全文获取类型
收费全文 | 442663篇 |
免费 | 8494篇 |
国内免费 | 1414篇 |
专业分类
电工技术 | 7907篇 |
综合类 | 374篇 |
化学工业 | 67756篇 |
金属工艺 | 14782篇 |
机械仪表 | 12434篇 |
建筑科学 | 12666篇 |
矿业工程 | 921篇 |
能源动力 | 11740篇 |
轻工业 | 48041篇 |
水利工程 | 3581篇 |
石油天然气 | 1976篇 |
武器工业 | 17篇 |
无线电 | 57236篇 |
一般工业技术 | 81858篇 |
冶金工业 | 82325篇 |
原子能技术 | 5720篇 |
自动化技术 | 43237篇 |
出版年
2021年 | 3379篇 |
2020年 | 2406篇 |
2019年 | 2995篇 |
2018年 | 5087篇 |
2017年 | 5150篇 |
2016年 | 5448篇 |
2015年 | 4301篇 |
2014年 | 6832篇 |
2013年 | 21396篇 |
2012年 | 12077篇 |
2011年 | 16397篇 |
2010年 | 12614篇 |
2009年 | 14051篇 |
2008年 | 15317篇 |
2007年 | 15514篇 |
2006年 | 13702篇 |
2005年 | 12733篇 |
2004年 | 12265篇 |
2003年 | 11909篇 |
2002年 | 11473篇 |
2001年 | 11414篇 |
2000年 | 10578篇 |
1999年 | 11176篇 |
1998年 | 26281篇 |
1997年 | 18953篇 |
1996年 | 14771篇 |
1995年 | 11503篇 |
1994年 | 10212篇 |
1993年 | 9802篇 |
1992年 | 7270篇 |
1991年 | 6945篇 |
1990年 | 6556篇 |
1989年 | 6273篇 |
1988年 | 6004篇 |
1987年 | 5008篇 |
1986年 | 4976篇 |
1985年 | 6126篇 |
1984年 | 5732篇 |
1983年 | 4927篇 |
1982年 | 4620篇 |
1981年 | 4583篇 |
1980年 | 4332篇 |
1979年 | 4179篇 |
1978年 | 3865篇 |
1977年 | 4695篇 |
1976年 | 6400篇 |
1975年 | 3180篇 |
1974年 | 3062篇 |
1973年 | 2959篇 |
1972年 | 2363篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Summary Electropolymerization of o-(CH3O)2C6H4 (Veratrole) in dry acetonitrile/tetrabutylammonium tetrafluoroborate at platinum electrode, yields a green polymeric conducting solid. As the pressure is increased, the material becomes more conductive, but sharp changes towards lower conductivity occur at 4.2 and 6.2 metric tons pressure, probably due to phase transitions. The photoconductivity shows an exponential increase as temperature is increased, but it shows reversible changes at 200 K, 225 K and 250 K during slow warming, also attributable to phase transitions. Photoconductivity of Polyveratrole increases with the radiation frequency, being higher under blue light than under red of infrared radiation and there is a linear increase of photoconductivity with radiation density. Conductivity transients show that the conductivity increases exponentially with time to the saturation point within few minutes, and it also decreases exponentially when the light is turned off and the circuit is open. This behaviour can be explained by formation and encounter of electron and hole as current carriers. A preliminary study indicates a piezoelectric effect for the material. absorption spectroscopy allowed the measurements of the material band gap. 相似文献
992.
Summary In this study, new hydrogels in rod shape were prepared from N-acryloyl-TRIS(hydroxymethyl)aminomethane (NAT) using ethylene glycol dimethacrylate (EGDMA) or N,N’methylenebisacrylamide (BIS) as crosslinking agent, dimethylformamide (DMF) as solvent and benzoyl peroxide (BPO) as initiator. In most cases, 2-hydroxyethyl methacrylate (HEMA), acrylamide (Aam) or acrylic acid (Aac) were used as co-monomers. The polymeric matrices obtained by free radical polymerization exhibited different properties by changing crosslinker, crosslinker concentration, co-monomer and initial NAT/co-monomer mole ratio. Besides, hydrogels from HEMA, Aam and Aac with BIS in absence of NAT were prepared under the same experimental reaction conditions in order to compare the properties of these products with those synthesized from NAT and the respective co-monomers. Some of the final products were selected to perform urea release assays, conducted through swelling-controlled release. Urea was chosen as “model” plant fertilizer agent. 相似文献
993.
A.P. Unwin I.M. Ward H. Ugail M.I.G. Bloor M.J. Wilson 《Polymer Engineering and Science》2005,45(5):694-703
In this study, we investigated the implementation of an automatic procedure for optimizing thermoformed thin‐walled structures. Such objects are created in great numbers, especially in the food packaging industry. The methodology for the optimal design of such structures is based on the use of a parameterized geometry model created within an interactive design environment. By varying the parameters associated with the computer‐aided design (CAD) model, one can create a rich variety of possible designs. One can then subject these designs to physical analysis to calculate their physical properties, and thus select an optimal design. The two distinct stages of this process—the prediction of the shape of the thermoformed structure, and the physical behavior of the structure—were validated by experiments. This article reports the experimental investigation of the deformation behavior of polystyrene, the mechanical behavior of specially prepared deformed polystyrene sheets, and the response to loading of a hemispherical structure (used in the validation). POLYM. ENG. SCI., 45:694–703, 2005. © 2005 Society of Plastics Engineers 相似文献
994.
Aqueous solutions of five selected non-ionic surfactants: Triton (i-octylphenolethoxylates), Tergitol (2,6,8-trimethyl-4-nonanoloxethylates), Symperonic (n-nonylphenol-oxethylates) and Brij (fatty alcohol ethoxylates) were investigated in this study. Using the bioluminescent bacteria Vibrio fischeri the toxicity of the surfactants solutions were determined. An attempt was made to relate rather low biodegradability of nonionic surfactant solutions measured by the BOD5/COD ratio (ranging from 1 to 17%) to their toxicity. The ozonation process was carried out in a 1.5 dm3 stirred cell reactor equipped with two ozone detectors. The following parameters were analyzed: pH, COD, BOD5, DOC, TOC, polarography as well as UV spectrum. The positive effect of ozonation, represented by decay of UV absorption, was visible in almost complete destruction of the surfactants, with exception of Triton X-705 (only 65% degradation after absorption of 2000 mgO3/dm3). The most striking results were obtained in toxicity tests for ozonated solutions of the non-ionic surfactants – an increase of the bacterial growth inhibition (1.5 to 4 times increase in toxicity due to ozonation). The obtained results were discussed taking into account the literature and our own experimental data on mechanisms of ozonation and biodegradation of non-ionic surfactants of the Triton-type and similar chemical structures. 相似文献
995.
A kinetic model for simulation of the MTO process over SAPO-18 catalyst in a wide range of operating conditions has been proposed. The kinetic model predicts the experimental evolution of reaction products with time on stream, which follows three consecutive periods: initiation (where olefin production increases), a period of maximum olefin production and a period in which this production decreases. The kinetic scheme takes into account these three steps that evolve with time on stream: formation of active intermediate compounds, an step where olefins are formed by reaction of oxygenates (methanol/DME) with these intermediates and deactivation of intermediates by degradation to coke. The presence of water in the reaction medium attenuates the reaction rate of these steps. Discrimination of kinetic equations and calculation of the parameters of best fit have been carried out by solving the mass conservation equations of the individual components of the kinetic scheme together with the kinetic equation for deactivation and taking into account the effect of water on the kinetics of each step. 相似文献
996.
On the learning control of a robot manipulator 总被引:1,自引:0,他引:1
D. M. Dawson Z. Qu J. F. Dorsey F. L. Lewis 《Journal of Intelligent and Robotic Systems》1991,4(1):43-53
This paper derives a learning control law to achieve trajectory following for a robot manipulator. The controller consists of two parts, a computed torque servo for the rigid body terms that can be modelled and a learning law for the unmodelled dynamics. An advantage of this method is that bounds can be assigned to the position and velocity tracking errors. 相似文献
997.
The increasing complecity of many expert system application areas calls for the integration of the knowledge of multiple experts. The use of multiple experts introduces some interesting new problems during the process of knowledge acquisition. The problems are further complicated when the experts are geographically dispersed or unavailable for face-to-face interactions.
This article discusses the motivations for acquiring the knowledge of multiple experts, the problems related to knowledge acquisition, new issues that arise whens multiple experts interact, solutions that can be brought to bear in building multiple expert systems (particularly when experts are geographically dispersed), and new tools for knowledge engineers to use when dealing with multiple experts. 相似文献
998.
PROMOT: a FORTRAN program to scan protein sequences against a library of known motifs 总被引:2,自引:0,他引:2
M J Sternberg 《Computer applications in the biosciences》1991,7(2):257-260
Information about the three-dimensional structure or function of a newly determined protein sequence can be obtained if the protein is found to contain a characterized motif or pattern of residues. Recently a database (PROSITE) has been established that contains 337 known motifs encoded as a list of allowed residue types at specific positions along the sequence. PROMOT is a FORTRAN computer program that takes a protein sequence and examines if it contains any of the motifs in PROSITE. The program also extends the definitions of patterns beyond those used in PROSITE to provide a simple, yet flexible, method to scan either a PROSITE or a user-defined pattern against a protein sequence database. 相似文献
999.
A logical design that describes the overall structure of proteins, together with a more detailed design describing secondary and some supersecondary structures, has been constructed using the computer-aided software engineering (CASE) tool, Auto-mate. Auto-mate embodies the philosophy of the Structured Systems Analysis and Design Method (SSADM) which enables the logical design of computer systems. Our design will facilitate the building of large information systems, such as databases and knowledgebases in the field of protein structure, by the derivation of system requirements from our logical model prior to producing the final physical system. In addition, the study has highlighted the ease of employing SSADM as a formalism in which to conduct the transferral of concepts from an expert into a design for a knowledge-based system that can be implemented on a computer (the knowledge-engineering exercise). It has been demonstrated how SSADM techniques may be extended for the purpose of modelling the constituent Prolog rules. This facilitates the integration of the logical system design model with the derived knowledge-based system. 相似文献
1000.
Knowledge elicitation is accepted as being one of the most problematic areas in the creation of a knowledge-based system.A large amount of research has already concentrated on finding more efficient and effective techniques for eliciting knowledge from an individual expert. However, little attention has been given to the involvement of more than one source of expertise in knowledge-based system development.This paper is based on the authors' practical experience gained when developing a knowledge-based system for the conceptual design of bridges. It shows that the use of more than one expert throughout the knowledge elicitation process can improve both the efficiency of the approach and the quality of the knowledge acquired. 相似文献