全文获取类型
收费全文 | 208964篇 |
免费 | 2277篇 |
国内免费 | 567篇 |
专业分类
电工技术 | 3632篇 |
综合类 | 145篇 |
化学工业 | 32457篇 |
金属工艺 | 10322篇 |
机械仪表 | 7061篇 |
建筑科学 | 4050篇 |
矿业工程 | 2163篇 |
能源动力 | 4721篇 |
轻工业 | 13096篇 |
水利工程 | 2926篇 |
石油天然气 | 7728篇 |
武器工业 | 22篇 |
无线电 | 21832篇 |
一般工业技术 | 45560篇 |
冶金工业 | 35065篇 |
原子能技术 | 6661篇 |
自动化技术 | 14367篇 |
出版年
2021年 | 2111篇 |
2019年 | 2019篇 |
2018年 | 3718篇 |
2017年 | 3746篇 |
2016年 | 4041篇 |
2015年 | 2266篇 |
2014年 | 3852篇 |
2013年 | 8851篇 |
2012年 | 5786篇 |
2011年 | 7330篇 |
2010年 | 5945篇 |
2009年 | 6583篇 |
2008年 | 6723篇 |
2007年 | 6590篇 |
2006年 | 5644篇 |
2005年 | 5210篇 |
2004年 | 4699篇 |
2003年 | 4644篇 |
2002年 | 4433篇 |
2001年 | 4536篇 |
2000年 | 4248篇 |
1999年 | 4260篇 |
1998年 | 10470篇 |
1997年 | 7340篇 |
1996年 | 5608篇 |
1995年 | 4205篇 |
1994年 | 3667篇 |
1993年 | 3907篇 |
1992年 | 3016篇 |
1991年 | 3023篇 |
1990年 | 2915篇 |
1989年 | 2896篇 |
1988年 | 2885篇 |
1987年 | 2530篇 |
1986年 | 2596篇 |
1985年 | 2872篇 |
1984年 | 2708篇 |
1983年 | 2575篇 |
1982年 | 2317篇 |
1981年 | 2292篇 |
1980年 | 2365篇 |
1979年 | 2381篇 |
1978年 | 2405篇 |
1977年 | 2514篇 |
1976年 | 2916篇 |
1975年 | 2197篇 |
1974年 | 2080篇 |
1973年 | 2175篇 |
1972年 | 1942篇 |
1971年 | 1760篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Twyford E.J. Jokerst N.M. Kohl P.A. Tayag T.J. 《Photonics Technology Letters, IEEE》1995,7(7):766-768
We have demonstrated and evaluated a grating array outcoupler fabricated by photoelectrochemical (PEC) etching, a manufacturable and practical approach for fabrication of grating-based III-V semiconductor waveguide devices. An array of submicron period gratings was etched into photolithographically delineated areas in a single PEC step. The fabricated devices are: 10-μm wide rib waveguides with 0.35-μm first-order outcoupling gratings; and 10-μm wide rib waveguides with 10 μm×10 μm pixellated areas of gratings. Device characterization demonstrates the effectiveness of outcoupling grating fabrication using PEC and that the pixellated grating outcoupler is an effective and simple means of generating an optical beam array 相似文献
992.
M Fortuin N Maine M Mendy A Hall M George H Whittle 《Canadian Metallurgical Quarterly》1995,89(3):326-329
A nation-wide cross-sectional survey of 816 children 3-4 years old was carried out in The Gambia between September 1990 and July 1991 to assess the seroprevalence of antibodies against 3 diseases included in the expanded programme on immunization: measles, poliomyelitis and tetanus. Among 689 children whose records were available, 94.5% were fully immunized. Measles vaccine was administered to 97% of the children and 91% of these had detectable antibodies at the time of the survey. Antibodies against type 1 and type 3 polioviruses, after up to 6 doses of oral polio vaccine, were present in 88.1% and 89.3% of the children respectively. Ninety-seven percent of the children who had received 4 doses of diphtheria-pertussis-tetanus vaccine (DPT) and 91% of those who received 3 doses had detectable tetanus toxoid antibodies at the age of 3-4 years. This study shows that serological responses to EPI vaccines given in infancy persist at very satisfactory levels throughout early childhood. 相似文献
993.
Éntin S. V. Anzheurov N. M. Krasovitskii Yu. V. Panova O. A. Shcheglova L. I. 《Refractories and Industrial Ceramics》2003,44(3):194-198
An experimental setup for studying filter-assisted dust collection is described. The role of various factors — hydraulic resistance, filtering velocity, dust concentration, pressure drop, efficiency of recovery — in the performance of filters intended for use in the production of refractory materials is considered. The dust removal method was tested under industrial conditions at the Semilukskii Refractory Plant and can be recommended for wider use in different branches of industry. 相似文献
994.
Cdc7 kinase is essential for initiation of DNA replication. Cdc7(-/-) mouse embryonic stem (ES) cells are non-viable but their growth can be rescued by an ectopically expressed transgene (Cdc7(-/-)tg). Here we report that, despite the normal growth capability of Cdc7(-/-)tg ES cells, the mice with the identical genetic background exhibit growth retardation. Concomi tantly, Cdc7(-/-)tg embryonic fibroblasts (MEFs) display delayed S phase entry and slow S phase progression. Furthermore, spermatogenesis of Cdc7(-/-)tg mice is disrupted prior to pachytene stage of meiotic prophase I. The impairment in spermatogenesis correlates with the extremely low level of Cdc7 protein in testes, and is rescued by introducing an additional allele of transgene, which results in increase of Cdc7 expression. The increased level of Cdc7 also recovers the growth of Cdc7(-/-)tg MEFs and mice, indicating that the developmental abnormalities of Cdc7(-/-)tg mice are due to insufficiency of Cdc7 protein. Our results indicate the requirement of a critical level of a cell-cycle regulator for mouse development and provide genetic evidence that Cdc7 plays essential roles in meiotic processes in mammals. 相似文献
995.
Astadjov D.N. Isaev A.A. Petrash G.G. Ponomarev I.V. Sabotinov N.V. Vuchkov N.K. 《Quantum Electronics, IEEE Journal of》1992,28(10):1966-1969
Populations of CuI ground state, resonance, and metastable levels have been measured in CuBr laser with and without hydrogen additives. Temporal evolution and spatial distribution populations have been studied during the excitation pulse and also during the interpulse period of time. Addition of hydrogen results in a smaller rate of resonance and metastable levels excitation and smaller depopulation of the ground state during the excitation pulse. Repopulation of the ground state after the excitation pulse with hydrogen is much faster. CuBr molecules have been detected only in the periphery of the active zone. The density of the CuBr molecules had no noticeable change either during the excitation pulse or in the interpulse period 相似文献
996.
A series of experiments examining the effects of welded boundaries on the localised blast load response of mild steel plates is presented in this short communication. Two types of welding are examined, TIG and MIG welding. The welded boundary response is compared with clamped boundary and straight and chamfer machined boundary responses. The overall trend of midpoint deflection–thickness ratio versus dimensionless damage number for all the plates follows previously reported empirical trends. The tearing threshold (Mode II*) in terms of the damage number is lowest for the machined plates followed by the welded plates with the clamped plates having the highest resistance to tearing. 相似文献
997.
We tested the effect of caffeine, on hyperoxia-induced seizures. Thirty-seven rats with chronic cortical electrodes were injected i.p. with caffeine (1.25, 2.5, and 10 mg/kg) or vehicle before exposure to 0.5 MPa oxygen and 17 rats to oxygen with 5% CO2 at 0.5 MPa. EEG monitoring and spectral analysis of EEG activity were carried out. Caffeine significantly prolonged the latent period to the onset of seizures (P < 0.05 in ANOVA), in a dose-related manner. Our results suggest that caffeine may be used in low doses for protection against hyperoxia-induced seizures. 相似文献
998.
M. Nyoka G. Akdogan R. H. Eric N. Sutcliffe 《Metallurgical and Materials Transactions B》2003,34(6):833-842
The process of mixing and solid-liquid mass transfer in a one-fifth scale water model of a 100-ton Creusot-Loire Uddeholm
(CLU) converter was investigated. The modified Froude number was used to relate gas flow rates between the model and its protoype.
The influences of gas flow rate between 0.010 and 0.018 m3/s and bath height from 0.50 to 0.70 m on mixing time were examined. The results indicated that mixing time decreased with
increasing gas flow rate and increased with increasing bath height. The mixing time results were evaluated in terms of specific
energy input and the following correlation was proposed for estimating mixing times in the model CLU converter: T
mix=1.08Q
−1.05
W
0.35, where Q (m3/s) is the gas flow rate and W (tons) is the model bath weight. Solid-liquid mass-transfer rates from benzoic acid specimens immersed in the gas-agitated
liquid phase were assessed by a weight loss measurement technique. The calculated mass-transfer coefficients were highest
at the bath surface reaching a value of 6.40 × 10−5 m/s in the sprout region. Mass-transfer coefficients and turbulence parameters decreased with depth, reaching minimum values
at the bottom of the vessel. 相似文献
999.
1000.