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   270740篇
  免费   10080篇
  国内免费   4993篇
电工技术   8815篇
技术理论   12篇
综合类   6600篇
化学工业   42810篇
金属工艺   12049篇
机械仪表   9918篇
建筑科学   9890篇
矿业工程   3032篇
能源动力   6213篇
轻工业   28258篇
水利工程   4035篇
石油天然气   7230篇
武器工业   838篇
无线电   29427篇
一般工业技术   45675篇
冶金工业   39817篇
原子能技术   5174篇
自动化技术   26020篇
  2023年   1871篇
  2022年   3507篇
  2021年   4978篇
  2020年   3944篇
  2019年   3740篇
  2018年   4553篇
  2017年   4904篇
  2016年   5000篇
  2015年   5165篇
  2014年   7317篇
  2013年   13319篇
  2012年   10880篇
  2011年   13272篇
  2010年   11021篇
  2009年   11120篇
  2008年   11792篇
  2007年   11541篇
  2006年   10138篇
  2005年   8650篇
  2004年   7196篇
  2003年   6509篇
  2002年   6524篇
  2001年   6367篇
  2000年   5659篇
  1999年   5421篇
  1998年   10687篇
  1997年   7955篇
  1996年   6089篇
  1995年   4603篇
  1994年   4207篇
  1993年   4123篇
  1992年   3261篇
  1991年   3055篇
  1990年   3090篇
  1989年   3021篇
  1988年   2833篇
  1987年   2421篇
  1986年   2444篇
  1985年   2709篇
  1984年   2633篇
  1983年   2463篇
  1982年   2160篇
  1981年   2342篇
  1980年   2108篇
  1979年   2313篇
  1978年   2116篇
  1977年   2237篇
  1976年   2944篇
  1975年   1958篇
  1973年   1862篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
从古村落民居空间看现代城市空间   总被引:1,自引:1,他引:1  
宋晶 《山西建筑》2006,32(23):36-37
以古村落宏村为例,从风水观念、宗族礼制两方面介绍了古代村落规划思想,探讨了民居建筑中的天井空间和街巷空间结构特点,对于现代城市空间的规划具有借鉴价值。  相似文献   
22.
The oxidation of protocatechuic acid with ozone in aqueous solution has been studied varying the temperature, pH, ozone partial pressure and initial acid concentration. A stoichiometric ratio of 2 mol of ozone consumed per mol of protocatechuic acid reacted is deduced from homogeneous experiments in a discontinuous tank reactor.In the ozonation experiments, the influence of the operating variables was established and the kinetic regime of ozone absorption was shown to be fast and pseudo m-order with respect to ozone, except at pH 2, when the reaction develops in the moderate kinetic regime. After applying a mass transfer with chemical reaction model, the reaction orders are deduced and the kinetic rate constants are determined as a function of pH and temperature. A modified Arrhenius expression is proposed to correlate them.  相似文献   
23.
Data from compartment tests of polyurethane foam seating assemblies are reported. Results show that the behavior of such assemblies during fires is strongly dependent on the type and position of interliners and upholstery fabrics. An incidental comparison of peak rate of heat release obtained in a compartment test with a predictive model equation is made and selected data from the tests are presented.  相似文献   
24.
Weak base phenol formaldehyde anion exchange resins were found to have an excellent capacity for removing the anionic organic species benzenesulfonate (BS), p-toluenesulfonate (PTS) and dodecylbenzenesulfonate (DBS) from pH 2 and 4 solutions. The capacity for DBS was generally greater than that of activated carbon and significantly greater than that of a polystyrene-divinylbenzene resin with no ion exchange functional groups. The maximum capacity of the weak base resins appeared to be limited by the exchange capacity of the resin. The mechanism of removal was strongly indicated as being either ion exchange at the amine functional groups in the acid form, or uptake of a proton by the amine groups in the free base form with the organic anion as the associated counter ion. Increasing concentrations of Cl decreased the amount of organic anion that could be sorbed.The swelling of one of the resins, Duolite A-7, also had a marked effect on its sorption capacity. The pores of the acid chloride or acid organic anion form of the resin were apparently larger than those of the free base form; thus, as conversion of the resin to the acid form took place, a greater amount of uptake was observed. The effect of swelling on the capacity of Duolite S-37 was much less, apparently owing to its larger pore structure, but its capacity for DBS at low pH was lower than that for A-7 because of its lower nitrogen content. Also, BS and DBS were easily removed from the weak base resins by washing them with NaOH. Four bed volumes of 1 m NaOH gave essentially complete regeneration of BS saturated A-7.  相似文献   
25.
The dynamics and optimal feedback control of biological waste treatment processes are examined. Modern feedback control theory is applied and results are obtained for both proportional and proportional plus integral control. A dynamic mathematical model of the system is developed by employing a Monod kinetic model, which is modified to account for endogenous metabolism, and a complete mixing flow model in the growth chamber. The feedback control is implemented via the flow rate so as to maintain desired effluent concentrations. External disturbances which cause changes in substrate concentration and/or cell concentration are considered and optimal control effort is applied. Results are also presented for impulse and step changes in the influent substrate concentration. It is found that by changing the values of the weighting factors in the objective function, it is possible to obtain good control of either the effluent substrate concentration or cell concentration.  相似文献   
26.
Toxicity bioassay of heavy metals in water using Tetrahymena pyriformis   总被引:1,自引:0,他引:1  
The toxicities of five heavy metal compounds on the survival of the ciliated protozoan Tetrahymena pyriformis were determined. Three-day old cultured cells were exposed to five concentrations of each test compound in one or more of three water series; distilled, soft, and hard water. A culture of each sample was placed in a plastic petri dish and covered with mineral oil to prevent evaporation. The cells were counted initially, then again at 0·5 h, 1 h, 2 h, 1 day, 2 days, 3 days, and on the fourth day. Lethal threshold concentrations and tolerance limit medians were determined by graphic methods. On a weight bases cadmium was most toxic, followed in decreasing toxicity by mercury, cobalt, zinc, and lead. Compared to similar fish data, all heavy metals were more toxic to T. pyriformis except lead. The toxicity of lead in soft versus hard water exemplified an antagonistic effect with greater than seven times the amount of lead necessary in hard water to produce comparable mortality as lead in soft water. On the other hand, the toxicity of mercury is about twice as great in hard water as in soft water (a synergistic effect). T. pyriformis appears to be a more sensitive indicator than fish of heavy metal contamination of water. T. pyriformis bioassay should prove a good means of determining the existence of many water pollutants.  相似文献   
27.
对于具有大面积玻璃围护结构的建筑中庭,由于太阳辐射强、室内空间大等问题,导致热环境的控制非常复杂。对于这类建筑的空调系统设计,计算流体力学(CFD)等数值模拟方法的应用有着广阔的发展前景,但已有的数值计算方法复杂,通用性差,仅能被少数计算流体力学领域专业研究人员所掌握,这一因素大大制约了数值模拟方法的在实际工程中的广泛应用。本文对相关数值模拟方法进行了分析和总结,建立了一套简单快速的并具有合理准确性的通用联合模拟方法,并开发出相应的联合模拟计算程序。通过将模拟计算结果与实验结果进行比较,该方法的准确性得到了验证,同时将该方法应用于中庭空调系统的优化设计,表明了其在实际工程应用中的可行性。  相似文献   
28.
Brominated flame retardants (BFRs) were investigated in juvenile common sole from nursery zones situated along the French coast in 2007, 2008 and 2009. Extensive identification was performed with regard to PBDEs, novel BFRs 1,2-bis(2,4,6-tribromophenoxy)ethane (BTBPE) and decabromodiphenylethane (DBDPE), and other non-PBDE BFRs, namely, hexabromobenzene (HBB) and 2,2′,4,4′,5,5′-hexabromobiphenyl (BB-153). Polybrominated diphenyl ether (PBDE) concentrations (Σ 14 congeners) ranged from 0.01 ng/g to 0.16 ng/g wet weight (ww) in muscle, and 0.07 ng/g to 2.8 ng/g ww in liver. Concentrations were in the lower range of those reported in the literature in other European locations. Lower PBDE concentrations, condition indices and lipid contents were observed in the Seine estuary in 2009, possibly in relation to a lower water flow. The PBDE patterns and ratios we observed suggested that juvenile sole have a relative high metabolic degradation capacity. Non-PBDE BFRs were detected at lower levels than PBDEs, i.e., within the < method detection limit - 0.005 ng/g ww range in muscle, and < method detection limit - 0.2 ng/g ww range in liver. The data obtained is of particular interest for the future monitoring of these compounds in the environment.  相似文献   
29.
L.F. Boswell  S.H. Zhang 《Thin》1983,1(4):353-383
The purpose of this paper is to present a formulation of a curved thin-walled box beam finite element having a variable cross-section with at least one vertical axis of symmetry.

To allow for the effects of warping and distortion, three degrees of freedom have been included in the formulation. These degrees of freedom have been designated as the rate of change of twisting angle, the distortional angle of the cross-section, and the rate of change of distortional angle. The effect of shear lag has also been included.

The element may be used for the elastic analysis of a variety of thin-walled structures and in particular for the preliminary analysis of box bridge decks where a three-dimensional analysis may be unnecessary. The accuracy of the element has been demonstrated by comparison of the results obtained with known results from other methods for some examples.  相似文献   

30.
An improved phenol-hypochlorite method for determining ammonia in water using nitroprusside as catalyst has been investigated. Unlike other recently published methods, the method is simple (and sufficiently sensitive for low-level analyses) and does not require accurate time intervals between reagents, not expensive equipment. The method is well suited for routine use.

Beer's Law was obeyed over the whole ammonia range investigated. Reproducibility was 1·3 per cent with 10 μg N, and 5·7 per cent with 1 μg N.  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