a freely programmable path‐synchronous movement of the two robots. The final shape is produced by the incremental infeed of the forming tool in depth direction and its movement along the contour in lateral direction at each level of the depth direction. The supporting tool with its simple geometry is used to support the sheet metal and follows the forming tool at the rear side of the sheet metal. The sheet metal components manufactured in first attempts are of simple geometry like frustum and frustum of pyramids as well as spherical cups. Among other things the forming results are improved by an adjustment of the movement strategy, a variation of individual process parameters and geometric modifications of the tools. In addition to a measurement of the form deviations of the sheet with a Coordinate Measurement Machine, screened and deformed sheets are used for deformation analyses. Furthermore, the incremental forming process is analysed with assistance of the finite element method. In total the results show that a robot‐based sheet metal forming with kinematic shape generation is possible and leads to acceptable forming results. In order to be able to use the potential of this process, a goal‐oriented process design is as necessary as specific process knowledge. In order to achieve process stability and safety, the essential process parameters and the process boundaries have to be determined.  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