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   907863篇
  免费   10398篇
  国内免费   2476篇
电工技术   16053篇
综合类   2999篇
化学工业   137485篇
金属工艺   37559篇
机械仪表   29377篇
建筑科学   20627篇
矿业工程   5942篇
能源动力   23534篇
轻工业   67770篇
水利工程   10667篇
石油天然气   18498篇
武器工业   46篇
无线电   101600篇
一般工业技术   182039篇
冶金工业   153090篇
原子能技术   19340篇
自动化技术   94111篇
  2021年   7681篇
  2020年   5792篇
  2019年   7390篇
  2018年   25687篇
  2017年   24757篇
  2016年   22479篇
  2015年   9118篇
  2014年   14512篇
  2013年   39380篇
  2012年   25300篇
  2011年   39096篇
  2010年   31917篇
  2009年   32527篇
  2008年   34183篇
  2007年   34590篇
  2006年   24390篇
  2005年   22967篇
  2004年   21893篇
  2003年   21721篇
  2002年   20174篇
  2001年   19386篇
  2000年   18483篇
  1999年   18670篇
  1998年   45705篇
  1997年   32366篇
  1996年   25140篇
  1995年   18961篇
  1994年   16979篇
  1993年   16781篇
  1992年   12532篇
  1991年   11935篇
  1990年   11777篇
  1989年   11362篇
  1988年   10812篇
  1987年   9762篇
  1986年   9390篇
  1985年   10725篇
  1984年   9760篇
  1983年   9123篇
  1982年   8302篇
  1981年   8471篇
  1980年   7915篇
  1979年   7975篇
  1978年   7817篇
  1977年   8835篇
  1976年   11245篇
  1975年   6918篇
  1974年   6640篇
  1973年   6735篇
  1972年   5693篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Impact ionization is a major limiting factor to the maximum operating voltage of InGaAs-based, high-speed transistors. In this work, data on the positive temperature dependence of the electron impact ionization coefficient αn in In0.53Ga0.47As at medium-low electric fields are reported for the first time. The increase of αn with temperature is opposite to the behavior normally observed in most semiconductors. This anomalous behavior implies the onset of a positive feedback between power dissipation and avalanche generation which may adversely affect the power handling capability of In0.53Ga 0.47As-based devices, and which should be taken into account in device thermal modeling. In the experimental procedure, based on the measurement of the multiplication factor M-1 in npn In0.53Ga 0.47As/InP Heterojunction Bipolar Transistors (HBT), particular care has been taken in order to rule out possible spurious, temperature-dependent contributions to the measured multiplication current  相似文献   
92.
This paper presents the design criteria, procedure, and implementation of a soft-switched power-factor-correction (PFC) circuit based on the extended-period quasi-resonant (EPQR) principles. All power electronic devices including switches and diodes in the circuit are fully soft switched. The design method is demonstrated in a prototype circuit. The operating principles are confirmed with computer simulation and experimental results. A comparison of the EP-QR operation and zero-voltage-transition (ZVT) pulse-width modulation (PWM) method  相似文献   
93.
The purpose of this paper is to evaluate two methods of assessing the productivity and quality impact of Computer Aided Software Engineering (CASE) and Fourth Generation Language (4GL) technologies: (1) by the retrospective method; and (2) the cross-sectional method. Both methods involve the use of questionnaire surveys. Developers' perceptions depend on the context in which they are expressed and this includes expectations about the effectiveness of a given software product. Consequently, it is generally not reliable to base inferences about the relative merits of CASE and 4GLs on a cross-sectional comparison of two separate samples of users. The retrospective method that requires each respondent to directly compare different products is shown to be more reliable. However, there may be scope to employ cross-sectional comparisons of the findings from different samples where both sets of respondents use the same reference point for their judgements, and where numerical rather than verbal rating scales are used to measure perceptions.  相似文献   
94.
95.
96.
Malignant bone pain: pathophysiology and treatment   总被引:1,自引:0,他引:1  
2-acetyl aminofluorene (AAF) reacts in acidic conditions with nitrous fume yielding N-nitroso-AAF (N-NO-AAF), as previously described, that exerts more toxic and mutagenic effects than its parental compound. In this study, the effect of sodium nitrite (NaNO2) on the tumorigenicity of AAF in rats fed with AAF and NaNO2 was observed. Wistar rats were divided into five groups: group I served as control; group II were treated with NaNO2 (0.3%); group III was given 0.02% AAF alone; groups IV and V received both AAF and NaNO2 (0.2 and 0.3% respectively) in their diet for 12 weeks. At the end of the experiment, all rats in groups III, IV and V developed early stage phenomena of hepatocellular carcinoma, including hepatomegaly with variable-sized foci and neoplastic nodules. Severe damage was observed in the rats treated with AAF and NaNO2. Feeding of AAF (0.02%) for 3 months elevated the levels of c-Fos, c-Jun and c-Myc proteins in the rat livers. The AAF-induced c-Jun, c-Fos and c-Myc expressions were significantly magnified (P < 0.001) by NaNO2. These data confirmed that the strengthening of AAF-induced hepatocarcinogenesis by NaNO2 should be associated with its enhancing effect on the AAF-induced increases in the expressions of c-Jun, c-Fos and c-Myc.  相似文献   
97.
The physical widths of reference features incorporated into electrical linewidth test structures patterned in films of monocrystalline silicon have been determined from Kelvin voltage measurements. The films in which the test structures are patterned are electrically insulated from the bulk-silicon substrate by a layer of silicon dioxide provided by SIMOX (Separation by the IMplantation of OXygen) processing. The motivation is to facilitate the development of linewidth reference materials for critical-dimension (CD) metrology-instrument calibration. The selection of the (110) orientation of the starting silicon and the orientation of the structures' features relative to the crystal lattice enable a lattice-plane-selective etch to generate reference-feature properties of rectangular cross section and atomically planar sidewalls. These properties are highly desirable for CD applications in which feature widths are certified with nanometer-level uncertainty for use by a diverse range of CD instruments. End applications include the development and calibration of new generations of CD instruments directed at controlling processes for manufacturing devices having sub-quarter-micrometer features  相似文献   
98.
A new commercially available diode model is described. This unified model is capable of simulating the widest range of diode technologies of any presently available. The emphasis of this paper is on describing the model's extensive features and flexibility in the different domains of operation and is of particular interest in power applications  相似文献   
99.
The capacitive idling converters derived from the Cuk, SEPIC, Zeta, and flyback topologies allow soft commutation of power switches without the need for additional circuitry, making it possible to increase the switching frequency while maintaining high efficiency  相似文献   
100.
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