全文获取类型
收费全文 | 4964篇 |
免费 | 97篇 |
国内免费 | 7篇 |
专业分类
电工技术 | 46篇 |
综合类 | 2篇 |
化学工业 | 935篇 |
金属工艺 | 66篇 |
机械仪表 | 102篇 |
建筑科学 | 217篇 |
矿业工程 | 16篇 |
能源动力 | 142篇 |
轻工业 | 351篇 |
水利工程 | 48篇 |
石油天然气 | 31篇 |
武器工业 | 1篇 |
无线电 | 285篇 |
一般工业技术 | 735篇 |
冶金工业 | 1399篇 |
原子能技术 | 53篇 |
自动化技术 | 639篇 |
出版年
2021年 | 45篇 |
2020年 | 38篇 |
2019年 | 56篇 |
2018年 | 62篇 |
2017年 | 67篇 |
2016年 | 57篇 |
2015年 | 58篇 |
2014年 | 75篇 |
2013年 | 275篇 |
2012年 | 172篇 |
2011年 | 240篇 |
2010年 | 145篇 |
2009年 | 184篇 |
2008年 | 221篇 |
2007年 | 212篇 |
2006年 | 214篇 |
2005年 | 188篇 |
2004年 | 148篇 |
2003年 | 141篇 |
2002年 | 125篇 |
2001年 | 76篇 |
2000年 | 102篇 |
1999年 | 91篇 |
1998年 | 87篇 |
1997年 | 85篇 |
1996年 | 77篇 |
1995年 | 88篇 |
1994年 | 83篇 |
1993年 | 93篇 |
1992年 | 91篇 |
1991年 | 65篇 |
1990年 | 70篇 |
1989年 | 67篇 |
1988年 | 63篇 |
1987年 | 83篇 |
1986年 | 80篇 |
1985年 | 88篇 |
1984年 | 71篇 |
1983年 | 71篇 |
1982年 | 74篇 |
1981年 | 67篇 |
1980年 | 50篇 |
1979年 | 52篇 |
1978年 | 62篇 |
1977年 | 55篇 |
1976年 | 50篇 |
1975年 | 57篇 |
1974年 | 38篇 |
1973年 | 51篇 |
1972年 | 35篇 |
排序方式: 共有5068条查询结果,搜索用时 0 毫秒
41.
Properties and compound types were examined in detail for tar-sand bitumens representative of four major deposits in Utah and Alberta. Methods for bitumen extraction, separation, and compound type (functional group) analysis are presented. Results of the analyses are compared with each other and with results in the literature for tar-sand bitumens and petroleum. Comparison of the results of Utah bitumens with the Athabasca bitumen provides a basis for evaluation of the Utah bitumens because considerable information about the processing characteristics of the Athabasca bitumen has been reported. Simulated distillation and Chromatographic separation data indicate that, in general, the tarsand bitumens most closely resemble Wilmington petroleum with respect to hydrocarbon/non-hydrocarbon distribution, especially in the non-distilling portion (the distillable portions, mainly hydrocarbons, are similar for all samples). Tar-sand bitumens were also shown to differ significantly from one another, principally in the amounts and kinds of heteroatomic compound types present. This information is important because of the effects that composition has on recovery and refining processes. Analyses of the chemical functionalities also provide a basis for developing correlations between bitumen composition and the recovery and processing characteristics of the bitumen. 相似文献
42.
Kenneth L. Stevens 《Journal of chemical ecology》1986,12(6):1205-1211
The allelopathic weed Russian knapweed (Centaurea repens) was found to contain polyacetylenes VIII–XIV in the roots. Dose vs. response of the root length elongation against lettuce, alfalfa, barnyard grass, and red millet showed IX to be active. Closely related isomers were not active. Examination of the soil surrounding the Russian knapweed roots revealed the presence of IX in sufficient concentration to have an appreciable effect on the surrounding plant community. 相似文献
43.
Recent laboratory research has indicated that coal fly ash derived from subbituminous and bituminous type coals is a sink for atmospheric mercury (Hg), however lignite-based ash was found to emit Hg to the air. Solids collected from systems with components that enhance Hg removal (i.e. activated carbon injection (ACI), flue gas desulfurization (FGD), and selective catalytic reduction (SCR) or selective non-catalytic reduction (SNCR)) may have higher Hg concentrations and therefore a higher potential for Hg release. For this study we investigated the potential for Hg release to the air and water from coal combustion products (CCPs) collected from coal-fired units with FGD equipment, SCR and SNCR equipment, and sorbent injection for Hg removal. In the laboratory study, most dry samples acted as sinks for atmospheric Hg in the dark at 25 °C. When exposed to light or increased temperature (45 °C), deposition of Hg to the fly ash substrates in most cases continued but decreased. Wet FGD samples emitted Hg. However, they became a sink for atmospheric Hg or exhibited low Hg emission rates when dried. Mercury flux in the dark at 25 °C was correlated with fly ash carbon content (LOI). Most liquid extracts derived using the synthetic precipitation leaching procedure (SPLP EPA method 1312) had very low Hg concentrations (<13 ng/l). 相似文献
44.
A new method has been devised to propagate stable subcritical mixed mode I–mode III cracks in soda–lime–silicate and borosilicate glasses. The mode I content was measured using an original optical tunneling technique and mode III content via multiple-beam interferometry. Measurement of the mode III displacement field allows values of mode III fracture toughness, KIII , to be derived. KIII vs crack velocity (v) diagrams have been constructed for both glasses, and, in the case of the soda–lime–silicate glass, v vs KIII relationships have been determined for a variety of humidities. Comparison has been made with published KI vs v data for this glass. 相似文献
45.
Liquid—liquid thermally induced phase separation of the polymer—diluent system isotactic poly(propylene)—diphenyl ether was studied under an optical microscope. It was found that as the system phase separated, droplets of a diluent-rich phase formed within a polymer-rich matrix. These droplets grew in size and decreased in number by a process known as coarsening. The scaling exponent relating droplet diameter and time was determined. The scaling exponent was found to be a strong function of the droplet phase volume fraction, and this strong dependence on volume fraction was relatively insensitive to temperature. The influence of the ratio of interfacial tension between the phases and the viscosity of the matrix phase was less significant on the growth rate of the droplets. 相似文献
46.
Ruan Ke-He; Milfeld Kent; Kulmacz Richard J.; Wu Kenneth K. 《Protein engineering, design & selection : PEDS》1994,7(11):1345-1351
A 3-D model of human thromboxane A2 synthase (TXAS) was constructedusing a homology modeling approach based on information fromthe 2.0 crystal structure of the hemoprotein domains of cytochromeP450BM-3 and P450cam. P450BM-3 is a bacterial fatty acid monooxygenaseresembling eukaryotic microsomal cytochrome P450s in primarystructure and function. TXAS shares 26.4% residue identity and48.4% residue similarity with the P450BM-3 hemoprotein domain.The homology score between TXAS and P450BM-3 is much higherthan that between TXAS and P450cam. Alignment between TXAS andthe P450BM-3 hemoprotein domain or P450cam was determined throughsequence searches. The P450BM-3 or P450cam main-chain coordinateswere spplied to the TXAS main chain in those sements where thetwo sequences were well aligned. These segments were linkedto one another using a fragment search method, and the sidechains were added to produce a 3-D model for TXAS. A TXAS substrate,prostaglandin H2 (PGH2) was docked into the TXAS cavity correspondingto the arachidonic acid binding pocket in P450BM-3 or camphorbinding site in P450cam. Regions of the heme and putative PGH2binding cavities in the TXAS model were identified and analyzed.The segments and residues involved in the active-site pocketof the TXAS model provide reasonable candidates for TXAS proteinengineering and inhibitor design. Comparison of the TXAS modelbased on P450BM-3 with another TXAS model based on the P450BM-3with another TXAS model based on the P450cam structure indicatedthat P450BM-3 is a more suitable template for homology modelingof TXAS. 相似文献
47.
A mathematical model was developed to describe the high-speed melt-spinning behavior crystallizable polymers. This model included the effects of acceleration, gravity, and air friction on the kinematics of the process; temperature and molecular orientation on the crystallization kinetics of the polymer; and temperature, molecular weight, and crystallinity on the elongational viscosity of the material. Experimental on-line diameter, birefringence, and temperature profiles were obtained for a 12,000 Mn nylon-66 at 2.5 g/min spun at take-up speeds ranging from 2800 to 6600 m/min. These profiles were qualitatively and reasonably quantitatively in agreement with the predicted profiles. They indicated that orientation induced crystallization occurs at spinning speeds greater than 4000 m/min. The experimental diameter and birefringence profiles were compared to those predicted by the model using Avrami indices of 3, 2, and 1. There was a small increase in the crystalline index at the lower speeds with decreasing index. The effect of the strain hardening was more significant at the higher speeds, this being shown by decreasing the exponent in the relationship for the crystallinity on the elongational viscosity. The model developed in this study indicates that high spinning speeds provide the high stress environment that increases the molecular orientation within the fiber. It is this higher molecular orientation that is the driving force for rapid crystallization on the spinline. This rapid crystallization causes a strain hardening, preventing any further drawdown in the fiber diameter and an abrupt rise in the birefringence. This behavior closely corresponds to the observed spinline profiles. 相似文献
48.
Small quantities (<2%) of various metal oxides are incorporated into the lattice of β-C2S during the high temperature preparation. The effect which these stabilizers have on the hydration rate is examined. A model involving the charge/radius ratio of the metal ion is developed to explain altered hydration rates. This charge/ radius ratio model also proves useful in describing the crystal-chemical stabilization of β-C2S. In addition, the solubility of Cr2O3 in β-C2S is determined by a novel differential thermal analysis technique. 相似文献
49.
Zheng Ya-Jun; Merz Kenneth M. Jr; Farber Gregory K. 《Protein engineering, design & selection : PEDS》1993,6(5):479-484
Two mechanisms for an aldoseketose isomerization havebeen examined using high level ab initio and semiempirical molecularorbital methods. The proton transfer pathway via an enediolintermediate is shown to be favored in the absence of a metalion, while the hydride transfer pathway becomes favored in thepresence of a metal ion. Our calculations explain why the protontransfer pathway is operative in most aldoseketose isomerizationreactions. These calculations also provide further support forthe previously proposed metal ion-mediated hydride transfermechanism for xylose isomerase. 相似文献
50.
Gordon B. A. Lim Kenneth S. McGuire Douglas R. Lloyd 《Polymer Engineering and Science》1993,33(9):537-542
Non-isothermal crystallization growth rates of nucleated and non-nucleated isotactic polypropylene (iPP) in dotriacontane were determined experimentally by thermal optical microscopy. Adipic acid was used as the nucleating agent. The non-isothermal growth rates of the nucleated and non-nucleated systems were compared with experimentally determined isothermal growth rates. The Lauritzen and Hoffman growth rate equation, originally developed for isothermal crystallization, was modified to describe the non-isothermal growth. The modified Lauritzen-Hoffman equation was used to predict isothermal growth rates from non-isothermal crystallization for the nucleated and non-nucleated polymer-diluent mixtures. This study is the first to deal with polymer-diluent-nucleating agent systems vital to membrane production. 相似文献