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   156221篇
  免费   1408篇
  国内免费   929篇
电工技术   2643篇
综合类   2358篇
化学工业   19807篇
金属工艺   11641篇
机械仪表   9164篇
建筑科学   3577篇
矿业工程   1301篇
能源动力   2172篇
轻工业   4498篇
水利工程   2105篇
石油天然气   1982篇
武器工业   1篇
无线电   16485篇
一般工业技术   36237篇
冶金工业   9827篇
原子能技术   1492篇
自动化技术   33268篇
  2022年   82篇
  2021年   166篇
  2020年   110篇
  2019年   171篇
  2018年   20810篇
  2017年   20523篇
  2016年   13285篇
  2015年   1277篇
  2014年   636篇
  2013年   1170篇
  2012年   5660篇
  2011年   16795篇
  2010年   15278篇
  2009年   12309篇
  2008年   12757篇
  2007年   15507篇
  2006年   917篇
  2005年   5284篇
  2004年   3502篇
  2003年   3043篇
  2002年   1521篇
  2001年   613篇
  2000年   705篇
  1999年   473篇
  1998年   793篇
  1997年   549篇
  1996年   504篇
  1995年   336篇
  1994年   293篇
  1993年   242篇
  1992年   175篇
  1991年   201篇
  1990年   127篇
  1989年   114篇
  1988年   95篇
  1968年   148篇
  1967年   135篇
  1966年   172篇
  1965年   142篇
  1964年   90篇
  1963年   72篇
  1962年   73篇
  1961年   78篇
  1960年   94篇
  1959年   74篇
  1958年   88篇
  1957年   146篇
  1956年   125篇
  1955年   155篇
  1954年   166篇
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
101.
102.
Laparoscopic retroperitoneal lymph node dissection is a new surgical procedure used to enhance staging in men with clinical stage I nonseminomatous germ cell tumors of the testis. The procedure has been performed in a limited number of patients at several centers with extensive laparoscopic experience. Laparoscopic retroperitoneal lymphadenectomy is a technically demanding procedure which can be successfully completed in the majority of patients. However, the risk of complications is greater than in patients who undergo standard open retroperitoneal lymph node dissection. The primary advantage of a laparoscopic approach is shortened hospitalization and rapid return to normal activity. The role of laparoscopy in the management of patients with testis malignancy has not been defined. The use of this staging procedure may help minimize the need for surveillance studies following surgery and may be best utilized in men with a lower likelihood of nodal metastases. Ultimately, prospective study in large groups of patients will be necessary to determine the role of laparoscopic retroperitoneal lymph node dissection in patients with testis cancer.  相似文献   
103.
104.
In this paper, we report the study of the electrical characteristics of GaN and AlGaN vertical p-i-n junctions and Schottky rectifiers grown on both sapphire and SiC substrates by metal-organic chemical-vapor deposition. For GaN p-i-n rectifiers grown on SiC with a relatively thin “i” region of 2 μm, a breakdown voltage over 400 V, and forward voltage as low as 4.5 V at 100 A/cm2 are exhibited for a 60-μm-diameter device. A GaN Schottky diode with a 2-μm-thick undoped layer exhibits a blocking voltage in excess of ∼230 V at a reverse-leakage current density below 1 mA/cm2, and a forward-voltage drop of 3.5 V at a current density of 100 A/cm2. It has been found that with the same device structure and process approach, the leakage current of a device grown on a SiC substrate is much lower than a device grown on a sapphire substrate. The use of Mg ion implantation for p-guard rings as planar-edge terminations in mesageometry GaN Schottky rectifiers has also been studied.  相似文献   
105.
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   
106.
The robust Schur stability of a polynomial with uncertain coefficients will be investigated. A formula for the stability radius of a Schur polynomial is established. The result is the counterpart of [1] for linear discrete-time systems  相似文献   
107.
Presuppositions of utterances are the pieces of information you convey with an utterance no matter whether your utterance is true or not. We first study presupposition in a very simple framework of updating propositional information, with examples of how presuppositions of complex propositional updates can be calculated. Next we move on to presuppositions and quantification, in the context of a dynamic version of predicate logic, suitably modified to allow for presupposition failure. In both the propositional and the quantificational case, presupposition failure can be viewed as error abortion of procedures. Thus, a dynamic assertion logic which describes the preconditions for error abortion is the suitable tool for analysing presupposition.  相似文献   
108.
109.
110.
Modeling and analysis of vias in multilayered integrated circuits   总被引:1,自引:0,他引:1  
A method for modeling and analyzing vias the multilayered integrated circuits is presented. The model is based on microwave network theory. The whole via structure is divided into cascaded subnetworks, including a vertical via passing through different layers and transitions from the microstrip line and/or striplines to the vertical via. The parameters of each subnetwork are obtained from electromagnetic field analysis. Numerical results in the frequency domain and the time domain are presented. Validation of the model has been carried out by both measurements and finite-difference-time-domain (FDTD) modeling. The results show good agreement with the measurements in the frequency range for which the components of the experimental model are within specification. The time domain simulation results also agree well with the FDTD results  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