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   266326篇
  免费   2818篇
  国内免费   1238篇
电工技术   4685篇
综合类   183篇
化学工业   40565篇
金属工艺   12774篇
机械仪表   9013篇
建筑科学   5390篇
矿业工程   2505篇
能源动力   6257篇
轻工业   18547篇
水利工程   3606篇
石油天然气   9265篇
武器工业   61篇
无线电   27353篇
一般工业技术   58065篇
冶金工业   44744篇
原子能技术   8623篇
自动化技术   18746篇
  2021年   2724篇
  2019年   2574篇
  2018年   4773篇
  2017年   4834篇
  2016年   5265篇
  2015年   2976篇
  2014年   4955篇
  2013年   11638篇
  2012年   7481篇
  2011年   9453篇
  2010年   7710篇
  2009年   8582篇
  2008年   8720篇
  2007年   8546篇
  2006年   7302篇
  2005年   6611篇
  2004年   6135篇
  2003年   6004篇
  2002年   5763篇
  2001年   5860篇
  2000年   5584篇
  1999年   5521篇
  1998年   13394篇
  1997年   9415篇
  1996年   7191篇
  1995年   5463篇
  1994年   4774篇
  1993年   5008篇
  1992年   3803篇
  1991年   3859篇
  1990年   3767篇
  1989年   3707篇
  1988年   3620篇
  1987年   3199篇
  1986年   3253篇
  1985年   3587篇
  1984年   3313篇
  1983年   3166篇
  1982年   2845篇
  1981年   2812篇
  1980年   2860篇
  1979年   2876篇
  1978年   2859篇
  1977年   3112篇
  1976年   3672篇
  1975年   2638篇
  1974年   2498篇
  1973年   2599篇
  1972年   2315篇
  1971年   2114篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
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   
32.
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   
33.
The value of flexible videoendoscopy as a biofeedback tool in conservative swallowing rehabilitation of patients suffering from aspiration after head and neck surgery was examined. For this purpose, the outcomes of conventional swallowing therapy and of swallowing therapy with support of videoendoscopic biofeedback were compared. In the first 40 days of swallowing therapy, videoendoscopic biofeedback significantly increased the chance of therapeutic success, shortening the period of functional rehabilitation in comparison to conventional swallowing therapy. Limitations of this visual biofeedback procedure are poor cognitive skills of the patient, rejection of the procedure by the patient and local factors that make the positioning of the endoscope difficult.  相似文献   
34.
Pipelining and bypassing in a VLIW processor   总被引:1,自引:0,他引:1  
This short note describes issues involved in the bypassing mechanism for a very long instruction word (VLIW) processor and its relation to the pipeline structure of the processor. The authors first describe the pipeline structure of their processor and analyze its performance and compare it to typical RISC-style pipeline structures given the context of a processor with multiple functional units. Next they study the performance effects of various bypassing schemes in terms of their effectiveness in resolving pipeline data hazards and their effect on the processor cycle time  相似文献   
35.
36.
37.
This paper presents a model of heterogenous diffusion in capillary porous materials during the process of drying. The governing heat and mass transfer equations have been established using the liquid as well as vapor flow. Two models have been presented. Model 1 does not consider the heat conduction while the model 2 has been established by considering the conduction. The developed models and the numerical solutions of the resulting differential equations can take into account the moisture and temperature dependent thermophysical properties of the product. All equations have been established in spherical coordinates but the programme written for the purpose of calculations can be used for other geometries also. Numerical calculations have been performed for gas concrete and tiles using model 1, while model 2 has been used for gas concrete only because of the lack of data for thermophysical properties of the tile. For gas concrete it was seen that conduction has only marginal effect on the drying process and the numerical predictions of the drying process were reasonably accurate.  相似文献   
38.
The effectiveness of the hybrid system sonication followed by wet oxidation (SONIWO) to treat otherwise refractory waste has been demonstrated. In such a hybrid system homogeneous CuSO4 catalyst was found to be very efficient.  相似文献   
39.
The technological process of cold forging applied for the first time in the production of the casing of the internal joint with races is described. The process operations of cold forging and the annealing and carburizing regimes for this part me described.  相似文献   
40.
It is not known whether impaired hematopoiesis noted during human immunodeficiency virus (HIV) infection results from infection of stem/progenitor cells or of cells of the bone marrow microenvironment. Normal adherent primary stromal layers were exposed to HIV to determine which of this mixture of endothelial cells, fibroblasts, and macrophages are susceptible to the virus. Viral p24 in supernatants was noted with monocytotropic HIV-1Ada, HIV-1Ba-L, and HIV-1JR-FL but not with lymphotropic HIV-1LAI nor HIV-1MN strain, and only stromal macrophages expressed the viral antigens. Coculture of the layers with PHA-activated normal lymphocytes failed to rescue lymphotropic virus. No p24 was produced when macrophage-depleted stromal cells were exposed to either HIV-1Ba-L or HIV-1LAI; proviral DNA was then amplified by PCR in cells exposed to either virus, though coculture with lymphocytes rescued only HIV-1Ba-L. Altogether, these data indicate that macrophages are the major targets of HIV in cultured stromal layers. As virus replication in macrophages did not affect the profile of major cytokines involved in regulating hematopoiesis, HIV infection could alter hematopoiesis by other as yet unspecified mechanisms.  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