全文获取类型
收费全文 | 2342篇 |
免费 | 360篇 |
国内免费 | 43篇 |
专业分类
化学 | 865篇 |
晶体学 | 26篇 |
力学 | 84篇 |
综合类 | 86篇 |
数学 | 511篇 |
物理学 | 1173篇 |
出版年
2023年 | 35篇 |
2022年 | 104篇 |
2021年 | 127篇 |
2020年 | 76篇 |
2019年 | 73篇 |
2018年 | 71篇 |
2017年 | 116篇 |
2016年 | 111篇 |
2015年 | 75篇 |
2014年 | 151篇 |
2013年 | 144篇 |
2012年 | 121篇 |
2011年 | 147篇 |
2010年 | 89篇 |
2009年 | 139篇 |
2008年 | 147篇 |
2007年 | 124篇 |
2006年 | 109篇 |
2005年 | 101篇 |
2004年 | 96篇 |
2003年 | 89篇 |
2002年 | 71篇 |
2001年 | 66篇 |
2000年 | 50篇 |
1999年 | 54篇 |
1998年 | 37篇 |
1997年 | 37篇 |
1996年 | 50篇 |
1995年 | 29篇 |
1994年 | 18篇 |
1993年 | 17篇 |
1992年 | 13篇 |
1991年 | 8篇 |
1990年 | 10篇 |
1989年 | 8篇 |
1988年 | 5篇 |
1987年 | 12篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 3篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1971年 | 1篇 |
1959年 | 1篇 |
排序方式: 共有2745条查询结果,搜索用时 140 毫秒
81.
82.
Thomas L. Theis Bhavik R. Bakshi Delcie Durham Vasilis M. Fthenakis Timothy G. Gutowski Jacqueline A. Isaacs Thomas Seager Mark R. Wiesner 《固体物理学:研究快报》2011,5(9):312-317
While significant advances in our understanding of the behavior of engineered nanoparticles in the environment continue, there remains a need to engage the nanoparticle research community directly in the development and evaluation of environmentally benign nanoparticles to ensure that nanomaterial‐based industries emerge as tools for sustainability rather than environmental liabilities. Current research efforts aimed at understanding the environmental implications of nanotechnology emphasize existing groups of nanoparticles and products already in commercial distribution. While this is clearly necessary, this approach fails to identify and address the many tradeoffs associated with product performance and environmental quality. We believe this to be a critical gap in the ongoing exploration of nanostructured materials and their properties and applications. We posit that a number of issues are not being holistically addressed, including resource availability and allocation, manufacturing energy requirements and embodied energy, material efficiency, environmental properties of nanomaterials and nanoproducts, and waste generation. An interdisciplinary approach to research, based on the life cycle paradigm and devoted to the identification, investigation, synthesis, testing, and analysis of groups of new, more environmentally conscious nanoparticles is needed. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
83.
Tao Li Kaijin Hu Libo Cheng Yin Ding Yuxiang Ding Jinling Shao Liang Kong 《Applied Mathematical Modelling》2011
This study aimed to evaluate continuous and simultaneous variations of dental implant diameter and length, and to identify their relatively optimal ranges in the posterior mandible under biomechanical consideration. A 3D finite element model of a posterior mandibular segment with dental implant was created. Implant diameter ranged from 3.0 to 5.0 mm, and implant length ranged from 6.0 to 16.0 mm. The results showed that under axial load, the maximum Von Mises stresses in cortical and cancellous bones decreased by 76.53% and 72.93% respectively, with the increasing of implant diameter and length; and under buccolingual load, by 83.97% and 84.93%, respectively. Under both loads, the maximum displacements of implant-abutment complex decreased by 58.09% and 75.53%, respectively. The results indicate that in the posterior mandible, implant diameter plays more significant roles than length in reducing cortical bone stress and enhancing implant stability under both loads. Meanwhile, implant length is more effective than diameter in reducing cancellous bone stress under both loads. Moreover, biomechanically, implant diameter exceeding 4.0 mm and implant length exceeding 12.0 mm is a relatively optimal combination for a screwed implant in the posterior mandible with poor bone quality. 相似文献
84.
Quality function deployment (QFD) is a useful analyzing tool in product design and development. To solve the uncertainty or imprecision in QFD, numerous researchers have applied the fuzzy set theory to QFD and developed various fuzzy QFD models. Three issues are investigated by examining their models. First, the extant studies focused on identifying important engineering characteristics and seldom explored the subsequent prototype product selection issue. Secondly, the previous studies usually use fuzzy number algebraic operations to calculate the fuzzy sets in QFD. This approach may cause a great deviation in the result from the correct value. Thirdly, few studies have paid attention to the competitive analysis in QFD. However, it can provide product developers with a large amount of valuable information. Aimed at these three issues, this study integrates fuzzy QFD and the prototype product selection model to develop a product design and selection (PDS) approach. In fuzzy QFD, the α-cut operation is adopted to calculate the fuzzy set of each component. Competitive analysis and the correlations among engineering characteristics are also considered. In prototype product selection, engineering characteristics and the factors involved in product development are considered. A fuzzy multi-criteria decision making (MCDM) approach is proposed to select the best prototype product. A case study is given to illustrate the research steps for the proposed PDS method. The proposed method provides product developers with more useful information and precise analysis results. Thus, the PDS method can serve as a helpful decision-aid tool in product design. 相似文献
85.
The application of a closed thick film flowing filtered water to immerse the ablation etching mechanism of an excimer laser poses interesting possibilities concerning debris control, modification of machined feature topography and modification of the ablation rate. Furthermore, these parameters have been shown to be dependent on flow velocity; hence, offering further user control of machining characteristics. However, the impact of this technique requires investigation. This contribution offers comparison of the calculated ablation pressure and the effect on feature surface characteristics given for laser ablation of bisphenol A polycarbonate using KrF excimer laser radiation in ambient air against laser ablation of the same substrate under closed thick film flowing filtered water immersion. Also, an impact of such immersion equipment on the optical performance of the micromachining centre used is quantified and reviewed. The pressure is calculated to have risen by a magnitude of 48, when using the liquid immersed ablation technique. This increase in pressure is proposed to have an increased surface roughness, promoting the number of asperities with a surface area lower than 16 μm2; resulting in a diffuse reflection of light and an apparent darkening of features. The focal length of the optical system was accurately predicted to increase by 2.958 mm, when using the closed flowing liquid immersion equipment. This equipment is predicted to have increased the optical depth of focus via reduction in the angle of convergence of the two defining image rays; yet the perceived focus, measured discretely by mean feature wall angle, was found to be 25% smaller when using the closed thick film flowing filtered water immersion technique instead of similar laser ablation in ambient air. A compressed plume interaction is proposed as a contributing factor in this change. 相似文献
86.
The paper considers various aspects of statistical quality control by means of sample data received on a ternary ordinal scale. A new method for evaluating quality level and dispersion, free of any latent numerical scale assumptions, is proposed. The emphasis is on working with large samples, which enable the statistical analysis, estimation and control by the use of approximate analytical expressions of these measures to be considerably simplified. Two complementary studies demonstrate the usage of the proposed approach. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
87.
Xia Liu Hang Song Guoqing MiaoHong Jiang Lianzhen Cao Dabing LiXiaojuan Sun Yiren Chen 《Applied Surface Science》2011,257(6):1996-1999
In0.82Ga0.18As epilayers were grown on InP substrates using a two-step growth technique by LP-MOCVD. A homogeneous low-temperature (450 °C) In0.82Ga0.18As buffer layer was introduced to improve the crystalline quality of epilayers. The influence of low-temperature buffer layer deposition condition, such as thermal annealing duration, on the crystalline quality of the In0.82Ga0.18As epilayer was investigated. Double-crystal X-ray diffraction measurement, Hall measurement, and Raman scattering spectrum were used to evaluate the In0.82Ga0.18As epilayers. Atomic force microscope was used to study the surface morphology. It is found that the In0.82Ga0.18As epilayer, with buffer layer thermal annealing for 5 min, exhibits the best crystalline quality. The change of the surface morphology of the buffer layer after thermal annealing treatment was suggested to explain the phenomenon. 相似文献
88.
89.
90.
本文推导出了双曲余弦高斯(ChG)列阵光束在湍流大气中的光束传输因子( M 2因子)的解析公式,并采用相对 M 2因子研究了湍流对 M 2因子的影响.研究表明,在湍流大气中 M 2因子不再是一个传输不变量,湍流使得 M 2因子增大.非相干合成情况下, M 2因子随着传输距离、光束参数、相对子光束间距和子光束数目的增大而增大.相干合成情况下, M 2因子随光束参数和相对子光束间距的增大呈现振荡上升.相干合成情况下的 M 2因子比
关键词:
M2因子)')" href="#">光束传输因子(M2因子)
光束质量
双曲余弦高斯列阵光束
大气湍流 相似文献