全文获取类型
收费全文 | 734656篇 |
免费 | 9359篇 |
国内免费 | 2365篇 |
专业分类
电工技术 | 13597篇 |
综合类 | 623篇 |
化学工业 | 110723篇 |
金属工艺 | 27386篇 |
机械仪表 | 22702篇 |
建筑科学 | 17997篇 |
矿业工程 | 2890篇 |
能源动力 | 20218篇 |
轻工业 | 63357篇 |
水利工程 | 6772篇 |
石油天然气 | 9390篇 |
武器工业 | 30篇 |
无线电 | 91217篇 |
一般工业技术 | 142249篇 |
冶金工业 | 138222篇 |
原子能技术 | 13472篇 |
自动化技术 | 65535篇 |
出版年
2021年 | 5977篇 |
2020年 | 4412篇 |
2019年 | 5686篇 |
2018年 | 9364篇 |
2017年 | 9438篇 |
2016年 | 10157篇 |
2015年 | 7050篇 |
2014年 | 11484篇 |
2013年 | 35513篇 |
2012年 | 19143篇 |
2011年 | 26592篇 |
2010年 | 20782篇 |
2009年 | 23651篇 |
2008年 | 24629篇 |
2007年 | 24644篇 |
2006年 | 22143篇 |
2005年 | 20079篇 |
2004年 | 19376篇 |
2003年 | 19033篇 |
2002年 | 18276篇 |
2001年 | 18152篇 |
2000年 | 16995篇 |
1999年 | 17930篇 |
1998年 | 43250篇 |
1997年 | 30846篇 |
1996年 | 23988篇 |
1995年 | 18317篇 |
1994年 | 16312篇 |
1993年 | 16024篇 |
1992年 | 11782篇 |
1991年 | 11173篇 |
1990年 | 10866篇 |
1989年 | 10324篇 |
1988年 | 9927篇 |
1987年 | 8549篇 |
1986年 | 8419篇 |
1985年 | 9899篇 |
1984年 | 9210篇 |
1983年 | 8133篇 |
1982年 | 7538篇 |
1981年 | 7586篇 |
1980年 | 7086篇 |
1979年 | 6883篇 |
1978年 | 6618篇 |
1977年 | 7795篇 |
1976年 | 10353篇 |
1975年 | 5613篇 |
1974年 | 5340篇 |
1973年 | 5295篇 |
1972年 | 4362篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
J Harbott M Mancini C Verellen-Dumoulin AV Moorman LM Secker-Walker 《Canadian Metallurgical Quarterly》1998,12(5):823-827
OBJECTIVE: Using receiver-operating characteristic (ROC) curves, we tried to determine the diagnostic threshold of amniotic fluid index (AFI) that will identify abnormal fetal size (birth weights under 2500 g or at least 4000 g) at 37 weeks or beyond. METHODS: We analyzed prospectively over 2 years all parturients with intact membranes and known AFI in early labor. Patients with the following conditions were excluded: pregestational or gestational diabetes, known anomalies, and preterm labor. Two ROC curves were constructed, and the areas (+/- standard error of the mean [SE]) under the curves were calculated. P < .05 was considered significant. RESULTS: Of the 1038 subjects meeting study criteria, 3.6% and 11.5% gave birth to infants who were small for gestational age (SGA) or macrosomic, respectively. Overall, 28.7% had oligohydramnios (AFI at most 5.0 cm) and 3.6% had hydramnios (AFI at least 24.0 cm). Small for gestational age was more common in patients with AFI at most 5.0 cm (6.4%) than in those with adequate fluid (AFI 5.1-23.9; 2.5%), or hydramnios (2.7%; P = .012). Macrosomic newborns were less likely to be born to women with oligohydramnios (7.7%) than to those with adequate amniotic fluid (13.1%) or hydramnios (13.5%). Areas under ROC curves are not significantly different from the area under the nondiagnostic line, indicating that AFI (0-34 cm) cannot differentiate between newborns under 2500 g and at or over 2500 g or under 4000 and at or more 4000 g. CONCLUSION: Intraparterium AFI appears to be a poor screening test to identify risk for delivery of SGA or macrosomic fetus. 相似文献
992.
993.
South Korea and Cuba are dissimilar in religion, economy, culture and attitudes toward premarital sexual relations. In 1960, Korea instituted a national family planning programme to combat rapid population growth. Cuba explicitly rejected Malthusian policies, but made family planning universally available in 1974 in response to health needs. Both countries have undergone rapid fertility declines and today have less than replacement level fertility. Both countries have also used a similar mixture of methods, including a high prevalence of female sterilisation. Abortion has played a major role in the fertility decline of both countries, rising in the first half of the fertility transition and then falling, although remaining a significant variable in the second half. It is concluded that access to contraception, voluntary sterilisation, and safe abortion has a direct impact on fertility and has been associated with a rapid fall in family size in two very different countries. 相似文献
994.
Drying of textile bobbins was carried out on two kinds of laboratory-scale dryers: in a cross circulation drying rig, atmospheric pressure heating air is used, while in the through circulation drying apparatus, the heating air is pressurized before the drying process takes place. In both cases, a bobbin was fitted with seven thermocouples, placed in a plane normal to the bobbin axis but at different radial distances, in order to provide temperature data during the drying process. In cross circulation drying, plots of temperature against time indicated the presence of two moving fronts, one starting from inside the bobbin, another from the outside, while in through circulation drying, similar plots showed the presence of an evaporation front, moving from inside the bobbin to the outside. In both canes, numerical models were developed to simulate the drying processes. 相似文献
995.
Keith S. Matlack Tomasz Labuda 《Journal of chemical technology and biotechnology (Oxford, Oxfordshire : 1986)》1995,62(1):91-93
A method for analyzing the fluorine content of glass using a microwave oven to digest the glass is presented. Analysis time and secondary waste generation are reduced using this method, without sacrificing accuracy. 相似文献
996.
Xie K. Zhao J.H. Flemish J.R. Burke T. Buchwald W.R. Lorenzo G. Singh H. 《Electron Device Letters, IEEE》1996,17(3):142-144
A 6H-SiC thyristor has been fabricated and characterized. A forward breakover voltage close to 100 V and a pulse switched current density of 5200 A/cm2 have been demonstrated. The thyristor is shown to operate under pulse gate triggering for turn-on and turn-off, with a rise time of 43 ns and a fall time of less than 100 ns. The forward breakover voltage is found to decrease by only 4% when the operating temperature is increased from room temperature to 300°C. It is found that anode ohmic contact resistance dominates the device forward drop at high current densities 相似文献
997.
The relationship between the distance properties of trellis codes and the computational effort and error performance of sequential decoding is studied and optimum distance profile (ODP) and optimum free distance (OFD) trellis codes are constructed for 8-PSK and 16 QAM modulation. A comparison of the performance of both the ODP and the OFD trellis codes reveals that neither class of codes results in the best trade-off between error performance and computational effort when sequential decoding is used. A new algorithm is then proposed to construct robustly good trellis codes for use with sequential decoding. New trellis codes with asymptotic coding gains up to 6.66 dB are obtained using this algorithm, and the new codes achieve nearly the same free distances as the OFD codes and nearly the same distance profiles as the ODP codes 相似文献
998.
The effect of duty cycle of pulsed dc currents on the critical length-current density product, (jlc), was measured using the Blech-Kinsbron edge-displacement technique [Thin Solid Films 25, 327 (1975)]. Unencapsulated Al edge-displacement segments mere stressed at various duty cycles and the critical lengths, the so-called “Blech lengths”, were measured. It was found that jlc increased with decreasing duty cycle. We measured a factor of 2.6 increase in jlc for the 25% duty cycle as compared to dc. This duty cycle dependence of Blech length implies that electromigration resistance for an integrated circuit would be increased for small duty cycle operation by increasing the fraction of interconnects which are sub-Blech-length and are not susceptible to EM damage 相似文献
999.
In previous work, an algorithm for matching geometric features was developed. Although the method worked well, it was demanding in computational resources when applied to large problems. The authors now present methods for reducing the computational requirements, without significantly affecting the reliability of the algorithm 相似文献
1000.
Schubert E.F. Hunt N.E.J. Malik R.J. Micovic M. Miller D.L. 《Lightwave Technology, Journal of》1996,14(7):1721-1729
Resonant-cavity light-emitting diodes (RCLED) are novel, high-efficiency light-emitting diodes which employ optical microcavities. These diodes have higher intensities and higher spectral purity as compared to conventional LEDs. Analytical formulas are derived for the enhancement of the spontaneous emission along the optical axis of the cavity. The design rules for high-efficiency operation of RCLEDs are established. The temperature dependence of the emission intensity is analyzed in the range 20-80° and it is described by an exponential dependence with a characteristic temperature of 112 K. The modulation characteristics of RCLEDs exhibit 3 dB frequencies of 580 MHz. Eye diagrams at transmission rates of 622 Mb/s are wide open indicating the suitability of RCLEDs for high-speed data transmission 相似文献