全文获取类型
收费全文 | 86787篇 |
免费 | 1033篇 |
国内免费 | 422篇 |
专业分类
电工技术 | 828篇 |
综合类 | 2323篇 |
化学工业 | 12010篇 |
金属工艺 | 4890篇 |
机械仪表 | 3115篇 |
建筑科学 | 2197篇 |
矿业工程 | 572篇 |
能源动力 | 1280篇 |
轻工业 | 3755篇 |
水利工程 | 1294篇 |
石油天然气 | 347篇 |
无线电 | 9730篇 |
一般工业技术 | 17046篇 |
冶金工业 | 2948篇 |
原子能技术 | 282篇 |
自动化技术 | 25625篇 |
出版年
2023年 | 42篇 |
2022年 | 80篇 |
2021年 | 162篇 |
2020年 | 85篇 |
2019年 | 94篇 |
2018年 | 14543篇 |
2017年 | 13432篇 |
2016年 | 10037篇 |
2015年 | 670篇 |
2014年 | 334篇 |
2013年 | 394篇 |
2012年 | 3246篇 |
2011年 | 9558篇 |
2010年 | 8393篇 |
2009年 | 5680篇 |
2008年 | 6892篇 |
2007年 | 7869篇 |
2006年 | 219篇 |
2005年 | 1285篇 |
2004年 | 1182篇 |
2003年 | 1229篇 |
2002年 | 586篇 |
2001年 | 155篇 |
2000年 | 237篇 |
1999年 | 107篇 |
1998年 | 157篇 |
1997年 | 87篇 |
1996年 | 97篇 |
1995年 | 60篇 |
1994年 | 74篇 |
1993年 | 67篇 |
1992年 | 67篇 |
1991年 | 79篇 |
1990年 | 46篇 |
1989年 | 32篇 |
1988年 | 39篇 |
1987年 | 32篇 |
1986年 | 31篇 |
1984年 | 32篇 |
1968年 | 44篇 |
1967年 | 35篇 |
1966年 | 42篇 |
1965年 | 45篇 |
1960年 | 31篇 |
1959年 | 36篇 |
1958年 | 38篇 |
1957年 | 36篇 |
1956年 | 34篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Punt J.B. Sparreboom D. Brouwer F. Prasad R. 《Vehicular Technology, IEEE Transactions on》1998,47(4):1302-1313
Dynamic channel selection (DCS) is an algorithm for flexible resource sharing in mobile radio systems. The digital enhanced cordless telecommunications (DECT) standard implements a version of DCS based on time as well as frequency multiplexing. In this paper, mathematical models are developed to evaluate the probabilities of channel availability, desired carrier power, and the carrier-to-interference ratio (CIR) for a constant traffic load. These results can be used to compute the call setup blocking probability. The models reported in this study are based on a decentralized DCS according to the DECT standard specifications. The results show that blocking due to availability of a channel (resource blocking), not interference blocking, generally is the most important factor in overall call blocking. Furthermore, it is seen that resource blocking is sensitive to the implementation of portable sets. Also, with homogeneous static traffic, this type of DCS ran be as good as fixed channel allocation (FCA) with respect to resource blocking 相似文献
92.
HE Montgomery P Clarkson CM Dollery K Prasad MA Losi H Hemingway D Statters M Jubb M Girvain A Varnava M World J Deanfield P Talmud JR McEwan WJ McKenna S Humphries 《Canadian Metallurgical Quarterly》1997,96(3):741-747
PURPOSE: To determine the radiologic characteristics of cystic dystrophy of the duodenal wall. MATERIALS AND METHODS: Ten patients with cystic dystrophy of the duodenal wall and chronic pancreatitis underwent ultrasonography (US) (n = 10), computed tomography (CT) (n = 10), endoscopic US (n = 5), and endoscopic retrograde cholangiopancreatography (ERCP) (n = 9). Cystic dystrophy of the duodenal wall was classified as either cystic or solid. The imaging findings were retrospectively analyzed and compared with findings at pancreatoduodenectomy (n = 10). RESULTS: The more frequent cystic type (n = 7) of cystic dystrophy of the duodenal wall was characterized by the presence of easily recognizable cystic lesions (diameter, more than 1 cm), located within the thickened wall of the second portion of the duodenum. The solid type (n = 3) of cystic dystrophy of the duodenal wall demonstrated fibrous thickening of the duodenal wall within which small cysts (diameter, less than 1 cm) were present. The intraduodenal cysts were usually elongated or bilobate with a thick wall. The thickening of the duodenal wall appeared as a solid layer between the duodenal lumen and the pancreas, hypoechoic at US, isoattenuating at unenhanced CT, and hypoattenuating in the early phase (after initiation of infusion of contrast material) and isoattenuating in the late phase (after completion of infusion) at contrast material-enhanced CT. Findings at retrospective analysis of CT and endoscopic US images were characteristic. CONCLUSION: Imaging modalities, notably CT and endoscopic US, helped establish the diagnosis of cystic dystrophy of the duodenal wall. 相似文献
93.
Four parameters are defined to measure the performance of a teletext system, namely, the probability of delivering a message within specified time, the transmission efficiency, the average number of errors per page, and the throughput. Each of them is derived and computational results are presented taking the UK teletext system as an example. These parameters are compared in their ability to characterize the performance of the teletext system 相似文献
94.
95.
In connection with the specific features of high velocity aerated flow generated by hydraulic engineering structures, the mathematical model is developed for high turbulence air-water two-phase flow with the use of twin flow theoretical model in this paper. Furthermore the numerical method is proposed to treat bubbled flows. In addition, on the basis of air-water stratified twin flow model, the new calculation methods and free surface tracking technique are proposed to describe complicated movements of the free surface. Finally, the proposed model is used to calculate artificial aerated flows. The computed results coincide quite well with experimental results. This means that the proposed method can provide solid basis for practical engineering design. 相似文献
96.
Although the dams produce remarkable social and economic benefits, the threat made by unsafe dams to the life and property of people who live in the lower river area is un-negligible. Based on the monitoring data which reflect the safety condition of dams, the risk degree concept is proposed and the analysis system and model for evaluating risk degree (rate) are established in this paper by combining the reliability theory and field monitoring data. The analysis method for risk degree is presented based on Bayesian approach. A five-grade risk degree system for dam operation risk and corresponding risk degree is put forward according to the safety condition of dams. The operation risks of four cascade dams on some river are analyzed by the model and approach presented here and the result is adopted by the owner. 相似文献
97.
Fanggao Chang Tao Li Yongxia Ge Zhenping Chen Zhongshi Liu Xiping Jing 《Journal of Materials Science》2007,42(17):7109-7115
DC resistivity, dielectric constant, dielectric loss and positron annihilation spectra of (Ba1−x
Ho
x
)TiO3 ceramics have been measured as a function of holmium concentration x. It has been found that the DC resistivity of (Ba1−x
Ho
x
)TiO3 is strongly dependent on the Ho content: it decreases three orders of magnitude and reaches a minimum at x = 0.4%. Doping with 0.6% holmium increases the permittivity of BaTiO3 by approximately three times (from ∼1,300 to ∼4,000), with only a slight increase in the corresponding dielectric loss. The
local electron density and defect concentration estimated using positron annihilation technique conforms well to the features
found in the dielectric and resistivity measurements. The results have been discussed in terms of a mixed compensation model. 相似文献
98.
Document caching and connection caching are extensively studied problems. In document caching, one has to maintain caches
containing documents accessible in a network. In connection caching, one has to maintain a set of open network connections
that handle data transfer. Previous work investigated these two problems separately while in practice the problems occur together:
In order to load a document, one has to establish a connection between network nodes if the required connection is not already
open. In this paper we present the first study that integrates document and connection caching. We first consider a very basic
model in which all documents have the same size and the cost of loading a document or establishing a connection is equal to
1. We present deterministic and randomized online algorithms that achieve nearly optimal competitive ratios unless the size
of the connection cache is extremely small. We then consider general settings where documents have varying sizes. We investigate
a FAULT model in which the loading cost of a document is 1 as well as a BIT model in which the loading cost is equal to the
size of the document. 相似文献
99.
Non-redundant data clustering 总被引:6,自引:6,他引:0
Data clustering is a popular approach for automatically finding classes, concepts, or groups of patterns. In practice, this
discovery process should avoid redundancies with existing knowledge about class structures or groupings, and reveal novel,
previously unknown aspects of the data. In order to deal with this problem, we present an extension of the information bottleneck
framework, called coordinated conditional information bottleneck, which takes negative relevance information into account by maximizing a conditional mutual information score subject to
constraints. Algorithmically, one can apply an alternating optimization scheme that can be used in conjunction with different
types of numeric and non-numeric attributes. We discuss extensions of the technique to the tasks of semi-supervised classification
and enumeration of successive non-redundant clusterings. We present experimental results for applications in text mining and
computer vision. 相似文献
100.
Shou-ren Wang Hao-ran Geng Lin-hai Hui Ying-zi Wang 《Journal of Materials Engineering and Performance》2007,16(1):113-118
A multiphase reticulated porous ceramic (RPC) as Si3N4–Al2O3–SiO2 was fabricated by replication techniques. Proper volumes of additives and twice sinter- twice immerse process endow the RPC
an excellent crack healing and submerging property. The compressive strength and fracture toughness improved owing to the
crack bridging behavior. The existence of pores in struts in RPC blunt the crack tip and increased the external force needed
to propagate the crack. The mechanisms play a beneficial role in enhancing the compressive strength and fracture strength.
Si3N4 RPC with additives of 5%Al and 5% Al2O3 yielded the compressive strength of 9.8 MPa and fracture toughness of 0.3 MPa m1/2. 相似文献