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   197801篇
  免费   2114篇
  国内免费   633篇
电工技术   4060篇
综合类   123篇
化学工业   26697篇
金属工艺   7507篇
机械仪表   5460篇
建筑科学   4050篇
矿业工程   747篇
能源动力   4657篇
轻工业   14380篇
水利工程   1695篇
石油天然气   2842篇
武器工业   7篇
无线电   27291篇
一般工业技术   37385篇
冶金工业   45379篇
原子能技术   3560篇
自动化技术   14708篇
  2021年   1246篇
  2019年   1280篇
  2018年   2124篇
  2017年   2105篇
  2016年   2173篇
  2015年   1490篇
  2014年   2619篇
  2013年   7952篇
  2012年   4465篇
  2011年   6110篇
  2010年   4931篇
  2009年   5771篇
  2008年   6145篇
  2007年   6195篇
  2006年   5633篇
  2005年   5303篇
  2004年   5249篇
  2003年   5088篇
  2002年   4898篇
  2001年   5307篇
  2000年   4915篇
  1999年   5435篇
  1998年   15617篇
  1997年   10311篇
  1996年   7929篇
  1995年   5783篇
  1994年   5013篇
  1993年   5021篇
  1992年   3385篇
  1991年   3260篇
  1990年   3216篇
  1989年   3039篇
  1988年   2784篇
  1987年   2234篇
  1986年   2301篇
  1985年   2606篇
  1984年   2312篇
  1983年   2063篇
  1982年   1901篇
  1981年   2043篇
  1980年   1790篇
  1979年   1659篇
  1978年   1656篇
  1977年   2021篇
  1976年   2702篇
  1975年   1422篇
  1974年   1369篇
  1973年   1316篇
  1972年   1124篇
  1971年   956篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
The glassforming region in the system was roughly outlined and liquidus data were obtained for the three joins LiPO3-BPO4, Li4P2O7-BPO4, and Li3PO4-Li2B4O7. Compatibility relations for the ternary subsystems Li4P2O7-BPO4-P2O5 and Li2O-Li3PO4-Li2B8O13 were established. Two ternary compounds with the probable compositions 22Li2O - 11B2O3 - 13P2O5 and 2Li2O 3B2O3 P2O5 were detected.  相似文献   
992.
Three electrical effects in an approximately stoichiometric propane-oxygen flame seeded with aluminum powder were studied: a) increase in flame current caused by the addition of aluminum, b) effect of transverse electrical field on the rate of aluminum oxide deposition onto a solid surface, c) effect of the field on the rate of heat transfer to a flat plate immersed in the flame. The maximum field was limited by breakdown to about 450 v/cm. It appears that the oxide deposition rate and the heat transfer rate can be influenced by relatively small electric fields.  相似文献   
993.
A 2-year field experiment was carried out in a sandy soil (Xeropsament-Torripsament) at Nir Yizhaq, Israel, where commercially grown peanut plants were sprayed with different NPKS solutions during the pod-filling period. All foliar sprays were applied in addition to the fertilizer which had been added to the soil before planting. The differences in yield between the treatments were not significant in either year. In the first year, there was a tendency toward increased yields of pods (up to 13%) and of hay (up to 16%) when four foliar fertilizer applications (10 kg N, 1 kg P, 3 kg K and 0.5 kg S/ha/application) were given, at one-week intervals. N was added mainly as urea, P and K as potassium polyphosphate, and S as ammonium sulphate. In the second year, the highest yield was obtained in the control plots and the differences between the treatments were not significant. Utilization of foliar nutrient application seems to be dependent upon availability of these nutrients in the soil.Contribution from the Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel. No. 1976-E. 1087 series.  相似文献   
994.
A number of lipid products are formed during the desaturation of stearyl CoA by hen liver microsomes. This article presents an analysis of the products formed when [1-14C] stearyl CoA is incubated with hen liver microsomes for various time periods. [1-14C] Oleyl CoA was the first radioactive unsaturated product formed. Synthesis of phospholipids containing [1-14C] oleic acid occurs only after the desaturase activity has begun to decline. The specific radioactivity of [1-14C] oleyl CoA was similar to the specific radioactivity of [1-14C] stearyl CoA at all time periods tested. The specific radioactivities of [1-14C] oleic acid and phospholipids containing [1-14C] oleic acid were much lower than that of the [1-14C] stearyl CoA.  相似文献   
995.
Rheological behavior of dispersed two-phase polymer melts has been investigated by means of a capillary rheometer. The two-phase systems chosen for study were blends of two polymers: high-density polyethylene and polystyrene, which are incompatible in the molten state. In order to investigate the state of dispersion, photographs were taken of the microstructure of extrudate samples, showing that the polystyrene forms long fibers or droplets as a discrete phase dispersed in the polyethylene which forms a continuous phase. Measurements were made of the axial pressure distributions of the two-phase molten polymers flowing through circular tubes, which permit one, according to the theory advanced by Han, to determine viscous and elastic properties of the melts. Also measured was melt die swell ratio. It has been found from the two independent experimental techniques that there exist a maximum and/or minimum elastic property at a certain blending ratio, and that the elastic property decreases first with L/D ratio of a capillary and then levels off.  相似文献   
996.
Palmqvist crack extension in WC-Co can be described using the center-loaded penny crack analogy. When Palmqvist cracks are represented by semicircles of diameter l (l=crack length measured from an impression corner), the stress intensity factor which controls crack extension differs from that of a half-penny crack of radius l+a (2a = impression size) by only a slowly varying factor m=21/2 (l/a)1/2; m≅1.0 to 2.4 for WC-Co and m≅0.8 to 1.4 for ceramics.  相似文献   
997.
Studies on voidage fluctuations, axial voidage profile and bed expansion are carried out by measuring the local void fraction using particles of wide ranging characteristics in liquid-solid inverse fluidized bed. The quality of fluidization is elucidated by the local voidage fluctuations. The RMS voidage fluctuation depicts a maximum with respect to average bed void fraction and increases with increase in Archimedes number. The fluidization quality has been quantified using average normalized RMS voidage fluctuation in terms of Transition number. The axial void fraction is almost uniform throughout the bed except for particles with size distribution. All the literature and present experimental data on bed expansion are unified in terms of Richardson and Zaki equation using experimental terminal velocities. A new correlation is proposed for predicting the wall effect corrected experimental terminal velocities, as a substitute for standard drag equation. The bed expansion data are also predicted using the drift flux model.  相似文献   
998.
Fluor公司开发的Fluor SO_2净化工艺具有广泛多样的工艺布局,可以通过经济的方式定制脱除烟气中的有害组成。该工艺几乎能去除全部SO_2并减少CO排放。Fluor SO_x净化工艺的主要步骤已在多套工业装置中得到成功的证实。除技术可行、经济合理之外,Fluor SO_x净化工艺不会产生任何有害副产物,也不会遇到与SO_3 有关的腐蚀问题。此外,该工艺生产高纯度可市售的元素硫。论述了Fluor SO_x净化工艺的特征、技术和成本优势、设计和操作的简易性以及实施的便利性。  相似文献   
999.
1000.
An approach to the analysis of 55 possible nonconjugated positional isomers of octadecadienoic acid is described and tested with mixtures of individual synthetic methyl esters. In the first example, by ozonolysis a sevencomponent mixture consisting ofcis,cis 5,12-, 6,10-, 6,11-, 6,12-, 7,12-, 8,12-, and 9,12-octadecadienoates was converted to aldehydes, aldehyde-esters and dialdehydes. These fragments were separated on a 50 m×0.2 mm free fatty acid phase (FFAP) vitreous silica capillary column. Equations for an arbitrarily restricted 12×15 matrix of linear simultaneous equations and a computer solution of the matrix provided the composition of the initial methyl octadecadienoate mixture. The power and significance of this method became apparent with the observation that only two of the seven isomers in the known mixture were resolved as single peaks by state-of-the-art capillary gas chromatography, but all seven were identified and estimated with acceptable error by the ozonolysis-capillary gas chromatography-computer procedure. In a generalized approach to the analysis of the 55 possible nonconjugated isomers, a computer program selects the appropriate matrix of linear simultaneous equations based on the aldehyde data supplied by the analyst. Twenty of 21 combinations of seven isomeric esters taken five at a time have been analyzed to assess the efficiency of the method. To illustrate applicability at this stage of development, the method has been used to analyze the diene products of the hydrazine reduction of γ-linolenic acid and the diene products from the biological desaturation of isomeric monoenes. The possibility of distinguishing geometric and positional isomers of 18∶2 has been opened by the observed separation ofcis-andtrans-unsaturated aldehydes and aldehyde-esters. Presented in part at the 16th World Congress of the International Society for Fat Research, October 4–7, 1983, in Budapest, Hungary, and at the American Oil Chemists' Society Meeting, May 15, 1986, in Honolulu, Hawaii.  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