全文获取类型
收费全文 | 54576篇 |
免费 | 5228篇 |
国内免费 | 2523篇 |
专业分类
电工技术 | 3492篇 |
技术理论 | 4篇 |
综合类 | 6325篇 |
化学工业 | 4408篇 |
金属工艺 | 1151篇 |
机械仪表 | 2489篇 |
建筑科学 | 6226篇 |
矿业工程 | 2265篇 |
能源动力 | 1386篇 |
轻工业 | 5838篇 |
水利工程 | 2313篇 |
石油天然气 | 4464篇 |
武器工业 | 918篇 |
无线电 | 3166篇 |
一般工业技术 | 5136篇 |
冶金工业 | 2043篇 |
原子能技术 | 408篇 |
自动化技术 | 10295篇 |
出版年
2024年 | 284篇 |
2023年 | 776篇 |
2022年 | 1362篇 |
2021年 | 1667篇 |
2020年 | 1818篇 |
2019年 | 1549篇 |
2018年 | 1512篇 |
2017年 | 1690篇 |
2016年 | 2055篇 |
2015年 | 2123篇 |
2014年 | 3627篇 |
2013年 | 3336篇 |
2012年 | 4164篇 |
2011年 | 4473篇 |
2010年 | 3283篇 |
2009年 | 3379篇 |
2008年 | 2966篇 |
2007年 | 3575篇 |
2006年 | 3085篇 |
2005年 | 2686篇 |
2004年 | 2216篇 |
2003年 | 1885篇 |
2002年 | 1447篇 |
2001年 | 1212篇 |
2000年 | 957篇 |
1999年 | 857篇 |
1998年 | 668篇 |
1997年 | 606篇 |
1996年 | 475篇 |
1995年 | 433篇 |
1994年 | 320篇 |
1993年 | 263篇 |
1992年 | 181篇 |
1991年 | 157篇 |
1990年 | 126篇 |
1989年 | 151篇 |
1988年 | 109篇 |
1987年 | 74篇 |
1986年 | 70篇 |
1985年 | 79篇 |
1984年 | 99篇 |
1983年 | 116篇 |
1982年 | 105篇 |
1981年 | 42篇 |
1980年 | 34篇 |
1979年 | 31篇 |
1978年 | 28篇 |
1977年 | 25篇 |
1959年 | 12篇 |
1955年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
The lead-free piezoelectric ceramics (Na.47Bi.47Ba.06)1-xCaxTiO3 (x?=?0, 0.01, 0.02, 0.03, 0.05, and 0.08, abbreviated as BNBTC/0, BNBTC/1, BNBTC/2, BNBTC/3, BNBTC/5, and BNBTC/8, respectively) were obtained using the solid-state reaction method. The structure, electric conductivity, and dielectric, ferroelectric, and piezoelectric properties of the Ca2+-doped (Na.47Bi.47Ba.06)TiO3 ceramics were thoroughly investigated. The ceramics sintered at 1200?°C exhibit dense microstructures, having relative densities higher than 96%. The X-ray diffraction results demonstrate that all ceramics have a pure perovskite structure. The mean grain sizes of the ceramics are related to the Ca2+ quantity. A small quantity of Ca2+ ions (x?≤?0.03) improves the piezoelectric and ferroelectric properties of the samples. The dielectric behavior of the samples is sensitive to the Ca2+ content and electric poling. The results demonstrate that the electrical properties of the (Na.47Bi.47Ba.06)TiO3 lead-free ceramics can be well tuned by varying the Ca2+ quantity. 相似文献
32.
Mohan B. Ponnuchamy G. Mohan Muralikrishna Venkateswara Rao Mannava G. Srinivas Reddy 《Ceramics International》2018,44(13):15019-15023
Nanocrystalline nickel oxide (NiO) was prepared from nickel hydroxide by Spark plasma sintering (SPS) and the mechanisms involved in the densification of NiO were studied. Reverse precipitated nickel hydroxide powders were SPS processed at 400, 600 and 700?°C with 70?MPa pressure. Pure NiO with 12?nm crystallite size formed after 400?°C sintering process. However NiO grains had grown to 18 and 38?nm after 600 and 700?°C sintering respectively. NiO pellets prepared using 600 and 700?°C SPS sintering schedules had relative densities of 83% and 94% respectively. Two displacement rate regimes were observed during densification of NiO in both 600 and 700?°C sintering processes. Decomposition of nickel hydroxide and particle sliding of NiO led to first displacement rate maximum while inverse Hall-Petch based plastic deformation facilitated densification during the constant second displacement rate regime. No densification occurred during sintering holding times indicating the limited role that diffusion played during densification. 相似文献
33.
34.
35.
36.
Mohamed Abdellatif 《Color research and application》2015,40(6):564-576
The spectral overlap of color‐sampling filters increases errors when using a diagonal matrix transform, for color correction and reduces color distinction. Spectral sharpening is a transformation of colors that was introduced to reduce color‐constancy errors when the colors are collected through spectrally overlapping filters. The earlier color‐constancy methods improved color precision when the illuminant color is changed, but they overlooked the color distinction. In this article, we introduce a new spectral sharpening technique that has a good compromise of color precision and distinction, based on real physical constraints. The spectral overlap is measured through observing a gray reference chart with a set of real and spectrally disjoint filters selected by the user. The new sharpening method enables to sharpen colors obtained by a sensor without knowing the camera response functions. Experiments with real images showed that the colors sharpened by the new method have good levels of color precision and distinction as well. The color‐constancy performance is compared with the data‐based sharpening method in terms of both precision and distinction. © 2014 Wiley Periodicals, Inc. Col Res Appl, 40, 564–576, 2015 相似文献
37.
38.
39.
Seong‐Hee Kim 《Quality and Reliability Engineering International》2015,31(6):923-934
We present a distribution‐free tabular cumulative sum chart for monitoring the variability of an autocorrelated process. A quantity known as the asymptotic variance parameter is employed as a measure of the variability, and a distribution‐free tabular cumulative sum chart is applied to variance estimates calculated from batches of nonoverlapping samples. The proposed chart is applicable to a stationary process with a general marginal distribution and a general autocorrelation structure. It also determines control limits analytically without trial‐and‐error simulations. The performance of the proposed chart is tested on stationary processes with both normal and nonnormal marginals with various autocorrelation structures. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
40.
Numerical dissipation acts as artificial viscosity to make smoke viscous. Reducing numerical dissipation is able to recover visual details smeared out by the numerical dissipation. Great efforts have been devoted to suppress the numerical dissipation in smoke simulation in the past few years. In this paper we investigate methods of combating the numerical dissipation. We describe visual consequences of the numerical dissipation and explore sources that introduce the numerical dissipation into course of smoke simulation. Methods are investigated from various aspects including grid variation, high-order advection, sub-grid compensation, invariant conservation, and particle-based improvement, followed by discussion and comparison in terms of visual quality, computational overhead, ease of implementation, adaptivity, and scalability, which leads to their different applicability to various application scenarios. 相似文献