全文获取类型
收费全文 | 24081篇 |
免费 | 1052篇 |
国内免费 | 318篇 |
专业分类
电工技术 | 517篇 |
综合类 | 552篇 |
化学工业 | 4039篇 |
金属工艺 | 582篇 |
机械仪表 | 802篇 |
建筑科学 | 1131篇 |
矿业工程 | 192篇 |
能源动力 | 686篇 |
轻工业 | 2402篇 |
水利工程 | 287篇 |
石油天然气 | 191篇 |
武器工业 | 27篇 |
无线电 | 2646篇 |
一般工业技术 | 3412篇 |
冶金工业 | 4050篇 |
原子能技术 | 188篇 |
自动化技术 | 3747篇 |
出版年
2023年 | 108篇 |
2022年 | 201篇 |
2021年 | 521篇 |
2020年 | 314篇 |
2019年 | 396篇 |
2018年 | 478篇 |
2017年 | 473篇 |
2016年 | 500篇 |
2015年 | 443篇 |
2014年 | 632篇 |
2013年 | 1292篇 |
2012年 | 1010篇 |
2011年 | 1244篇 |
2010年 | 954篇 |
2009年 | 1000篇 |
2008年 | 1049篇 |
2007年 | 1032篇 |
2006年 | 912篇 |
2005年 | 760篇 |
2004年 | 787篇 |
2003年 | 1010篇 |
2002年 | 1307篇 |
2001年 | 1088篇 |
2000年 | 664篇 |
1999年 | 597篇 |
1998年 | 1374篇 |
1997年 | 906篇 |
1996年 | 676篇 |
1995年 | 471篇 |
1994年 | 344篇 |
1993年 | 368篇 |
1992年 | 223篇 |
1991年 | 170篇 |
1990年 | 162篇 |
1989年 | 151篇 |
1988年 | 144篇 |
1987年 | 126篇 |
1986年 | 124篇 |
1985年 | 166篇 |
1984年 | 105篇 |
1983年 | 97篇 |
1982年 | 92篇 |
1981年 | 110篇 |
1980年 | 104篇 |
1979年 | 75篇 |
1978年 | 56篇 |
1977年 | 100篇 |
1976年 | 194篇 |
1975年 | 55篇 |
1973年 | 51篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
71.
Georgios D. Styliaras Georgios K. Tsolis Chris M. Papaterpos 《International Journal on Digital Libraries》2007,8(1):61-78
In this paper, AssetCollector is presented, which is a system for managing collections of cultural assets. AssetCollector
covers the needs of collection curators towards defining, populating and searching a collection in a flexible way, while supporting
them in generating reports based on the collection’s assets and reusing them in order to build web sites and CD-ROMs. In order
to support the above functionality, the system provides the content structuring subsystem, the content input subsystem, the
search subsystem and the report subsystem. The use of the subsystems is straightforward and requires no technical skills from
the curators. AssetCollector has been successfully applied for organizing various collections of cultural assets in Greece,
such as archaeological sites, museums and published books. In the future, an evaluation procedure is planned in order to further
refine the use of the system according to the targeted users’ needs. Furthermore, more import and export facilities will be
provided, which will make the system compliant with widely accepted standards. 相似文献
72.
Here, we describe the development of a reacting flow multi-species/combustion methodology, implemented as an extension to the differential reduced ejector analysis (DREA) computer program [Mathematical and computer modeling, vol. 31, 2000, p. 21; Appl. Math. Model. 25 (2001) 427; Comput. Math. Appl. 43(10–11); NASA Contractor Report, 1998]. Use of the single fluid IVP solver framework that was developed for the original DREA model has been directly coupled into the combustion formulation. With these modifications, the analysis has an elementary single step reaction Fuel+Oxidizer→Product combustion capability. Though approximate in nature, the simplicity and efficiency of the DREA formulation make it suitable for its original niche, namely design and preliminary design environments where more complex and expensive models may be inappropriate. 相似文献
73.
This paper presents a hyperstability theorem for a class of hybrid dynamic systems composed of coupled differential and difference equations subject to (possibly) time-varying nonlinearities satisfying a Popov-type inequality. The nonlinear controller generates the plant input at all times from its sampled values by defining an extended discrete system. The hyperstability results are obtained from this discrete system of special type whose state consists of the sampled continuous substate and the digital substate of the given hybrid system. Some corollaries and related physical interpretations are also given 相似文献
74.
We present a two-sorted algebra, called aPeirce algebra, of relations and sets interacting with each other. In a Peirce algebra, sets can combine with each other as in a Boolean algebra, relations can combine with each other as in a relation algebra, and in addition we have both a set-forming operator on relations (the Peirce product of Boolean modules) and a relation-forming operator on sets (a cylindrification operation). Two applications of Peirce algebras are given. The first points out that Peirce algebras provide a natural algebraic framework for modelling certain programming constructs. The second shows that the so-calledterminological logics arising in knowledge representation have evolved a semantics best described as a calculus of relations interacting with sets. 相似文献
75.
AL Basso E Ricca C Caruso L Ferrara M De Felice 《Canadian Metallurgical Quarterly》1993,144(7):539-545
Acetohydroxy acid synthase (AHAS) and threonine deaminase (TD) activities were found in Streptococcus bovis and shown to be involved in the biosynthesis of the branched chain amino acids isoleucine, leucine and valine. Apparent lack of repression of AHAS synthesis by the end-products and reduced sensitivity of S. bovis growth to analogues of the branched chain amino acids suggested that secretion of isoleucine, leucine and valine in the growth medium may be a consequence of the regulatory features of AHAS. A glycyl-leucine-resistant mutant with reduced TD activity secreted a reduced amount of isoleucine and an increased amount of valine, which might be a result of the reduced rate of synthesis of the isoleucine precursor alpha-ketobutyrate and of a consequent preferential carbon flow through the valine branch of the pathway. 相似文献
76.
U Aguglia A Gambardella RL Oliveri G De Sarro M Zappia A Quattrone 《Canadian Metallurgical Quarterly》1995,10(3):233-238
A 80-year-old man with cobalamin deficiency and no history of epilepsy developed a partial complex epileptic confusional status (ECS) unresponsive to acute i.v. diazepam. Brain CT scan and MRI investigation ruled out a focal cerebral lesion. Therapy with high doses (10,000 micrograms i.m. daily) of cobalamin alone was started, and the patient fully recovered in the following 72-hour. Control EEGs repeatedly performed days and weeks later showed progressive disappearance of the frontal interictal spiking, while the patient was on monotherapy with cobalamin (5,000 micrograms i.m. weekly). A month later the patient unfortunately discontinued replacement therapy and 13 weeks later he developed a fatal convulsive epileptic status. To our knowledge the association of ECS and cobalamin deficiency has not been previously reported. 相似文献
77.
L De Petrocellis P Orlando M Gavagnin M Ventriglia G Cimino V Di Marzo 《Canadian Metallurgical Quarterly》1996,52(9):874-877
Five novel 1,2-sn-diacylglycerols with diterpenoid acyl moieties in the sn-1 position were isolated and characterized, together with the corresponding 1,3-sn-diacylglycerols, from three species of dorid nudibranchs molluscs. Their potent activity as morphogens in vivo in the Hydra tentacle regeneration assay and their parallel activity as activators of rat brain protein kinase C (PKC) in vitro are reported here. Our findings promote the use of these compounds as useful molecular probes for both in vivo and in vitro studies on the participation of PKC in cell development. 相似文献
78.
M. Zandian J. G. Pasko J. M. Arias R. E. De Wames S. H. Shin 《Journal of Electronic Materials》1995,24(5):681-684
Measurements of 77K RoA and 300K reverse bias dynamic impedance (RdA) products at one volt reverse bias has been carried out to assess the degree of correlation of this figure of merit. Planar
P-on-n heterostructures were grown on near lattice-matched CdZnTe substrates with Hg1-xCdxTe (0.20< x <0.30) by molecular beam epitaxy. These devices were passivated with CdTe and doped with indium and arsenic as
n- and p-type dopants, respectively. Current-voltage characteristic of these devices exhibit thermally generated dark currents
at small and modest reverse bias. We have observed that RoA values of these long wavelength infrared P-on-n heterostructure photodiodes at 77K correlate with room temperature RdA values. Diode arrays with high room temperature RdA values at one volt reverse bias also have high RoA values at 77K. Similarly, low RdA values at room temperature indicate poor performance at 77K where deviation from diffusion current occurs at reverse bias
of 0.2 to 1 volt at room temperature. The results presented here, for a small samples of devices, demonstrate that room temperature
measurements of current-voltage characteristics to evaluate Hg1-xCdxTe (0.22< x <0.28) diode performance and array uniformity at lower temperatures can be used. This provides an acceptable criteria
for further study at lower temperatures. 相似文献
79.
Luginbuhl P. Collins S.D. Racine G.-A. Gretillat M.-A. De Rooij N.F. Brooks K.G. Setter N. 《Journal of microelectromechanical systems》1997,6(4):337-346
The fabrication using silicon micromachining and characterization of an acoustic Lamb wave actuator is presented. The intended use of the device is for mass transport and sensor applications. The device consists of dual interdigitated transducers patterned on a thin-film composite membrane of silicon nitride, platinum, and a sol-gel-derived piezoelectric ceramic (PZT) thin film. The acoustic properties of the device are presented along with preliminary applications to mechanical transport and liquid delivery systems. Improved acoustic signals and improved mass transport are achieved with PZT over present Lamb wave devices utilizing ZnO or AlN as the piezoelectric transducer 相似文献
80.
A Nu?ez C De la Roza ML Rodrigo-Angulo W Bu?o F Reinoso-Suárez 《Canadian Metallurgical Quarterly》1997,754(1-2):1-11
This study analyses the relationship between the use of rotating instruments, the production of a smear layer and the presence of alterations to enamel microstructures. The rotating instruments used were carbide (8-12 blade) and diamond tipped (30-15 m) cutters. Cavities were made in extracted teeth. Subsequently, half the sample was analysed using the rugosimeter before and after the application of ortophosphoric acid at 35% for 15 and the other half suing a Scansion Electronic microscope (SEM). The results obtained showed on the one hand that carbide cutters leave a smoother surfacer than diamond tipped cutters, and on the other that the smear layer is eliminated better by carbide cutters compared to diamond tipped cutters. Moreover, there are no major traumatic-type alterations at the level of the enamel affecting the microstructure after the use of carbide cutters. 相似文献