全文获取类型
收费全文 | 316481篇 |
免费 | 3804篇 |
国内免费 | 723篇 |
专业分类
电工技术 | 6248篇 |
综合类 | 562篇 |
化学工业 | 47495篇 |
金属工艺 | 10395篇 |
机械仪表 | 9038篇 |
建筑科学 | 8074篇 |
矿业工程 | 886篇 |
能源动力 | 8675篇 |
轻工业 | 31711篇 |
水利工程 | 2622篇 |
石油天然气 | 3680篇 |
武器工业 | 8篇 |
无线电 | 40212篇 |
一般工业技术 | 59177篇 |
冶金工业 | 59402篇 |
原子能技术 | 5521篇 |
自动化技术 | 27302篇 |
出版年
2021年 | 2327篇 |
2019年 | 2188篇 |
2018年 | 3652篇 |
2017年 | 3515篇 |
2016年 | 3590篇 |
2015年 | 2580篇 |
2014年 | 4523篇 |
2013年 | 14528篇 |
2012年 | 7485篇 |
2011年 | 10285篇 |
2010年 | 8071篇 |
2009年 | 9313篇 |
2008年 | 9805篇 |
2007年 | 9759篇 |
2006年 | 8685篇 |
2005年 | 8038篇 |
2004年 | 7885篇 |
2003年 | 7632篇 |
2002年 | 7485篇 |
2001年 | 7660篇 |
2000年 | 7310篇 |
1999年 | 7705篇 |
1998年 | 18289篇 |
1997年 | 13233篇 |
1996年 | 10467篇 |
1995年 | 8124篇 |
1994年 | 7416篇 |
1993年 | 7094篇 |
1992年 | 5374篇 |
1991年 | 5155篇 |
1990年 | 5030篇 |
1989年 | 4883篇 |
1988年 | 4774篇 |
1987年 | 3979篇 |
1986年 | 4086篇 |
1985年 | 4817篇 |
1984年 | 4338篇 |
1983年 | 4093篇 |
1982年 | 3658篇 |
1981年 | 3826篇 |
1980年 | 3516篇 |
1979年 | 3480篇 |
1978年 | 3279篇 |
1977年 | 3904篇 |
1976年 | 5016篇 |
1975年 | 2840篇 |
1974年 | 2703篇 |
1973年 | 2723篇 |
1972年 | 2252篇 |
1971年 | 1998篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
941.
In this work, we co‐formulated an oil‐borne copper naphthenate/permethrin wood preservative system with synthetic polymer‐based fire‐retardant additives prior to the impregnation of Pinus radiata sapwood. We evaluated what effect, if any, the preservative had upon the fire performance properties of the fire retardants and whether the fire retardants impacted on the fungicidal and termiticidal efficacy of the preservative. The fire retardants included halogenated and phosphorus‐based systems. A mass loss calorimeter, in conjunction with a thermopile, was used to measure the time to ignition and the peak heat release rate (PHRR) from which the fire performance index (FPI) was determined. The preservative properties were evaluated using termite and soil‐block decay bioassays. In summary, we found that the rate of fire growth was reduced when the fire retardants were used in combination with the wood preservative. We also found that the PHRR was a better determinant of fire performance than the FPI. The performance of the wood preservative was enhanced against fungal decay and termite attack when used in combination with the fire retardants. The fire retardants also demonstrated some wood preservative properties of their own. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
942.
Wax esters enriched in ω-3 fatty acids have been recently shown to be readily absorbed by rats after dietary supplementation. Wax esters are less prone to oxidation and can be better formulated than liquid ω-3 derivatives. All these characteristics suggest a possible use of wax esters enriched in ω-3 as food supplements. However, some naturally occurring wax esters are defined as scarcely digestible (jojoba oil, spermaceti). On the other hand, wax esters have been found in several sources of marine origin, which can enter normal foodstuff. In order to better understand the digestibility of wax esters, we have analysed some foods from marine sources. The content of triglycerides and wax esters of the foods has been determined. Moreover, wax esters have been analysed for their composition in alcohols and fatty acids. Wax esters have been shown to be a part of the neutral lipid extract of the foods analysed. Their content, if compared with the content in triglycerides, shows striking differences. In the case of two botargo preparations, it has been shown that wax esters are more than 90% of the total neutral lipid extract. The content of fatty acids and alcohols found in the foods analysed is consistent with published data. 相似文献
943.
Current understanding of the mechanisms involved in osseointegration following implantation of a biomaterial has led to adhesion quantification being implemented as an assay of cytocompatibility. Such measurement can be hindered by intra-sample variation owing to morphological changes associated with the cell cycle. Here we report on a new scanning electron microscopical method for the simultaneous immunogold labelling of cellular focal adhesions and S-phase nuclei identified by BrdU incorporation. Prior to labelling, cellular membranes are removed by tritonization and antigens of non-interest blocked by serum incubation. Adhesion plaque–associated vinculin and S-phase nuclei were both separately labelled with a 1.4 nm gold colloid and visualized by subsequent colloid enhancement via silver deposition. This study is specifically concerned with the effects microgroove topographies have on adhesion formation in S-phase osteoblasts. By combining backscattered electron (BSE) imaging with secondary electron (SE) imaging it was possible to visualize S-phase nuclei and the immunogold-labelled adhesion sites in one energy 'plane' and the underlying nanotopography in another. Osteoblast adhesion to these nanotopographies was ascertained by quantification of adhesion complex formation. 相似文献
944.
R. M. Hwang K. C. Cha 《The International Journal of Advanced Manufacturing Technology》2008,37(11-12):1093-1104
Learning and prediction capability of the backpropagation neural network (BPNN) have been used to build the prediction model for the structural stability of a surface grinder. The Lagrange energy method is applied to derive the dynamic equation of the lumped parameter model of the surface grinder. The major factors influencing the structural stability of the system can be determined after the ratio of kinetic energy of the sub-structure and the ratio of potential energy of the sub-structure interface are obtained. An orthogonal rotatable central composite design is adopted to dispose the treatment combinations of the major factors. The BPNN model is constructed by the treatment combinations of the training patterns and verified by the treatment combinations of the test patterns. In this paper, a 3-layer BPNN model with a 10-neuron hidden layer which converged after 4,072 learning cycles is selected to predict the structural stability of a surface grinder within the planned ranges. The percentage residuals of both training patterns and test patterns are all within 3.41%, thus the prediction accuracy of the BPNN model is excellent so that the engineering demands are well satisfied. 相似文献
945.
Carbon based multilayer systems for highly loaded forming tools Amorphous hydrogenated carbon (metal‐free a‐C:H and metal‐containing a‐C:H:Me) films respond very sensitively to local overloads. For example during forming tool operations, hard abrasive particles and locally high stresses on the coating surface can cause crack initiation and early coating failure. Compared to the high hardness, wear resistance and excellent friction properties, in many cases the adhesion of a‐C:H films is relatively insufficient. Adhesion and overload resistance of a‐C:H and a‐C:H:Me, prepared by reactive sputtering, can be influenced in a wide range by different interlayer systems. In the present report the wear mechanism of amorphous carbon coatings and the influence of different metallic, metal nitride and metal carbide interlayers on the growth structure, the adhesion and the load resistance will be reported. Two well adapted multi‐coating systems, successfully tested for highly loaded tools and components, will be presented. 相似文献
946.
Multimedia signal processing is more than simply “putting together” text, audio, images, and video. It is the integration and interaction among these different media that creates new systems and new research challenges and opportunities. In multimodal communication where speech is involved, audio-visual interaction is particularly significant 相似文献
947.
Capital cost correlations are given for oil-water separators, equalization basins, primary clarifiers, clarifiers and clarifier mechanisms, reverse osmosis unit, ultrafiltration units, gravity filters and miroscreens. Data are included for raw sewage, intermediate and recirculation pumping stations and for preliminary treatment (or bar screens, grit removal, overflow and bypass chamber and Parshall flume), for grit removal, comminution and gas stripping. The correlations are accurate to ± 30%, expressed in Canadian dollars and adjusted to a Marshall and Swift value of 1000. Details as to what components are included in the cost are given. Data are given for the labour and material costs to create a working process module. For inflation indices, a critique of the Engineering News Record, EPA small city, Marshall and Swift, Chemical Engineering and Southam inflation indices suggested that the Marshall and Swift index is most appropriate for the construction and process equipment studied here. Concerning the currency exchange, for the equipment and processes in this study, equipment costs the same whether expressed directly in US or Canadian dollars. To put the current data into perspective, several hundred sources of cost data were analyzed. The major correlations are compared, on a consistent basis, with current data. 相似文献
948.
Accumulating evidence suggests that angiotensin-(1-7)(Ang-(1-7)) is an important component of the renin-angiotensin system and that the actions of the peptide may either contribute to or oppose those of Ang II. Ang-(1-7) can be converted directly from Ang I bypassing prerequisite formation of Ang II. Formation of Ang-(1-7) is under the control of at least three endopeptidases depending on the tissue compartment and include neprilysin, thimet oligopeptidase and prolyl oligopeptidase. Both neprilysin and thimet oligopeptidase are also involved in the metabolism of bradykinin and the atrial natriuretic peptide. Moreover, recent studies suggest that in addition to Ang I and bradykinin, Ang-(1-7) is an endogenous substrate for angiotensin converting enzyme. These enzymatic pathways may contribute to a complex relationship between the hypertensive actions of Ang II and various vasodepressor peptides from either the renin-angiotensin system or other peptide systems. Ang-(1-7) is devoid of the vasoconstrictor, central pressor, or thirst-stimulating actions associated with Ang II. In fact, new findings reveal depressor, vasodilator, and antihypertensive actions that may be more apparent in hypertensive animals or humans. Thus, Ang-(1-7) may oppose the actions of Ang II directly or as a result of increasing prostaglandins or nitric oxide. In this review, we examine the mechanisms by which Ang-(1-7) may contribute to cardiovascular regulation. 相似文献
949.
An unconditionally stable two-stage CMOS amplifier 总被引:1,自引:0,他引:1
This paper describes a two-stage CMOS amplifier that is stable for any capacitive load. This is achieved through the use of an optimized cascoded compensation topology. A new level shifting technique allows independent optimization of drive capability, noise and systematic offset voltage. The circuit is 0.1 mm2 in a 2 μm technology and has a quiescent current consumption of 110 μA 相似文献
950.
GS Wengler A Lanfranchi T Frusca R Verardi A Neva D Brugnoni S Giliani M Fiorini P Mella F Guandalini E Mazzolari S Pecorelli LD Notarangelo F Porta AG Ugazio 《Canadian Metallurgical Quarterly》1996,348(9040):1484-1487
BACKGROUND: X-linked severe combined immunodeficiency (SCIDXI) is an inherited immune defect which leads to death in infancy from severe infections. The defect is caused by mutations of the IL-2RG gene that encodes for the common gamma chain shared by several cytokine receptors. The disease is characterised by lack of T and NK cells with normal numbers of B cells. SCIDXI can be cured by bone marrow transplantation (BMT) or prevented by abortion after prenatal diagnosis. METHODS: A male fetus was diagnosed as having SCIDXI by molecular, immunophenotypic, and functional analyses. The fetus was injected intraperitoneally under ultrasound guidance with CD34 haematopoietic progenitor cells purified from paternal bone marrow and T-cell depleted by E rosetting. Chimerism analysis was by HLA-DQ alpha typing and gamma-chain staining on cord blood. FINDINGS: A healthy 3.6 kg boy was delivered by caesarean section at 38 weeks of gestation with no clinical or laboratory signs of graft-versus-host disease. Engraftment of donor-derived CD2 cells was found at birth. At 3.5 months of age the infant is well and his T-cell counts and function are normal. INTERPRETATION: In-utero transplantation of haematopoietic progenitor cells allowed immune reconstitution of a fetus with SCIDXI and may be an alternative to elective abortion. Our report should encourage applications of this method to other inherited disorders curable by BMT. 相似文献