全文获取类型
收费全文 | 2839篇 |
免费 | 151篇 |
国内免费 | 2篇 |
专业分类
电工技术 | 12篇 |
综合类 | 3篇 |
化学工业 | 737篇 |
金属工艺 | 31篇 |
机械仪表 | 16篇 |
建筑科学 | 131篇 |
矿业工程 | 4篇 |
能源动力 | 46篇 |
轻工业 | 482篇 |
水利工程 | 28篇 |
石油天然气 | 19篇 |
武器工业 | 1篇 |
无线电 | 130篇 |
一般工业技术 | 478篇 |
冶金工业 | 504篇 |
原子能技术 | 18篇 |
自动化技术 | 352篇 |
出版年
2023年 | 15篇 |
2022年 | 76篇 |
2021年 | 118篇 |
2020年 | 59篇 |
2019年 | 47篇 |
2018年 | 59篇 |
2017年 | 88篇 |
2016年 | 86篇 |
2015年 | 85篇 |
2014年 | 109篇 |
2013年 | 157篇 |
2012年 | 162篇 |
2011年 | 195篇 |
2010年 | 148篇 |
2009年 | 129篇 |
2008年 | 162篇 |
2007年 | 160篇 |
2006年 | 144篇 |
2005年 | 102篇 |
2004年 | 87篇 |
2003年 | 86篇 |
2002年 | 67篇 |
2001年 | 54篇 |
2000年 | 33篇 |
1999年 | 51篇 |
1998年 | 48篇 |
1997年 | 46篇 |
1996年 | 39篇 |
1995年 | 26篇 |
1994年 | 35篇 |
1993年 | 37篇 |
1992年 | 28篇 |
1991年 | 19篇 |
1990年 | 28篇 |
1989年 | 22篇 |
1988年 | 20篇 |
1987年 | 21篇 |
1986年 | 17篇 |
1985年 | 21篇 |
1984年 | 8篇 |
1983年 | 16篇 |
1982年 | 9篇 |
1981年 | 4篇 |
1980年 | 14篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 10篇 |
1974年 | 7篇 |
1973年 | 4篇 |
1960年 | 5篇 |
排序方式: 共有2992条查询结果,搜索用时 15 毫秒
141.
142.
Synthesis and photovoltaic properties of mono-substituted quaterthiophenes bearing strong electron-withdrawing group 总被引:1,自引:0,他引:1
Abderrahim Yassar Christine Videlot Abdelhafid. Jaafari 《Solar Energy Materials & Solar Cells》2006,90(7-8):916-922
We report here on the synthesis and properties of a monosubstituted quaterthiophene derivative asymmetrically functionalized by an electron-withdrawing group using the palladium-catalyzed Stille's coupling reaction. The UV–vis absorption spectrum evidences a strong intramolecular charge transfer transition. Thus oligothiophenes act as electron donors and substituent group as acceptor. The diodes in which monosubstituted quaterthiophenes behave as organic semiconductors, exhibit a rectifying behavior. Photovoltaic measurements show moderate power conversion efficiency. 相似文献
143.
An enormous amount of work has been carried out into the ways in which computers can support collaboration in the service of learning. Less attention has been paid to computer support for learning when collaboration is non-problematic, yet this is arguably of equal significance. Recognizing this, the present paper reports software which was developed to help collaborating pupils test hypotheses, an activity thought by many to be relevant to conceptual learning. The software addressed hypotheses relating to: (a) the factors which influence the pressure of water; and (b) the factors which influence the formation of shadows. It required collaborating pupils to formulate hypotheses about such factors, decide which factors must be manipulated to check correctness, formulate predictions about outcomes prior to testing, observe the results of tests, and draw conclusions across test series. Pupils were asked to input their decisions at key points as they used the software, and they received prompts to the extent that their decisions were inappropriate. The prompts became increasingly explicit as inappropriate decisions persisted. The paper describes the use of the software by 9–14-year-old pupils, comparing their activity with that of similarly aged pupils who worked with otherwise equivalent software which lacked any prompts. Evidence is presented for the value of prompting, with implications not simply for the use of hypothesis testing as an instructional strategy but also for the design of computer-based support for other complex and co-ordinated activities. 相似文献
144.
Two phenolic acids were identified in the collembolan Ceratophysella denticulata: 3-hydroxy-4,5 dimethoxy benzoic acid and 4-hydroxy-3,5-dimethoxybenzoic acid (syringic acid). These are localized on or in the integument of the springtail, in field-collected animals, in a ratio of 47:100 (v/v). Springtails kept under different temperature regimes showed differences in production and ratio of the benzoic acid derivatives. At 20 degrees C, C. denticulata produced only syringic acid, whereas at 10 degrees C both isomers in a ratio of 100:61 (v/v) were detected. Bioassays with C. denticulata as well as with the specialized collembolan predator Stenus comma (Staphylinidae) were carried out. Staphylinid beetles topically treated with the acids try to clean their mouthparts by rubbing them on the ground significantly more often than do control beetles. Both compounds individually and as a natural mixture have deterrent effects towards the predator S. comma. 相似文献
145.
Mezreb N Charrueau C Boy P Allain P Chaumeil JC 《Drug development and industrial pharmacy》2004,30(5):481-490
Pellets obtained by extrusion-spheronization represent multiparticulate dosage forms whose interest in intestinal drug delivery can be potentiated and targeted through bioadhesive properties. However, adhesion itself makes the process difficult or even impossible. The problem of tackiness encountered with bioadhesive wet masses was previously eliminated by the use of electrolytes such as CaCl2. This approach is known to reduce the viscosity of polyacrylic acids by disturbing the interactions between carboxylate groups on adjacent polymer molecules, thereby decreasing their bioadhesive properties. The present study aimed at producing pellets containing carbomers without addition of electrolytes in order to maintain their bioadhesive potentiality at its maximum. Carbopol® 974P (10%, 15% and 20%) and Carbopol® 971P (10%) were used in combination with Avicel® PH101. The extrusion speed (30, 45, 60, 90, and 150 rpm), spheronizer speed (350, 700, 960, 1000, and 1300 rpm), spheronization time (5, 10, 15, and 20 minutes) and amount of water (45%, 50%, 54%, and 58%) were optimized in order to obtain the highest yield of spherical pellets ranging 710-1000 µm in diameter. For pellets containing 10%, 15% Carbopol® 974P or 10% Carbopol® 971P and 45% water content, 30 rpm extrusion speed, 960 rpm, and 10 minutes spheronization speed and time led to the highest yields and sphericities, respectively, 72% and 0.91, 67% and 0.78, and 76% and 0.80. Production of pellets with 20% Carbopol® 974P could be achieved through the increase of the water content up to 58% and implementation of 30 rpm extrusion speed, 1300 rpm, and 10 minutes spheronization speed and time. The yield and sphericity were 42% and 0.78 respectively. 相似文献
146.
Colleges and universities conduct student satisfaction studies for many important policy making reasons. However the differences in instrumentation and the use of students' self-reported ratings of satisfaction makes such decisions sample-, instrument-, and institution-dependent. A common metric of student satisfaction would assist decision makers by providing a richness of information not typically obtained. The present study investigated the extent to which two nationally known instruments of student satisfaction could be scaled on the same quantitative metric. Pseudo-common item equating (Fisher, 1997) based on five link items of low and high endorsability enabled comparisons of "similar, but not identical items, from different instruments, calibrated on different samples" (p. 87). Results suggest that both instruments measured similar constructs and could be reasonably used to create a single, common metric. While samples used in the experiment were less than ideal, results clearly demonstrated the usefulness and reasonability of the pseudo-common item equating process. 相似文献
147.
148.
Scarpulla John Mozulay Robert Ausnit Christine Hogan Edward W. Casey Richard H. 《IEEE transactions on nuclear science》1980,27(6):1442-1448
Rapid annealing test results show that the hardened RCA/Sandia 1802 Bulk CMOS Microprocessor will function immediately after application of intense ionizing radiation pulses. This result was established in LINAC tests using a comprehensive test procedure which exercised the ?P in logarithmic time intervals from 10 ?s to 1000 s. It was found that no loss of functionality occurs for doses up to about 280 KRads. At 790 KRads, the device initially loses functionality but anneals in 600 ms. At 1.6 MRads, the device loses functionality for 4 s, and at 2.4 MRads, the device loses functionality for at least 1000 s. A sample size of five devices was used. The 1802 ?P was tested using hardware and software especially designed for this purpose. The microprocessor was exercised using software routines of exponentially increasing length starting at 10 ?s after the pulse. A measure of ?P functionality was therefore provided at logarithmically increasing sample times, After 2 seconds was reached, the routines repeated every two seconds up to 1000 seconds when the test ended. The hardware and software are described in detail. 相似文献
149.
Transfer of volatile substances from water to the atmosphere 总被引:3,自引:0,他引:3
Mass transfer coefficients and the degree of saturation of the exit gas are important parameters in transferring volatile organic substances from water to the atmosphere. Aeration systems in batch and continuous flow reactors are discussed based on theoretical analysis and experimental data. Three different ranges of the degree of saturation of the exit gas are introduced and their significance for model experiments and technical operations is evaluated. The effect of the aeration system and of surface active agents on the mass transfer coefficient of tetrachloroethylene is studied. Mass transfer coefficients for six nonpolar volatile organic compounds are presented. The stripping efficiencies of different types of gas-liquid contact devices used in water and waste water treatment are assessed. 相似文献
150.