全文获取类型
收费全文 | 599080篇 |
免费 | 7272篇 |
国内免费 | 1855篇 |
专业分类
电工技术 | 11665篇 |
综合类 | 484篇 |
化学工业 | 85702篇 |
金属工艺 | 26707篇 |
机械仪表 | 21096篇 |
建筑科学 | 12573篇 |
矿业工程 | 3235篇 |
能源动力 | 15276篇 |
轻工业 | 39904篇 |
水利工程 | 6298篇 |
石油天然气 | 10658篇 |
武器工业 | 24篇 |
无线电 | 73203篇 |
一般工业技术 | 123173篇 |
冶金工业 | 112133篇 |
原子能技术 | 12089篇 |
自动化技术 | 53987篇 |
出版年
2021年 | 4727篇 |
2020年 | 3631篇 |
2019年 | 4694篇 |
2018年 | 13762篇 |
2017年 | 14068篇 |
2016年 | 11811篇 |
2015年 | 5745篇 |
2014年 | 9026篇 |
2013年 | 25870篇 |
2012年 | 16311篇 |
2011年 | 25464篇 |
2010年 | 21404篇 |
2009年 | 22560篇 |
2008年 | 23190篇 |
2007年 | 24418篇 |
2006年 | 16434篇 |
2005年 | 17362篇 |
2004年 | 15611篇 |
2003年 | 15186篇 |
2002年 | 13862篇 |
2001年 | 13579篇 |
2000年 | 12779篇 |
1999年 | 13458篇 |
1998年 | 35229篇 |
1997年 | 24100篇 |
1996年 | 18614篇 |
1995年 | 13797篇 |
1994年 | 12131篇 |
1993年 | 12072篇 |
1992年 | 8575篇 |
1991年 | 8156篇 |
1990年 | 8038篇 |
1989年 | 7646篇 |
1988年 | 7166篇 |
1987年 | 6137篇 |
1986年 | 6121篇 |
1985年 | 6855篇 |
1984年 | 6216篇 |
1983年 | 5623篇 |
1982年 | 5139篇 |
1981年 | 5339篇 |
1980年 | 4814篇 |
1979年 | 4649篇 |
1978年 | 4609篇 |
1977年 | 5376篇 |
1976年 | 7027篇 |
1975年 | 3968篇 |
1974年 | 3765篇 |
1973年 | 3781篇 |
1972年 | 3208篇 |
排序方式: 共有10000条查询结果,搜索用时 484 毫秒
991.
Ubiquitin has been shown by immunohistochemical studies to be a component of many of the filamentous inclusion bodies that are known in neuropathology. In the current study, we examined the expression of ubiquitin in 14 cases of typical inclusion body myositis, in skeletal muscle specimens from four cases of typical amyotrophic lateral sclerosis, and in muscle specimens from three normal controls. In the cases of inclusion body myositis, rimmed vacuoles were ubiquitin immunoreactive in all cases. Intrasarcoplasmic inclusions were positive in the nine cases that had them. In four cases, there were positive intranuclear inclusions, and in seven, there was homogeneous staining of nuclei. Atrophic fibers and necrotic fibers were positive in 11 and nine cases, respectively. In the cases of amyotrophic lateral sclerosis, atrophic fibers were positive in three cases, and focal nuclear staining was seen in two. In one of the three control cases, a few atrophic fibers had faint sarcoplasmic positivity; no other staining was seen. We conclude that ubiquitin is a component of the inclusions that characterize inclusion body myositis. However, ubiquitin expression in skeletal muscle disease is not pathognomonic of inclusion body myositis. 相似文献
992.
The expression patterns of laminin A, B1, B2, and nidogen genes were identified by in situ hybridization in postimplantation mouse extraembryonic tissues and maternal decidua during the period when the chorioallantoic placenta is established. Laminin and nidogen genes were not coordinately expressed either in the decidua or in trophoblast cells, indicating that these genes are regulated independently in these cell types during the establishment of the placenta. Laminin A mRNA was absent from the decidua except in the outer layer of cells adjacent to the myometrium and in the central decidual zone adjacent to the remnant of the uterine epithelium on Day 9. At this stage laminin B1, B2, and nidogen genes were strongly expressed in these cells and also in other regions of the decidua. Laminin B1 mRNA was present at higher levels in the decidua capsularis than in the decidua basalis, while nidogen mRNA showed highest expression in the decidua basalis. Laminin B2 mRNA was produced uniformly throughout the decidua at very high levels, suggesting that laminin B2 chains may be an important component of the decidual matrix. By Day 11, the nidogen gene was expressed only in endothelial cells lining the maternal blood spaces within the decidua. Laminin B1 and nidogen mRNAs were found at high levels within trophoblast giant cells at all stages, while laminin A mRNA was detected in trophoblast giant cells at later stages and laminin B2 mRNA was not produced in high levels by these cells. The patterns of gene expression show a very high degree of regional specialization, suggesting that the extracellular matrices in different regions of the decidua and extraembryonic membranes are likely to be composed of quite different ratios of laminin and nidogen polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
993.
S. Ahmad B.E. Bonner W. Gere G.S. Mutchler P. Rambo S. Toshkov E. Platner H.J. Crawford J.M. Engelage 《Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment》1993,330(3):416-421
The performance of Bicron BC404, BC408 and BC420 scintillators coupled to a Hamamatsu R3432 proximity mesh dynode photomultiplier tube was studied using pion beams from the TRIUMF cyclotron and the Brookhaven National Laboratory AGS accelerator. A system resolution of better than 100 ps seems feasible. This will satisfy the requirement for charged particle identification in the STAR detector at RHIC. 相似文献
994.
Martensite transformations proceeding in mechanically loaded TiNi-based alloys account for an “anomalous” character of the acoustic emission from the material, whereby cyclic transformations during the growth of mechanical stress in the course of the direct transition is accompanied by an increase, rather than by a decrease, in the acoustic emission energy. This behavior of the acoustic emission is evidence of a significant influence of the external stresses on the martensite transformations and the related energy dissipation process. 相似文献
995.
The state of the surface of n-GaAs crystals upon high-vacuum microwave plasmachemical (HVMWPC) etching in various gas mixtures and the influence of the semiconductor surface condition on the photoelectric characteristics of related metal-semiconductor-metal structures with double Schottky barrier (MSMDSB structures) are investigated. Dependence of the HVMWPC etching rate of the GaAs surface on the gas mixture composition and substrate temperature is determined. It is shown that the HVMWPC etching regime strongly influences the photoelectric properties of MSMDSB structures: the treatment can lead to either growth or drop in photosensitivity of the samples. Optimum etching regimes are established for which good semiconductor surface quality and high photosensitivity of the MSMDSB structures are retained at a high etching rate. 相似文献
996.
A Linear Cross-Coupled Control System for High-Speed Machining 总被引:4,自引:0,他引:4
Q. Zhong Y. Shi J. Mo S. Huang 《The International Journal of Advanced Manufacturing Technology》2002,19(8):558-563
We present a linear cross-coupled controller to improve highspeed contouring accuracy independently of tracking accuracy in
a biaxial machine tool feed drive servomechanism. Unlike conventional cross-coupled controllers, the cross-coupled controller
presented here is a linear system, so it is very easy to perform the stability and steady-state error analysis, and to optimise
the controller parameters. The proposed controller is evaluated experimentally on a CNC LOM machine and compared to an uncoupled
controller and a conventional cross-coupled controller. Controller performance is evaluated for a circular contour at a feedrate
of 30 m min _1 . The experimental results show that the proposed controller can greatly reduce the contour error at large feedrates. The
linear cross-coupled controller is simple to implement and is practical. 相似文献
997.
998.
Bagdonavicius V.B. Gerville-Reache L. Nikulin M.S. 《Reliability, IEEE Transactions on》2002,51(1):27-31
Applications of the additive accumulation of damages (AAD) or the accelerated failure time (AFT) and the proportional hazards (PH) models in accelerated life testing with step-stresses are discussed. A new model including AAD and PH models is proposed. It is more reasonable than the PH model and wider then the AAD model. Constructing the maximum likelihood function is discussed 相似文献
999.
Effect of Thermohydrogen Treatment on the Structure and Properties of Titanium Alloy Castings 总被引:2,自引:0,他引:2
Il'in A. A. Skvortsova S. V. Mamonov A. M. Permyakova G. V. Kurnikov D. A. 《Metal Science and Heat Treatment》2002,44(5-6):185-189
Metal Science and Heat Treatment - 相似文献
1000.
Yamanaka N. Kawano R. Oki E. Yasukawa S. Okazaki K. 《Advanced Packaging, IEEE Transactions on》2002,25(1):65-72
A 640-Gb/s high-speed ATM switching system that is based on the technologies of advanced MCM-C, 0.25-μm CMOS, and optical wavelength-division-multiplexing (WDM) interconnection is fabricated for future broadband backbone networks. A 40-layer, 160×114 mm ceramic MCM forms the basic ATM switch module with 80-Gb/s throughput. It consists of 8 advanced 0.25-μm CMOS LSIs and 32 I/O bipolar LSIs. The MCM has a 7-layer high-speed signal line structure having 50-Ω strip lines, high-speed signal lines, and 33 power supply layers formed using 50-μm thick ceramic layers to achieve high capacity. A uniquely structured closed-loop-type liquid cooling system for the MCM is used to cope with its high power dissipation of 230 W. A three-stage ATM switch is made using the optical WDM interconnection between high-performance MCMs. For WDM interconnection, newly developed compact 10-Gb/s, 8-WDM optical transmitter and receiver modules are used. These modules are each only 80×120×20 mm and dissipate 9.65 W and 22.5 W, respectively. They have a special chassis for cooling, which contains high-performance heat-conductive plates and micro-fans. An optical WDM router based on an arrayed waveguide router is used for mesh interconnection of boards. The optical WDM interconnect has 640-Gb/s throughput and simple interconnection 相似文献