全文获取类型
收费全文 | 2789篇 |
免费 | 54篇 |
国内免费 | 12篇 |
专业分类
电工技术 | 45篇 |
综合类 | 3篇 |
化学工业 | 558篇 |
金属工艺 | 162篇 |
机械仪表 | 71篇 |
建筑科学 | 148篇 |
矿业工程 | 10篇 |
能源动力 | 98篇 |
轻工业 | 226篇 |
水利工程 | 26篇 |
石油天然气 | 62篇 |
武器工业 | 1篇 |
无线电 | 207篇 |
一般工业技术 | 691篇 |
冶金工业 | 214篇 |
原子能技术 | 80篇 |
自动化技术 | 253篇 |
出版年
2021年 | 30篇 |
2020年 | 21篇 |
2019年 | 23篇 |
2018年 | 66篇 |
2017年 | 53篇 |
2016年 | 55篇 |
2015年 | 41篇 |
2014年 | 45篇 |
2013年 | 105篇 |
2012年 | 103篇 |
2011年 | 160篇 |
2010年 | 131篇 |
2009年 | 131篇 |
2008年 | 94篇 |
2007年 | 113篇 |
2006年 | 113篇 |
2005年 | 64篇 |
2004年 | 64篇 |
2003年 | 84篇 |
2002年 | 79篇 |
2001年 | 90篇 |
2000年 | 60篇 |
1999年 | 49篇 |
1998年 | 100篇 |
1997年 | 92篇 |
1996年 | 60篇 |
1995年 | 50篇 |
1994年 | 50篇 |
1993年 | 55篇 |
1992年 | 39篇 |
1991年 | 42篇 |
1990年 | 30篇 |
1989年 | 36篇 |
1988年 | 34篇 |
1987年 | 38篇 |
1986年 | 44篇 |
1985年 | 32篇 |
1984年 | 35篇 |
1983年 | 14篇 |
1982年 | 18篇 |
1981年 | 26篇 |
1980年 | 15篇 |
1979年 | 21篇 |
1978年 | 14篇 |
1977年 | 21篇 |
1976年 | 32篇 |
1975年 | 14篇 |
1974年 | 18篇 |
1973年 | 16篇 |
1971年 | 13篇 |
排序方式: 共有2855条查询结果,搜索用时 218 毫秒
141.
We address the problem of image similarity in the compressed domain, using a multivariate statistical test for comparing color distributions. Our approach is based on the multivariate Wald-Wolfowitz test, a nonparametric test that assesses the commonality between two different sets of multivariate observations. Using some pre-selected feature attributes, the similarity measure provides a comprehensive estimate of the match between different images based on graph theory and the notion of minimal spanning tree (MST). Feature extraction is directly provided from the JPEG discrete cosine transform (DCT) domain, without involving full decompression or inverse DCT. Based on the zig-zag scheme, a novel selection technique is introduced that guarantees image's enhanced invariance to geometric transformations. To demonstrate the performance of the proposed method, the application on a diverse collection of images has been systematically studied in a query-by-example image retrieval task. Experimental results show that a powerful measure of similarity between compressed images can emerge from the statistical comparison of their pattern representations. 相似文献
142.
Th. Zahner Ch. Stoiber E. Zepezauer H. Lengfellner 《Journal of Infrared, Millimeter and Terahertz Waves》1999,20(6):1103-1111
Thermoelectricity is investigated in a material designed to show anisotropic transport properties: Copper/constantan/copper $ \cdot \cdot \cdot $ multilayer structures were prepared by sintering of a compressed stack of alternating foils of these materials and are described by effective “in-plane” properties $S_\parallel $ and $k_\parallel $ for the Seebeck-coefficient and the thermal conductivity along the layers, and “out-of-plane”-properties $S_ \bot $ and $k_ \bot $ along the stack axis. Samples in form of thin slabs prepared by cutting the stack obliquely to the stack axis showed thermoelectric fields transverse to temperature gradients across the slab due to off-diagonal elements in the Seebeck-tensor, and may be used as devices for detection of laser radiation. 相似文献
143.
J. Reyes-Gasga R. García G O. Alvarez-Fregoso J. A. Chávez-Carvayar L. E. Vargas-Ulloa 《Journal of Materials Science》1999,34(9):2183-2188
When human tooth enamel is heated either in vacuum or air it presents drastic changes in electrical susceptibility, conductivity and structural properties. In this paper we report an insulator-conductive transition which is observed in air around 350°C where enamel conductivity changes drastically and its electrical resistance decreases from 1015 to 105 that is, it goes from an insulator to a super-ionic ceramic behavior. This transition, first evidenced in vacuum by electron microscopy observations, is now completely determined by a.c. impedance spectroscopy technique and its characterization was carried out as a function of the frequency and temperature. X-ray in situ heating diffraction experiments show that there is no structural phase transition during a wide range of temperature including the one where the conductivity transition occurs. 相似文献
144.
Thermally induced chain-scission and redistribution reactions in as-polymerized and fractionated bisphenol-A polycarbonate (PC) materials made by melt-transesterification and interfacial polymerization were studied at temperatures between 200 and 300 °C in open systems, where volatile reaction products are continuously removed, and in closed systems, where these products are retained. Under the applied conditions, PC made via interfacial polymerization shows no measurable susceptibility towards redistribution. This is the result of an extremely low concentration of hydroxyl end-groups. Upon similar thermal treatment, PC made by melt-transesterification undergoes fast redistribution which leads to post-condensation in open systems and strong changes of molecular weight distributions (MWDs) for fractions in all cases. Redistribution effects were visualized through changes in the MWD of mixed fractions. The initial stages of redistribution were simulated using a Monte Carlo method based on a random sampling technique. From comparison between experimental and simulation results, it can be concluded that approximately half a percent of all carbonate linkages is involved in a redistribution reaction per minute at 250 °C for the studied samples. 相似文献
145.
Transferability of the properties of fatigue in the local approach This article deals with the uncertainty of lifetime prediction. There are to reasons for still unsatisfying calculation results, known as transferability problems. The first appears when using strength under constant amplitude loading for the prediction of operational loaded structures. The second transferability problem exists only in the local strain approach where the strength of the unnotched small specimen is assumed to be valid for the real component as well. This difficulty is dealt with by separating the independent influences and its individual consideration. The microstructural mechanics influence is derived by a statistical evaluation of experimental data. The proposed concept for regarding the microstructural support effect allows o derive a strain‐life‐curve for an individual component. The results of verification look promising. 相似文献
146.
147.
John‐Jules Ch. Meyer 《国际智能系统杂志》2006,21(6):601-619
In this article we discuss the role of emotions in artificial agent design, and the use of logic in reasoning about the emotional or affective states an agent can reside in. We do so by extending the KARO framework for reasoning about rational agents appropriately. In particular, we formalize in this framework how emotions are related to the action monitoring capabilities of an agent. © 2006 Wiley Periodicals, Inc. Int J Int Syst 21: 601–619, 2006. 相似文献
148.
Seasonal affective disorder (SAD) is characterized by annual major depressive episodes. It occurs most commonly in young women during autumn and winter with full remission during the following spring. The patient's mood is a combination of depression and anxiety accompanied by fatigue, loss of libido, and a reduction of socialization. Most of these patients complain of atypical vegetative symptoms (e.g. hypersomnia, carbohydrate craving, and weight gain). Hypotheses on the underlying mechanisms of these behavioural disorders indicate that environmental variables, e.g. climate, latitude, light, and changes in neurotransmitter function that occur naturally with the seasons, may be important. Phototherapy is being used increasingly for the treatment of SAD. The antidepressant response is contingent on the exposure of the patients' eyes to light. The biological basis of the diverse psychological and biological changes in SAD and the underlying mechanism of action of phototherapy are still unclear and require further study. 相似文献
149.
Rypniewski W.R.; Hastrup S.; Betzel Ch.; Dauter M.; Dauter Z.; Papendorf G.; Branner S.; Wilson K.S. 《Protein engineering, design & selection : PEDS》1993,6(4):341-348
The trypsin from Fusarium oxysporum is equally homologous totrypsins from Streptomyces griseus, Streptomyces erythraeusand to bovine trypsin. A DFP (diisopropylfluorophosphate) inhibitedform of the enzyme has been crystallized from 1.4 M Na2SO4,buffered with citrate at pH 5.05.5. The crystals belongto space group P21 with cell parameters a=33.43 Å, b=67.65Å, c=39.85 Å and ß=107.6°. There isone protein molecule in the asymmetric unit. X-ray diffractiondata to a resolution of 1.8 Å were collected on film usingsynchrotron radiation. The structure was solved by molecularreplacement using models of bovine and S.griseus trypsins andrefined to an R-factor of 0.141. The overall fold is similarto other trypsins, with some insertions and deletions. Thereis no evidence of the divalent cation binding sites seen inother trypsins. The covalently bound inhibitor molecule is clearlyvisible. 相似文献
150.
At 225℃. caprolactone has been polymerized in the presence of succinic acid under dry nitrogen atmosphere. Characterizations of the polymer through IR and molecular weight measurements by 1H-NMR and end group titration have shown that the polycaprolactone obtained is of two carboxyl end groups. The molecular weight of it increases with decreasing of the acid content in the reaction mixture under the same polymerization conditions. With a certain ratio of acid to caprolactone. the maximum of molecular weight of the polymer will be reached at the reaction time of 3h. 相似文献