全文获取类型
收费全文 | 72722篇 |
免费 | 5586篇 |
国内免费 | 2727篇 |
专业分类
电工技术 | 3872篇 |
技术理论 | 6篇 |
综合类 | 4410篇 |
化学工业 | 12657篇 |
金属工艺 | 3925篇 |
机械仪表 | 4802篇 |
建筑科学 | 5173篇 |
矿业工程 | 2383篇 |
能源动力 | 2229篇 |
轻工业 | 4766篇 |
水利工程 | 989篇 |
石油天然气 | 5240篇 |
武器工业 | 460篇 |
无线电 | 7576篇 |
一般工业技术 | 9337篇 |
冶金工业 | 3789篇 |
原子能技术 | 751篇 |
自动化技术 | 8670篇 |
出版年
2024年 | 230篇 |
2023年 | 1198篇 |
2022年 | 2073篇 |
2021年 | 2869篇 |
2020年 | 2144篇 |
2019年 | 1904篇 |
2018年 | 2246篇 |
2017年 | 2470篇 |
2016年 | 2051篇 |
2015年 | 2683篇 |
2014年 | 3420篇 |
2013年 | 4201篇 |
2012年 | 4345篇 |
2011年 | 4858篇 |
2010年 | 4173篇 |
2009年 | 3949篇 |
2008年 | 3853篇 |
2007年 | 3713篇 |
2006年 | 3792篇 |
2005年 | 3433篇 |
2004年 | 2284篇 |
2003年 | 2083篇 |
2002年 | 1869篇 |
2001年 | 1636篇 |
2000年 | 1804篇 |
1999年 | 2036篇 |
1998年 | 1757篇 |
1997年 | 1406篇 |
1996年 | 1397篇 |
1995年 | 1176篇 |
1994年 | 938篇 |
1993年 | 710篇 |
1992年 | 540篇 |
1991年 | 420篇 |
1990年 | 323篇 |
1989年 | 249篇 |
1988年 | 220篇 |
1987年 | 138篇 |
1986年 | 101篇 |
1985年 | 89篇 |
1984年 | 59篇 |
1983年 | 35篇 |
1982年 | 42篇 |
1981年 | 28篇 |
1980年 | 22篇 |
1979年 | 13篇 |
1977年 | 7篇 |
1976年 | 8篇 |
1975年 | 5篇 |
1945年 | 4篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
142.
Y.H. Sun 《Materials Research Bulletin》2004,39(9):1247-1255
Effects of Ca and Zr substitution upon the dielectric properties of Ba5LaTi3Ta7O30 ceramics were investigated together with the structural characterization. All the samples of Ba5La(ZrxTi1−x)3Ta7O30 formed a filled tungsten-bronze structures, whereas the solid solution limit was determined as x=0.25 in (CaxBa1−x)5LaTi3Ta7O30. Beyond this limit secondary phase of CaTa2O6 was detected and it would become the major phase for the Ca-rich compositions. The temperature coefficient of dielectric constant was improved with increasing Zr content while the dielectric constant decreased and the low dielectric loss varied little (in the order of 10−4). In the case of (CaxBa1−x)5LaTi3Ta7O30, small temperature coefficient of dielectric constant could be obtained with increasing Ca content while the dielectric constant decreased significantly, and a small amount substitution of Ca for Ba induced decrease in dielectric loss. 相似文献
143.
Dongchang Sun Liyong Tong 《International journal for numerical methods in engineering》2004,60(11):1911-1932
A novel finite element model is presented for static and dynamic analysis of composite plates integrated with a laminated piezoelectric layer, a host laminated composite plate and an adhesive layer between them. A new adhesive element is developed which includes both peel and shear effects in the adhesive layer based on first‐order shear deformation plate theory. The thin adhesive layer between the piezoelectric layer and the host plate is modelled by assuming that it carries constant shear and peel strains throughout its thickness. In addition, a weighted static shape control scheme for finding the optimal voltage distribution for static shape control is given. By selecting different weighting matrices, a variety of items such as displacements, slopes, curvatures, strains and even generalized forces, can be included in finding the optimal actuating voltage for static shape control. The present model is validated by comparing with those results available in the literature. The numerical results show that the weighted linear least method can give a satisfactory voltage distribution to best match the desired shape. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
144.
145.
146.
移动代理计算模式较传统的分布式计算模式有明显的优势,而伴随它有诸多安全问题出现,这使得它没有得到十分广泛的应用。本文针对代理自身的安全问题,分析了代理可能受到的攻击,并提出了一个安全方案。 相似文献
147.
This letter extends a low-density parity-check code construction using maximum-length linear congruential sequences by Prabhakar and Narayanan. The corresponding bipartite graphs of their construction were guaranteed to have a girth larger than four by a sufficient condition. However, their sufficient condition was limited to regular codes and data-node degree equal to three. The extension in this letter allows arbitrary data-node degrees and is applicable to irregular codes. Further, simpler sufficient conditions are derived and larger girths are addressed. 相似文献
148.
火焰喷涂是激光熔覆前合金粉末的预置方法之一。本文在实验的基础上,探讨了工艺过程,并讨论了影响质量的因素。 相似文献
149.
The stress intensity factors for a short crack partially penetrating an inclusion of arbitrary shape
Some approximate solutions for predicting the stress intensity factor of a short crack penetrating an inclusion of arbitrary
shape have been developed under mode I and mode II loading conditions. The derivation of the fundamental formula is based
on the transformation toughening theory. The transformation strains in the inclusion are induced by the crack-tip field and
remotely applied stresses, and approximately evaluated by the Eshelby equivalent inclusion theory. As validated by detailed
finite element (FE) analyses, the developed solutions have good accuracy for different inclusion shape and for a wide range
of modulus ratio between inclusion and matrix material. 相似文献
150.
Xiaojun Xie Yonghong Cheng Hong Wang Qian Wang Xiaolin Chen Caixin Sun 《Ceramics International》2008,34(4):689-693
In the ideal A2B2O6O′ pyrochlore structure, the x-value of O atom position is a variable parameter. In Bi1.5ZnNb1.5−xTaxO7 (BZNT) cubic pyrochlores, the x-values alter with the different compositions of Nb/Ta. In this work, a series of initial models for BZNT were established by analyzing X-ray diffraction data. Then three structure modifying methods, including Rietveld refinement, Rietveld refinement with energy and geometry optimization based on quantum mechanics, were employed to obtain the precise models using Materials Studio. Moreover, the reflectivities of BZNT were computed by quantum mechanical simulation based on the refined models. Comparing the simulation results from different modifying models with the experimental results, it is found that Rietveld refinement with energy optimization is the most accurate method for BZNT pyrochlores. According to the simulation results, the different reflectivities correspond well with various x-values of O atom positions in BZNT pyrochlores. 相似文献