全文获取类型
收费全文 | 16796篇 |
免费 | 268篇 |
国内免费 | 25篇 |
专业分类
电工技术 | 211篇 |
综合类 | 4篇 |
化学工业 | 2272篇 |
金属工艺 | 420篇 |
机械仪表 | 365篇 |
建筑科学 | 567篇 |
矿业工程 | 72篇 |
能源动力 | 347篇 |
轻工业 | 1306篇 |
水利工程 | 112篇 |
石油天然气 | 119篇 |
无线电 | 1655篇 |
一般工业技术 | 2572篇 |
冶金工业 | 5271篇 |
原子能技术 | 171篇 |
自动化技术 | 1625篇 |
出版年
2021年 | 111篇 |
2019年 | 110篇 |
2018年 | 158篇 |
2017年 | 111篇 |
2016年 | 122篇 |
2015年 | 119篇 |
2014年 | 248篇 |
2013年 | 641篇 |
2012年 | 341篇 |
2011年 | 463篇 |
2010年 | 382篇 |
2009年 | 377篇 |
2008年 | 526篇 |
2007年 | 492篇 |
2006年 | 483篇 |
2005年 | 434篇 |
2004年 | 370篇 |
2003年 | 400篇 |
2002年 | 346篇 |
2001年 | 353篇 |
2000年 | 339篇 |
1999年 | 421篇 |
1998年 | 1415篇 |
1997年 | 913篇 |
1996年 | 665篇 |
1995年 | 465篇 |
1994年 | 417篇 |
1993年 | 444篇 |
1992年 | 298篇 |
1991年 | 259篇 |
1990年 | 287篇 |
1989年 | 240篇 |
1988年 | 237篇 |
1987年 | 227篇 |
1986年 | 219篇 |
1985年 | 285篇 |
1984年 | 244篇 |
1983年 | 232篇 |
1982年 | 228篇 |
1981年 | 201篇 |
1980年 | 216篇 |
1979年 | 145篇 |
1978年 | 154篇 |
1977年 | 262篇 |
1976年 | 345篇 |
1975年 | 146篇 |
1974年 | 151篇 |
1973年 | 117篇 |
1972年 | 92篇 |
1970年 | 91篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
101.
Cortical implants for the blind 总被引:4,自引:0,他引:4
Arrays of stimulating electrodes can be placed in the brain itself, in the visual cortex, to bring vision to the profoundly blind. However, when designing such devices one needs to take into consideration the fact that the visual pathway maps images onto cortical structures in a complex and unpredictable way 相似文献
102.
103.
R.M. Lammert G.M. Smith S. Hughes M.L. Osowski A.M. Jones J.J. Coleman 《Photonics Technology Letters, IEEE》1996,8(6):797-799
The design and operation of multiple-quantum-well (MQW) wavelength-tunable distributed Bragg reflector (DBR) InGaAs QW lasers with nonabsorbing gratings and monolithically integrated external cavity electroabsorption modulators fabricated by selective-area MOCVD are presented. Uncoated devices exhibit CW threshold currents as low as 10.5 mA with slope efficiencies of 0.21 W/A from the laser facet. Wavelength tuning of 7 nm is obtained by injection current heating of the DBR section. These devices also exhibit extinction ratios of 18 dB from the modulator facet at a low modulator bias of 1 V, when measured with a broad-area detector. When coupled to a singlemode fiber, these devices exhibited high extinction ratios of 40 dB at a modulator bias of 1.25 V. 相似文献
104.
Chen L.R. Benjamin S.D. Smith P.W.E. Sipe J.E. 《Quantum Electronics, IEEE Journal of》1998,34(11):2117-2129
We discuss theoretical and experimental studies on the propagation of ultrashort pulses through fiber Bragg gratings. We also consider several applications in optical communications to be found by combining ultrashort pulses and fiber Bragg gratings: a multiwavelength source for wavelength-division-multiplexed systems and a means for implementing optical code-division multiple access 相似文献
105.
Low-temperature-grown GaAs (LT-GaAs) is a promising material for all-optical switching devices due to its outstanding optical characteristics. In this paper, we outline a simplified model we have developed to describe the dynamics of the carriers in this material. We also report the results of a series of measurements that we have performed to characterize the optical properties of the material. Specifically, we present the first measurements of the two-photon absorption coefficient and the refractive index changes as a function of the growth and annealing temperatures in LT-GaAs. Finally, we show how our model can be used to optimize the material for applications in all-optical switching 相似文献
106.
107.
D.Z.-Y. Ting X. Cartoixà T.C. McGill D.L. Smith J.N. Schulman 《Journal of Computational Electronics》2002,1(1-2):147-151
The Rashba effect resonant tunneling diode is a candidate for achieving spin polarizing under zero magnetic field using only conventional non-magnetic III–V semiconductor heterostructures. We point out the challenges involved based on simple arguments, and offer strategies for overcoming these difficulties. We present modeling results that demonstrate the benefits of the InAs/GaSb/AlSb-based asymmetric resonant interband tunneling diode (a-RITD) for spin filtering applications. 相似文献
108.
This study examined the relationship of several maternal variables to the duration of exclusive breast-feeding and the total duration of breast-feeding, along with attitudes, perceptions, and beliefs about breast-feeding among women living in poor neighborhoods of Managua, Nicaragua. The field work was carried out in December 1992 and January 1993 using qualitative and quantitative methods. A structured questionnaire was administered in interviews with 556 mothers of children under 12 months of age, and meetings of four directed discussion groups were held, in which a total of 20 women participated. At one week of age, almost all the children of the mothers who were surveyed had been breast-fed, but only 45% had been exclusively breast-fed. At 12 weeks old, 30% were already completely weaned. The discussion groups revealed the coexistence of positive opinions about both breast-feeding and bottle-feeding. However, exclusive breast-feeding was considered harmful for the mother, and breast milk was not thought to be sufficient nourishment for the child. Previous experience was strongly related to the duration of exclusive breast-feeding and to total breast-feeding duration. Attitudes, social support, and work situation were important factors influencing the total length of time women breast-fed a child. In general, the results obtained through the interviews and in the discussion groups were in agreement and showed that the elements needed to promote exclusive breast-feeding were social support, a favorable community environment, and policies that dealt with problems faced by working mothers. The complementary research methods were useful for obtaining information about the relative importance of different factors that determine the duration of breast-feeding and for understanding that practice in greater depth from the mother's point of view. 相似文献
109.
110.
Mineralocorticoid action is facilitated by 11 beta-hydroxysteroid dehydrogenase type II (11 beta HSD2), which metabolizes glucocorticoids and allows aldosterone to bind to the nonselective mineralocorticoid receptor. We have recently demonstrated the presence of the 11 beta HSD2 protein in a wide range of human epithelia, suggesting that it is the sole isoform endowing specificity in man. In the present study we have used an immunopurified polyclonal antibody (RAH23) raised against a C-terminal peptide derived from the cloned rat 11 beta HSD2 protein to perform immunohistochemical and molecular analysis in rat tissues. In frozen sections of rat kidney, strong staining was seen with the RAH23 antibody in the distal tubule; weaker staining was observed in the thick ascending loop of Henle and the medullary and papillary collecting ducts. Punctate cortical staining was observed in the fetus at 20 days gestation and in 8-day-old rats, with a noticeable increase in the staining pattern at 16 days of age. The kidney did not attain the adult pattern of staining until 28 days of age. Epithelia of ileum and colon also stained with RAH23, as did excretory ducts of the submandibular gland. Intrahepatic and excretory bile ducts displayed strong immunoreactivity in the epithelial lining. Rat adrenal glands showed evidence of the 11 beta HSD2 antigen in the zona fasciculata and zona reticularis, but not in the zona glomerulosa or medulla. Western blot analysis with the RAH23 antibody revealed strong bands in the kidney, colon, adrenal gland, and submandibular gland at 40 kDa, colinear with the migration of the cloned 11 beta HSD2 enzyme. A band of medium intensity was also seen at this size in the pancreas, whereas a band of moderate intensity was seen in the bile duct, and weaker bands were noticed in the stomach, small intestine, and liver, with a diffuse band at 36-42 kDa in the prostate. Strong bands were seen in the pancreas and prostate at 78 kDa, with weaker signals in the colon, adrenal, stomach, and bile duct. A number of tissues also displayed multiple bands at about 30 kDa. Enzymatic assays on tissue homogenates showed extensive conversion of corticosterone to its 11-dehydro product in an NAD-dependent manner in the submandibular gland, adrenal gland, and kidney, but not in the pancreas or prostate. This study confirms the ubiquitous presence of 11 beta HSD2 in sodium-transporting epithelia, demonstrates the high level of 11 beta HSD2 protein and enzyme activity in the rat adrenal, and suggests a possible role for the enzyme in the biliary system. Further studies are required to determine the relevance of the various molecular species to the activity, latency, and processing of the enzyme. 相似文献