全文获取类型
收费全文 | 1375篇 |
免费 | 44篇 |
国内免费 | 7篇 |
专业分类
电工技术 | 66篇 |
综合类 | 2篇 |
化学工业 | 241篇 |
金属工艺 | 30篇 |
机械仪表 | 36篇 |
建筑科学 | 27篇 |
矿业工程 | 1篇 |
能源动力 | 109篇 |
轻工业 | 102篇 |
水利工程 | 12篇 |
石油天然气 | 13篇 |
武器工业 | 1篇 |
无线电 | 140篇 |
一般工业技术 | 234篇 |
冶金工业 | 189篇 |
原子能技术 | 9篇 |
自动化技术 | 214篇 |
出版年
2024年 | 4篇 |
2023年 | 22篇 |
2022年 | 46篇 |
2021年 | 73篇 |
2020年 | 40篇 |
2019年 | 56篇 |
2018年 | 56篇 |
2017年 | 34篇 |
2016年 | 52篇 |
2015年 | 29篇 |
2014年 | 41篇 |
2013年 | 86篇 |
2012年 | 40篇 |
2011年 | 57篇 |
2010年 | 34篇 |
2009年 | 41篇 |
2008年 | 36篇 |
2007年 | 38篇 |
2006年 | 25篇 |
2005年 | 20篇 |
2004年 | 32篇 |
2003年 | 21篇 |
2002年 | 21篇 |
2001年 | 19篇 |
2000年 | 22篇 |
1999年 | 19篇 |
1998年 | 60篇 |
1997年 | 39篇 |
1996年 | 36篇 |
1995年 | 26篇 |
1994年 | 29篇 |
1993年 | 21篇 |
1992年 | 14篇 |
1991年 | 27篇 |
1990年 | 25篇 |
1989年 | 24篇 |
1988年 | 18篇 |
1987年 | 13篇 |
1986年 | 10篇 |
1985年 | 12篇 |
1984年 | 12篇 |
1983年 | 13篇 |
1982年 | 12篇 |
1981年 | 9篇 |
1979年 | 9篇 |
1978年 | 9篇 |
1976年 | 6篇 |
1973年 | 4篇 |
1972年 | 7篇 |
1966年 | 4篇 |
排序方式: 共有1426条查询结果,搜索用时 0 毫秒
141.
Acyl analogs of PAF are the major products synthesized during agonist stimulation of endothelial cells. We have previously shown that PAF: 1-acyl-2-lyso-sn-glycero-3-phosphocholine transacetylase in calf pulmonary artery endothelial cells is activated by ATP through protein phosphorylation, and the increase in transacetylase activity by ATP contributes to the biosynthesis of acyl analogs of PAF (J. Biol. Chem. 272, 17431-17437, 1997). To understand the mechanisms(s) by which ATP stimulates acyl analogs of PAF production, we have identified the subtypes of the purinergic receptor that are linked to the activation of two enzymes involved in the generation of acyl analogs of PAF, namely, transacetylase and phospholipase A2. Experiments with transient transfection of the cells with antisense and sense thio-oligonucleotide to cytosolic phospholipase A2 (cPLA2) were also performed to evaluate whether downstream activation of cPLA2 is involved in ATP-receptor mediated induction of arachidonate release and synthesis of radylacetyl-GPC. We found that the P2u/P2Y2 receptor, which recognizes a pyrimidine nucleotide, UTP, as well as purine nucleotides, shows a potency profile of UTP > ATP = ATP gamma S > 2-methylthio-ATP in mediating the activation of PAF: lysophospholipid transacetylase. On the other hand, ADP beta S and 2-methylthio-ATP have similar potencies as ATP but have lower potencies than UTP and ATP gamma S in stimulating the release of arachidonate. These results suggest that both P2u/P2Y2 and P2y/P2Y1 receptor subtypes promote arachidonate release. In addition, transient transfection of endothelial cells with cPLA2 antisense but not the sense thio-oligonucleotide inhibited the stimulation of arachidonate release and [3H]acetate incorporation into radyl[3H]acetyl-GPC. Thus, our data suggest that a receptor-mediated process is involved in the activation of transacetylase for the induced synthesis of acyl analogs of PAF in endothelial cells. Furthermore, it is likely that cPLA2 supplies the lysophospholipids as substrates for the transacetylation reaction. 相似文献
142.
A Amirkhosravi T Meyer G Warnes M Amaya Z Malik JP Biggerstaff FA Siddiqui P Sherman JL Francis 《Canadian Metallurgical Quarterly》1998,80(4):598-602
The aim of this study was to compare the performance of CT and MRI in the diagnosis of longitudinal stress fracture of the tibia (LSFT). A retrospective study of imaging findings was performed in 15 patients with LSFT. The CT and MR images were compared for detection of fracture line, callus, bone marrow edema, and soft tissues changes. The CT and MRI techniques allowed the detection of the fracture line in 82 and 73 % of cases, respectively. The callus was always visualized with CT or MRI. The MRI technique had a markedly higher sensitivity than CT in the detection of bone marrow edema (73 vs 18 %) and soft tissue lesions (87 vs 9 %). This may cause a misleading aggressive appearance on MRI. Computed tomography remains the best imaging modality for diagnosis of LSFT. However, MRI findings should be known to obviate the performance of CT or bone biopsy. 相似文献
143.
This article describes the results of an investigation concerning the failure of economizer tubes of a high-pressure boiler
in a dual-purpose power/water cogeneration plant. The failure was observed in the form of rupturing of one tube and a macrohole
or pinhole in another tube. The boiler had an operating period of 116,123 h since its inception. For approximately the first
100,000 h, the fuel for the boiler was crude oil, which was replaced by Bunker C oil. The boiler tube is fabricated from carbon
steel SA 210A1. The location of the failure was determined by on-site visual inspection of the boiler. Detailed macro- and
microexaminations of inner and outer scales on the tube were begun to determine the cause of the rupture. The composition
of the fire- and waterside scale and ash deposited on the outer surface of the tubes was analyzed by energy-dispersive x-ray
(EDX) technique. The reduction percentage of wall thickness of the tube facing inside and outside the furnace was calculated.
The cause of the failure of the economizer tube appears to be H2SO4 dew-point corrosion. The relatively low temperature of feedwater lowered the tube metal temperature and promoted the condensation
of H2SO4. The external deposits on the tubes, as a result of bunker oil firing, further helped to lower the tube metal temperature,
thus promoting H2SO4 condensation over the deposit and subsequent corrosion of the tube wall. Recommendations are given to prevent/minimize such
failures. 相似文献
144.
145.
146.
Malik R.J. Chand N. Nagle J. Ryan R.W. Alavi K. Cho A.Y. 《Electron Device Letters, IEEE》1992,13(11):557-559
Temperature-dependent measurements from 25 to 125°C have been made of the DC I -V characteristics of HBTs with GaAs and In0.53Ga0.47As collector regions. It was found that the GaAs HBTs have very low output conductance and high collector breakdown voltage BV CEO>10 V at 25°C, which increases with temperature. In striking contrast, the In0.53Ga0.47As HBTs have very high output conductance and low BV CEO~2.5 V at 25°C, which actually decreases with temperature. This different behavior is explained by the >104 higher collector leakage current, I CO, in In0.53Ga0.47As compared to GaAs due to bandgap differences. It is also shown that device self-heating plays a role in the I -V characteristics 相似文献
147.
Construction of linear voltage-controlled resistors and conductors using i.e. multipliers is demonstrated. Experimental results from a network of such elements corroborates the realisation of nonlinear amnesic driving-point and transfer curves by systematic synthesis. Applicability of linear network sensitivity calculations to such networks is experimentally verified. 相似文献
148.
149.
A novel type of AlGaAs/GaAs heterojunction bipolar transistor (HBT) which uses a two-dimensional (2-D) hole gas base formed by planar doping using molecular-beam epitaxy (MBE) has been demonstrated. The base consists of a submonolayer of Be atoms of sheet concentration 0.5-5×1013 cm-2 which is deposited during growth interruption by MBE. The transistor structure exhibits DC current gains up to 700. The effective base transit time is negligible in these transistors and it is postulated that very high-speed nonequilibrium transport may occur in the collector region 相似文献
150.
Solar irradiance data obtained from the Meteorological Department of Civil Aviation, Ministry of Communication, Brunei Darussalam for a period of 10 years was analysed to study the diffuse radiation over four districts, namely Brunei/Muara, Belait, Tutuong and Temburong. It was observed that including more than one measured variable improves the accuracy of the estimation of diffuse radiation. 相似文献