全文获取类型
收费全文 | 49638篇 |
免费 | 15012篇 |
国内免费 | 58篇 |
专业分类
电工技术 | 938篇 |
综合类 | 45篇 |
化学工业 | 18678篇 |
金属工艺 | 634篇 |
机械仪表 | 1279篇 |
建筑科学 | 2160篇 |
矿业工程 | 8篇 |
能源动力 | 1236篇 |
轻工业 | 7751篇 |
水利工程 | 337篇 |
石油天然气 | 90篇 |
武器工业 | 3篇 |
无线电 | 9505篇 |
一般工业技术 | 13244篇 |
冶金工业 | 2393篇 |
原子能技术 | 123篇 |
自动化技术 | 6284篇 |
出版年
2023年 | 96篇 |
2022年 | 69篇 |
2021年 | 401篇 |
2020年 | 1550篇 |
2019年 | 3290篇 |
2018年 | 3274篇 |
2017年 | 3559篇 |
2016年 | 4109篇 |
2015年 | 4138篇 |
2014年 | 4174篇 |
2013年 | 5587篇 |
2012年 | 3307篇 |
2011年 | 3022篇 |
2010年 | 3105篇 |
2009年 | 3103篇 |
2008年 | 2636篇 |
2007年 | 2450篇 |
2006年 | 2207篇 |
2005年 | 1853篇 |
2004年 | 1765篇 |
2003年 | 1725篇 |
2002年 | 1648篇 |
2001年 | 1449篇 |
2000年 | 1387篇 |
1999年 | 796篇 |
1998年 | 888篇 |
1997年 | 529篇 |
1996年 | 366篇 |
1995年 | 318篇 |
1994年 | 234篇 |
1993年 | 255篇 |
1992年 | 142篇 |
1991年 | 146篇 |
1990年 | 114篇 |
1989年 | 108篇 |
1988年 | 90篇 |
1987年 | 87篇 |
1986年 | 86篇 |
1985年 | 95篇 |
1984年 | 57篇 |
1983年 | 48篇 |
1982年 | 53篇 |
1981年 | 59篇 |
1980年 | 44篇 |
1979年 | 35篇 |
1978年 | 34篇 |
1977年 | 38篇 |
1976年 | 66篇 |
1975年 | 23篇 |
1973年 | 27篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
101.
Nucleation and growth mechanisms and kinetics of crystals of an amino acid salt were investigated in a methanol‐water system by measuring and evaluating the induction time as a function of the supersaturation ratio and temperature in batch salting out crystallization experiments. Discrimination between the possible crystallization mechanisms, and estimation of the kinetic parameters were carried out using nonlinear parameter identification. The results concerning the growth mechanism obtained were checked additionally by measuring the induction time as a function of number density of seed crystals. 相似文献
102.
Conducting poly(aniline‐co‐o‐anisidine) (PAS) films with different ratios of aniline units in the polymer chain were prepared by oxidative polymerization of different molar ratios of aniline and o‐anisidine in 1 M HCl using cyclic voltammetry. Due to the much higher reactivity of o‐anisidine, the structure and properties of PASs were found to be dominated by the o‐anisidine units. The polymerization of poly‐o‐anisidine and PASs followed zero‐order kinetics with respect to formation of the polymer (film thickness) and the autocatalytic polymerization of aniline was completely inhibited. In contrast to polyaniline, a decrease in the polymerization temperature was found to increase the amount of copolymer formed and its redox charge. The presence of aniline units in PASs led to a pronounced increase in the molecular weight and conductivity, and a decrease in the solubility in organic solvents. Repetitive charging/discharging cycles showed that PASs resist degradation more than polyaniline. Copyright © 2003 Society of Chemical Industry 相似文献
103.
Data mining extracts implicit, previously unknown, and potentially useful information from databases. Many approaches have been proposed to extract information, and one of the most important ones is finding association rules. Although a large amount of research has been devoted to this subject, none of it finds association rules from directed acyclic graph (DAG) data. Without such a mining method, the hidden knowledge, if any, cannot be discovered from the databases storing DAG data such as family genealogy profiles, product structures, XML documents, task precedence relations, and course structures. In this article, we define a new kind of association rule in DAG databases called the predecessor–successor rule, where a node x is a predecessor of another node y if we can find a path in DAG where x appears before y. The predecessor–successor rules enable us to observe how the characteristics of the predecessors influence the successors. An approach containing four stages is proposed to discover the predecessor–successor rules. © 2006 Wiley Periodicals, Inc. Int J Int Syst 21: 621–637, 2006. 相似文献
104.
A central event in the life of a cellular system is the interaction between the exterior and the interior compartments. Biochemical signals arrive at the cellular surface, bind to their membrane bound receptor followed by a conformational change triggering the release of an internal chemical or electrical signal.This basic principle is followed by all our perceptive abilities like sense of smell or taste, but also by different signal transduction pathways involved in nerve conductivity, vision, sense of touch or hearing. To follow and mimic this principle of parallel registration is one of the aims of modern nanobiotechnology. If we are able to specifically biofunctionalize small arrays of a solid surface, which could be an electrode or a semiconductor, this approach will enable us to build up devices called “biochips” or “biosensors” that allow the determination of bioactive molecules with high specificity at lowest concentrations. Potential pharmacological active substrates might be screened as well as new receptors may be determined. Applications in genomics as well as proteomics are realistic. The major prerequisite for such a broad spectrum of applications is the fabrication of receptive surfaces. Biomolecules have to be surface‐adsorbed in a highly reproducible, oriented and well organised fashion, a task which in biology is taken by the cellular membranes as external or internal receptive surfaces. The physical principles like hydrogen bonds, electrostatic or hydrophobic interactions that lead to such an organized surface are well known. To synthesize molecular building blocks and to position them onto an otherwise unspecific surface is one of the challenges of nanobiotechnology combining biological knowledge and chemical skills with biophysical techniques that allow to handle or analyze even single molecules. 相似文献
105.
From its foundation until 2004, ETRI has registered over 1,000 US patents. This letter analyzes the characteristics of these patents and addresses the explanatory factors affecting their citation counts. For explanatory variables, research team related variables, invention specific variables, and geographical domain related variables are suggested. Zero‐altered count data models are used to test the impact of independent variables. A key finding is that technological cumulativeness, the scale of invention, outputs in the electronic field, and the degree of dependence on the US technology domain positively affect the citation counts of ETRI‐invented US patents. The magnitude of international presence appears to negatively affect the citation counts of ETRI‐invented US patents. 相似文献
106.
107.
A route to synthesize ZSM‐5 crystals with a bimodal micro/mesoscopic pore system has been developed in this study; the successful incorporation of the mesopores within the ZSM‐5 structure was performed using tetrapropylammonium hydroxide (TPAOH)‐impregnated mesoporous materials containing carbon nanotubes in the pores, which were encapsulated in the ZSM‐5 crystals during a solid rearrangement process within the framework. Such mesoporous ZSM‐5 zeolites can be readily obtained as powders, thin films, or monoliths. 相似文献
108.
Derivation of a second-order switching surface in the boundary control of buck converters 总被引:1,自引:0,他引:1
A second-order switching surface in the boundary control of buck converters is derived in this letter. The formulated switching surface can make the overall converter exhibit better steady-state and transient behaviors than the one with a first-order switching surface. The switching surface is derived by estimating the state trajectory movement after a switching action, resulting in a high state trajectory velocity along the switching surface. This phenomenon accelerates the trajectory moving toward the target operating point. The proposed control scheme has been successfully applied to a 120-W buck converter. The large-signal performance and a comparison with the first-order switching surface have been studied. 相似文献
109.
Inerfield M. Skones W. Nelson S. Ching D. Cheng P. Wong C. 《Solid-State Circuits, IEEE Journal of》2003,38(9):1524-1532
To date, the development of multifunction multicarrier digital receivers for cellular base station and military communications applications has been limited by the demanding dynamic range requirements for the analog-to-digital converter (ADC). The use of oversampling delta-sigma modulators provides a promising approach to overcoming the dynamic range barriers Nyquist-rate converters face in the same applications. This paper discusses issues involved in the design of high-speed high dynamic range wide-band delta-sigma ADCs for such communications applications. Test results of prototype designs are also presented. The delta-sigma modulators described in this paper operate at sampling frequencies ranging from 1 to 2.5 GHz with center frequencies ranging from dc to 100 MHz, providing between 74 and 84.2 dB signal-to-noise ratio (12 and 13.7 bits) for bandwidths of 25 and 12.5 MHz, respectively. The loop filters are continuous-time low-pass and bandpass implementations of order 6 and 10, and were fabricated in an InP heterojunction bipolar (HBT) technology. A typical tenth-order design consumes 6 W of power and occupies a die area of 23.5 mm/sup 2/. 相似文献
110.
Generalized multiuser orthogonal space-division multiplexing 总被引:8,自引:0,他引:8
Zhengang Pan Kai-Kit Wong Tung-Sang Ng 《Wireless Communications, IEEE Transactions on》2004,3(6):1969-1973
This paper addresses the problem of performing orthogonal space-division multiplexing (OSDM) for downlink, point-to-multipoint communications when multiple antennas are utilized at the base station (BS) and (optionally) all mobile stations (MS). Based on a closed-form antenna weight solution for single-user multiple-input multiple-output communications in the presence of other receiver points, we devise an iterative algorithm that finds the multiuser antenna weights for OSDM in downlink or broadcast channels. Upon convergence, each mobile user will receive only the desired activated spatial modes with no cochannel interference. Necessary and sufficient conditions for the existence of OSDM among the number of mobile users, the number of transmit antennas at the BS, and the number of receive antennas at the MS, are also derived. The assumption for the proposed method is that the BS knows the channels for all MS's and that the channel dynamics are quasi-stationary. 相似文献