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   112248篇
  免费   8559篇
  国内免费   4247篇
电工技术   5979篇
技术理论   11篇
综合类   6670篇
化学工业   18968篇
金属工艺   5979篇
机械仪表   6802篇
建筑科学   8690篇
矿业工程   3119篇
能源动力   3137篇
轻工业   6758篇
水利工程   1735篇
石油天然气   6200篇
武器工业   738篇
无线电   14046篇
一般工业技术   14290篇
冶金工业   5789篇
原子能技术   1245篇
自动化技术   14898篇
  2024年   382篇
  2023年   1716篇
  2022年   2721篇
  2021年   4061篇
  2020年   3077篇
  2019年   2607篇
  2018年   2989篇
  2017年   3394篇
  2016年   2961篇
  2015年   3953篇
  2014年   4993篇
  2013年   6343篇
  2012年   6702篇
  2011年   7317篇
  2010年   6326篇
  2009年   6237篇
  2008年   6243篇
  2007年   5812篇
  2006年   6105篇
  2005年   5483篇
  2004年   3730篇
  2003年   3223篇
  2002年   2879篇
  2001年   2776篇
  2000年   2897篇
  1999年   3384篇
  1998年   2905篇
  1997年   2540篇
  1996年   2278篇
  1995年   1913篇
  1994年   1532篇
  1993年   1214篇
  1992年   959篇
  1991年   730篇
  1990年   590篇
  1989年   492篇
  1988年   381篇
  1987年   285篇
  1986年   195篇
  1985年   167篇
  1984年   93篇
  1983年   76篇
  1982年   86篇
  1981年   58篇
  1980年   46篇
  1979年   32篇
  1978年   25篇
  1977年   25篇
  1976年   40篇
  1973年   11篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
吴军  路春生 《安徽化工》2002,28(4):14-16
以有机锡(Fascat4102)作催化剂,苯酐和辛醇(2-乙基己醇)酯化反应生成邻苯二甲酸二辛酯(DOP),其最佳工艺条件为:酯化温度215~225℃,催化剂加入量2.0‰,过量醇含量17%,脱醇真空为20KPa(绝压),脱醇温度为155℃,活性炭加入量为1‰,省却中和水洗和汽提工序,产品质量可以达到优级品.  相似文献   
212.
影响模具使用寿命的因素固然很多,但采用合理的热处理工艺相当重要。我厂采用真空高压气淬热处理工艺取得了很好的效果,尤其是成型模具。经过处理后的模具无氧化脱碳,变形小,表面非常洁净光亮。模具比一般热处理具有更高的强度、耐磨性、韧性和疲劳强度,使用寿命提高2~3倍以上。真空高压气淬原理:工件在抽成一定真空度的炉内,在负压气氛条件下加热和保温后,充入高压惰性气体,通过大功率风机进行冷却淬火。具有如下特点:(1)无氧化脱碳。由于被抽成真空,炉内氧化性和脱碳性气体极少,可防止氧化、脱碳,起到保护作用。炉内氧…  相似文献   
213.
吴立新 《石油沥青》2002,16(1):46-51
以灌溉、防洪为主的峡口水库大坝,为沥青混凝土面板堆石坝,为研究低温抗裂性能,对沥青混凝土面板破损取样,进行了配比和多种力学性能试验,并与原面板设计指标进行了对比。由于面板经受住了-20℃以下的低温考验,其所用沥青指标、破损取样试验结果,特别是冻断低温指标,对确定水工沥青混凝土低温抗裂检验标准有重要参考价值。  相似文献   
214.
In this paper, the influence of viewing distance on subjective assessment of the impairment in video sequences is investigated. Subjective tests using the double-stimulus impairment scale variant II (DSIS II) method have been conducted at viewing distances of 5H (where H stands for the screen height) and 3H, respectively. Several statistical measures have been used to analyze the influence, including correlations and ANOVA (analysis of variance) tests. The results reveal that there is a very high correlation between the subjective scores, the variances are similar under the two viewing distances, the means of subjective data at these two viewing distances are the same, and there is no interaction between the viewing distance and the other two factors, i.e., the codec system and the source sequence. Throughout the tests, there is no evidence that a closer viewing distance such as 3H will vary the subjective test result statistically significantly.  相似文献   
215.
216.
针对武钢热轧厂层流冷却系统能耗高,水压不稳,系统反应慢,供水泵工作效率低等缺陷,进行系统改造。提高了系统的控制精度和产品质量,节能降耗取得了明显的效果。  相似文献   
217.
A series of Al-Cu-Ni alloys of various compositions were made and annealed at 800 °C. The equilibrium phases were studied by metallography, X-ray diffraction (XRD) analysis, and electron probe microanalysis. The isothermal section of the ternary Al-Cu-Ni system at 800 °C was then determined based on these experimental results and the available phase relationship knowledge of the three constituent binary systems. No ternary compound was found. All three phases, AlNi3, AlNi, and Al3Ni2, have very high ternary solubility, especially the AlNi phase, which almost reaches the binary Al-Cu side. However, no continuous solid solution was formed between the AlNi phase and any of the binary Al-Cu phases. Interfacial reactions of Al/Ni, Al/Cu, Al-Cu/Ni, and Al-Ni/Cu at 800 °C were investigated by using reaction couple techniques. The results showed that Al3Ni and Al3Ni2 phases were formed in the Al/Ni couples; β-AlCu4, γ 1-Al4Cu9, and ɛ 2-Al2Cu3 phases were formed in the Al/Cu couples. As for the results in the Al-2 at. pct Ni/Cu, Al-5 at. pct Ni/Cu, and Al-2 at. pct Cu/Ni, Al-4.5 at. pct Cu/Ni, and Al-6 at. pct Cu/Ni were similar to those in the binary Al/Cu and Al/Ni couples, respectively. A different reaction path was found in the Al-7.5 at. pct Cu/Ni couples, and an AlNi solid solution layer was formed instead of the Al3Ni and Al3Ni2 phases.  相似文献   
218.
Highly efficient orange and green emission from single‐layered solid‐state light‐emitting electrochemical cells based on cationic transition‐metal complexes [Ir(ppy)2sb]PF6 and [Ir(dFppy)2sb]PF6 (where ppy is 2‐phenylpyridine, dFppy is 2‐(2,4‐difluorophenyl)pyridine, and sb is 4,5‐diaza‐9,9′‐spirobifluorene) is reported. Photoluminescence measurements show highly retained quantum yields for [Ir(ppy)2sb]PF6 and [Ir(dFppy)2 sb]PF6 in neat films (compared with quantum yields of these complexes dispersed in m‐bis(N‐carbazolyl)benzene films). The spiroconfigured sb ligands effectively enhance the steric hindrance of the complexes and reduce the self‐quenching effect. The devices that use single‐layered neat films of [Ir(ppy)2sb]PF6 and [Ir(dFppy)2sb]PF6 achieve high peak external quantum efficiencies and power efficiencies of 7.1 % and 22.6 lm W–1) at 2.5 V, and 7.1 % and 26.2 lm W–1 at 2.8 V, respectively. These efficiencies are among the highest reported for solid‐state light‐emitting electrochemical cells, and indicate that cationic transition‐metal complexes containing ligands with good steric hindrance are excellent candidates for highly efficient solid‐state electrochemical cells.  相似文献   
219.
A three-phase model for mixed columnar-equiaxed solidification   总被引:1,自引:0,他引:1  
A three-phase model for mixed columnar-equiaxed solidification is presented in this article. The three phases are the parent melt as the primary phase, as well as the solidifying columnar dendrites and globular equiaxed grains as two different secondary phases. With an Eulerian approach, the three phases are considered as spatially coupled and interpenetrating continua. The conservation equations of mass, momentum, species, and enthalpy are solved for all three phases. An additional conservation equation for the number density of the equiaxed grains is defined and solved. Nucleation of the equiaxed grains, diffusion-controlled growth of both columnar and equiaxed phases, interphase exchanges, and interactions such as mass transfer during solidification, drag force, solute partitioning at the liquid/solid interface, and release of latent heat are taken into account. Binary steel ingots (Fe-0.34 wt pct C) with two-dimensional (2-D) axis symmetrical and three-dimensional (3-D) geometries as a benchmark were simulated. It is demonstrated that the model can be used to simulate the mixed columnar-equiaxed solidification, including melt convection and grain sedimentation, macrosegregation, columnar-to-equiaxed-transition (CET), and macrostructure distribution. The model was evaluated by comparing it to classical analytical models based on limited one-dimensional (1-D) cases. Satisfactory results were obtained. It is also shown that in order to apply this model for industrial castings, further improvements are still necessary concerning some details.  相似文献   
220.
Nitrification is a key step for reliable biological nitrogen removal. In order to enhance nitrification in the activated sludge (AS) process, membrane-attached biofilm (MAB) was incorporated in a conventional activated sludge tank. Simultaneous organic carbon removal and nitrification of the MAB incorporated activated sludge (AS + MAB) process was investigated with continuous wastewater treatment. The effluent TOC concentration of AS and the AS + MAB processes were about 6.3 mg/L and 7.9 mg/L, respectively. The TOC removal efficiency of both AS and AS + MAB were above 95% during the wastewater treatment, indicating excellent organic carbon removal performance in both processes. Little nitrification occurred in the AS process. On the contrary, successful nitrification was obtained with the AS + MAB process with nitrification efficiency of about 90%. The volumetric and surface nitrification rates were about 0.14 g/Ld and 6.5 g/m2d, respectively. The results clearly demonstrated that nitrification in the conventional AS process was boosted by MAB. Furthermore, the microfaunal population in the AS + MAB process was different from that in the AS process. The high concentration of rotifers in the AS + MAB process was expected to decrease the generation of excess sludge in the process.  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