全文获取类型
收费全文 | 5080篇 |
免费 | 111篇 |
国内免费 | 5篇 |
专业分类
电工技术 | 45篇 |
综合类 | 4篇 |
化学工业 | 808篇 |
金属工艺 | 57篇 |
机械仪表 | 69篇 |
建筑科学 | 125篇 |
矿业工程 | 13篇 |
能源动力 | 79篇 |
轻工业 | 386篇 |
水利工程 | 26篇 |
石油天然气 | 26篇 |
无线电 | 491篇 |
一般工业技术 | 671篇 |
冶金工业 | 1801篇 |
原子能技术 | 55篇 |
自动化技术 | 540篇 |
出版年
2022年 | 37篇 |
2021年 | 50篇 |
2020年 | 32篇 |
2019年 | 33篇 |
2018年 | 57篇 |
2017年 | 40篇 |
2016年 | 46篇 |
2015年 | 36篇 |
2014年 | 92篇 |
2013年 | 208篇 |
2012年 | 140篇 |
2011年 | 136篇 |
2010年 | 127篇 |
2009年 | 135篇 |
2008年 | 158篇 |
2007年 | 159篇 |
2006年 | 134篇 |
2005年 | 137篇 |
2004年 | 119篇 |
2003年 | 128篇 |
2002年 | 106篇 |
2001年 | 94篇 |
2000年 | 98篇 |
1999年 | 122篇 |
1998年 | 462篇 |
1997年 | 266篇 |
1996年 | 218篇 |
1995年 | 152篇 |
1994年 | 145篇 |
1993年 | 152篇 |
1992年 | 90篇 |
1991年 | 60篇 |
1990年 | 77篇 |
1989年 | 65篇 |
1988年 | 70篇 |
1987年 | 59篇 |
1986年 | 63篇 |
1985年 | 76篇 |
1984年 | 64篇 |
1983年 | 76篇 |
1982年 | 57篇 |
1981年 | 53篇 |
1980年 | 56篇 |
1979年 | 49篇 |
1978年 | 45篇 |
1977年 | 64篇 |
1976年 | 86篇 |
1975年 | 39篇 |
1973年 | 33篇 |
1972年 | 27篇 |
排序方式: 共有5196条查询结果,搜索用时 0 毫秒
31.
Etchable thick-film multi-chip-module (MCM) technology has led to the possibility of fabricating microwave integrated circuits (MICs) with performance similar to MICs produced using more expensive conventional thin-film MCM-D techniques. However, little data is available on the loss characteristics of the technology at microwave frequencies. This paper describes an experimental investigation into the loss properties of high-definition etchable thick-film MCM microstrip lines formed on a variety of high dielectric constant (high-/spl epsiv//sub r/) ceramic substrates. Substrates investigated comprise 96% alumina (/spl epsiv//sub r/=9.5), (Zr,Sn)TiO/sub 4/(/spl epsiv//sub r/=36.6) and BaO-PbO-Nd/sub 2/O/sub 3/-TiO/sub 2/ (/spl epsiv//sub r/=90.9). Microstrip loss properties are determined by fabricating a series of loosely coupled half-wave resonators on each substrate, with a range of characteristic impedance values. Measurements to 6 GHz are compared to those for similar lines fabricated using conventional thin-film MCM-D technology. The results demonstrate that etchable thick-film MCM technology provides many of the advantages of thin-film MCM-D technology, such as low-loss and high-definition conductors, and is suitable for the cost-effective fabrication of miniaturised high-performance microstrip MICs in high volume. 相似文献
32.
33.
South Africa’s Reconstruction and Development Programme set ambitious goals for providing basic services to all, including housing and electrification. More efficient use of energy has the potential to socially and politically support these goals, particularly when it is targeted at low-income communities lacking adequate energy services. This paper examines the economics of energy efficiency for the urban poor from the perspective of society, utilities and poor consumers, using five example programmes. While the five energy efficiency programmes generally have significant economic and environmental benefits from a social perspective, they may not be as attractive to utilities and consumers. Also examined are the policy options for overcoming the significant barriers to energy efficiency, and ways government can bridge the gap between what is good for society and what is good for the electricity industry. 相似文献
34.
Goldman Susan R.; Mertz Davis L.; Pellegrino James W. 《Canadian Metallurgical Quarterly》1989,81(4):481
Practice on addition basic facts and changes in solution strategies were examined in 3rd- and 4th-grade students, practicing over 3 months. Response latencies decreased, indicating positive practice effects. Each of 3 student groups demonstrated different solution–strategy mixtures. Different mixtures led to different practice effects, indexed by shifts to more efficient counting strategies and more direct retrieval. Expectations about effects of practice and instructional strategies for optimizing practice must be sensitive to a child's actual strategy pattern. Most students had decreasing latency practice functions. However, quantitative functions fit to the practice session data indicated a high proportion of cases where the latency pattern over sessions was nonmonotonic. Trends over a small number of practice sessions may be inadequate for decisions about the utility of further practice, because such data may poorly represent long-term, cumulative effects on modifying strategy usage. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
35.
36.
Hitchcock Janice M.; Sananes Catherine B.; Davis Michael 《Canadian Metallurgical Quarterly》1989,103(3):509
Bilateral electrolytic lesions of the central, but not the lateral, nucleus of the amgydala blocked shock sensitization of startle (the increase in startle produced by presentation of ten 0.6-mA footshocks in rapid succession). Lesions of the central nucleus also decreased reactivity to shock (jumping and flinching) during shock presentation. However, this decrease in reactivity cannot account for the blockade of shock sensitization, because when a higher shock intensity (1.0 mA) was used, producing equivalent reactivity to that of controls at 0.6 mA, central nucleus lesions still blocked shock sensitization. Moreover, lesions of the caudal part of the ventral amygdalofugal pathway, which carries central nucleus efferents to the startle reflex pathway, also blocked shock sensitization. It is hypothesized that shock activates the central nucleus of the amygdala, which increases startle through modulation of the startle pathway. Activation of the amygdala by shock may be the unconditioned response relevant for fear conditioning. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
37.
The available surgical and non-surgical therapy options for treatment of gallstone disease are presented. Conventional cholecystectomy is regarded as standard therapy of symptomatic cholecystolithiasis. Other modes of therapy may be indicated under certain circumstances, depending on the results of imaging procedures. In this context conventional X-ray examination, oral and intravenous cholecystography, sonography, computed tomography, endoscopic retrograde cholangiography/cholecystography, and magnetic resonance imaging are discussed and their influence on therapeutic decisions is explained. 相似文献
38.
Stevenson D. Morow H. Blatecky A. Montgomery Davis P. Richardson P. Courter N.L.T. 《IEEE network》1994,8(6):32-38
The North Carolina Research and Education Network (NC-REN), formerly known as CONCERT, is an existing video and data network, owned and operated by MCNC. NC-REN's purpose has been to provide network-based support for collaboration in the research community within the state since 1985. The first major application planned for the North Carolina Information Highway (NCIH) is an interactive video distance learning system. A secondary application proposed is the use of Switched Multimegabit Data Service (SMDS) as a means of providing data communications services over NCIH. The user community currently supported by NC-REN has come to expect high levels of service, reliability, interoperability and performance for data and video communications. MCNC is committed to see that NC-REN users receive service that is equal to or better than what is currently provided by the existing network. The prospect of migrating NC-REN-provided services to NCIH-provided transport has raised challenging technical and unique service issues 相似文献
39.
The effects of lipid peroxidation and the antioxidant vitamin E contained in LDL isolated from control plasma (LDL--) and from plasma preincubated with 0.5 mmol/ml alpha-tocopherol (LDL+) on the proliferation of estrogen-receptor positive (ER+ : ZR-75, T-47-D, MCF-7) and negative (ER--: HBL-100, MDA-MB-231) human breast cancer cells were studied. Human skin fibroblasts served as controls. Incubation of plasma with 0.5 mmol/ml alpha-tocopherol resulted in a 3-fold increase of its content and a significant reduction in lipid hydroperoxides and conjugated dienes in LDL. Incubation of fibroblasts or ER+ tumor cells with LDL- or LDL+ had an effect on neither cell proliferation nor on the cellular levels of peroxidation products as compared to control incubations in the absence of LDL. In ER- cells, however, LDL+ stimulated the proliferation, whereas LDL- yielded a cytotoxic effect. Moreover, LDL- supplementation resulted in an increase in the content of hydroperoxides and conjugated dienes. LDL+ supplemented cells exhibited hydroperoxide levels in these tumor cells comparable to the basal levels measured in the absence of LDL. Our data suggested that peroxidation products in LDL are cytotoxic to estrogen-receptor negative breast tumor cells and vitamin E counteracts this effect. 相似文献
40.
We report an unusual case of a 1 cm papillary renal cell carcinoma arising from the outer surface of the free wall of a radiographically simple renal cyst. Most malignancies arising from the wall of a renal cyst are present along the inner surface, usually adjacent to underlying renal parenchyma. While this is a rare occurrence, it does reinforce the need to explore even small solid renal masses. 相似文献