全文获取类型
收费全文 | 50273篇 |
免费 | 6403篇 |
国内免费 | 3532篇 |
专业分类
电工技术 | 4946篇 |
技术理论 | 1篇 |
综合类 | 3906篇 |
化学工业 | 7339篇 |
金属工艺 | 2922篇 |
机械仪表 | 3230篇 |
建筑科学 | 4087篇 |
矿业工程 | 1736篇 |
能源动力 | 1462篇 |
轻工业 | 3830篇 |
水利工程 | 1333篇 |
石油天然气 | 2431篇 |
武器工业 | 660篇 |
无线电 | 6315篇 |
一般工业技术 | 5650篇 |
冶金工业 | 1918篇 |
原子能技术 | 512篇 |
自动化技术 | 7930篇 |
出版年
2024年 | 415篇 |
2023年 | 1406篇 |
2022年 | 2687篇 |
2021年 | 3643篇 |
2020年 | 2533篇 |
2019年 | 1905篇 |
2018年 | 1915篇 |
2017年 | 2114篇 |
2016年 | 1865篇 |
2015年 | 2529篇 |
2014年 | 2959篇 |
2013年 | 3352篇 |
2012年 | 3852篇 |
2011年 | 3947篇 |
2010年 | 3384篇 |
2009年 | 3190篇 |
2008年 | 3085篇 |
2007年 | 2787篇 |
2006年 | 2484篇 |
2005年 | 2208篇 |
2004年 | 1393篇 |
2003年 | 1055篇 |
2002年 | 964篇 |
2001年 | 818篇 |
2000年 | 743篇 |
1999年 | 666篇 |
1998年 | 432篇 |
1997年 | 376篇 |
1996年 | 333篇 |
1995年 | 278篇 |
1994年 | 230篇 |
1993年 | 140篇 |
1992年 | 133篇 |
1991年 | 88篇 |
1990年 | 61篇 |
1989年 | 46篇 |
1988年 | 37篇 |
1987年 | 28篇 |
1986年 | 35篇 |
1985年 | 13篇 |
1984年 | 10篇 |
1983年 | 3篇 |
1982年 | 14篇 |
1981年 | 8篇 |
1980年 | 12篇 |
1979年 | 12篇 |
1975年 | 1篇 |
1973年 | 1篇 |
1959年 | 8篇 |
1951年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 4 毫秒
1.
申浩 《苏州科技学院学报(工程技术版)》2015,(2):61-65
解释了跨界借鉴的含义,并阐述了其在建筑立面创新设计中运用的可能性和积极意义。通过案例分析,提出建筑立面跨界借鉴的内容和方式。 相似文献
2.
We propose an analytical model to evaluate the lightpath blocking performance for a single ROADM node with intra-node add/drop contention,in which the number of lightpaths that can be added/dropped with the same wavelength is limited by the add/drop contention factor.Different models of traffic load per nodal degree are considered to validate the effectiveness of the analytical model.The simulation results show that the proposed analytical model is effective in predicting the performance for different values of add/drop contention factor C and for variable offered loads at the node.The add/drop contention factor shows an important impact on the lightpath blocking performance and properly raising the contention factor can significantly improve the lightpath blocking performance.When the add/drop contention factor C exceeds a certain level,the performance of a ROADM with intra-node contention is close to that of a contentionless ROADM. 相似文献
3.
4.
In this work,the ionospheric variability is analyzed by applying the wavelet decomposition technique to the noontime fo F2,F10.7,interplanetary magnetic field(IMF)Bz,Ap,and lower thermospheric temperature at pressure of 10?4 h Pa in 2002.Results show that the variance of periodic oscillations in the ionosphere is largest in the 2–4-day period and declines with the increase of the period.The maximum variance of the periodic oscillations in solar irradiation is in the 16–32-day period.For geomagnetic activities,most of the variance is about equally distributed on intervals of periods shorter than 32 days.Variance distributions of IMF Bz and lower thermospheric temperature are similar to those of the ionosphere.They show the maximum in the 2–4-day period and decline with the increase of the period.By analyzing the distributions of the variances,the potential connections between the ionosphere and the external sources are discussed. 相似文献
5.
6.
Hao Li Pengcheng Zhang Shengquan Yu Hongyu Yang Bin Tang Fuhai Li Shuren Zhang 《Ceramics International》2019,45(9):11639-11647
Mg2(Ti1-xSnx)O4 (x?=?0–1) ceramics were prepared through conventional solid-state method. This paper focused on the dependence of microwave dielectric properties on crystal structural characteristics via crystal structure refinement, Raman spectra study and complex chemical bond theory. XRD spectrums delineated the phase information of a spinel structure, and structural characteristic of these compositions were achieved with the help of Rietveld refinements. Raman spectrums were used to depict the correlations between vibrational phonon modes and dielectric properties. The variation of permittivity is ascribed to the Mg2(Ti1-xSnx)O4 average bond covalency. The relationship among the B-site octahedral bond energy, tetrahedral bond energy and temperature coefficient are discussed by defining on the change rate of bond energy and the contribution rate of octahedral bond energy. The quality factor is affected by systematic total lattice energy, and the research of XPS patterns illustrated that oxygen vacancies can be effectively restrained in rich oxygen sintering process. Obviously, the microwave dielectric properties of Mg2(Ti1-xSnx)O4 compounds were obtained (= 12.18, ?=?170,130?GHz, ?=??53.1?ppm/°C, x?=?0.2). 相似文献
7.
8.
研究了无磁钻铤用0Cr20Mn21Ni2N奥氏体不锈钢圆棒锻件横截面的室温强度、硬度和韧性等力学性能,并从微观角度进行了分析。在圆棒表层区域观察到了锻造变形态晶粒组织,而直径四分之一区域为再结晶组织,平均晶粒尺寸为80μm,芯部再结晶晶粒尺寸粗化为150μm,这些差异是由不同的变形温度和降温速度造成的。这种组织差异导致的室温拉伸性能和冲击韧性也有相应的变化趋势。研究结果表明,在900、1000℃以0.5 s-1的应变速率压缩的0Cr20Mn21Ni2N试验钢样品,其应力应变曲线具有一定的加工硬化特征,其芯部硬度比应变速率为0.1 s-1的样品分别提升了28、15 HB。在700、800℃以20%以上的变形量进行压缩时,峰值抗力显著提高,而在900、1000℃以10%~50%的变形量进行压缩时,峰值抗力均无明显提高,可保证顺利的变形过程。 相似文献
9.
10.
Biao Chen Hao Wang 《Signal Processing, IEEE Transactions on》2004,52(7):2047-2057
Blind deterministic estimation of the orthogonal frequency division multiplexing (OFDM) frequency offset via oversampling is proposed in this paper. This method utilizes the intrinsic phase shift of neighboring sample points incurred by the frequency offset that is common among all subcarriers. The proposed method is data efficient - it requires only a single OFDM symbol to achieve reliable estimation, hence making it more suitable to systems with stringent delay requirement and mobility-induced channel variation. The proposed scheme is devised to perfectly retrieve frequency offset in the absence of noise. Quite remarkably, we show that in the presence of channel noise, this intuitive scheme is indeed the maximum likelihood estimate of the carrier frequency offset. The possible presence of virtual carriers are also accommodated in the system model, and some interesting observations are obtained. The Cramer-Rao lower bound is derived for the oversampling-based signal model, and we show through numerical simulation that the proposed algorithm is efficient. Practical issues such as identifiability, the front-end filter bandwidth, and the possible presence of correlated noises are also carefully addressed. 相似文献