全文获取类型
收费全文 | 3550篇 |
免费 | 40篇 |
专业分类
公路运输 | 601篇 |
综合类 | 1091篇 |
水路运输 | 1025篇 |
铁路运输 | 462篇 |
综合运输 | 411篇 |
出版年
2024年 | 2篇 |
2023年 | 4篇 |
2022年 | 23篇 |
2021年 | 47篇 |
2020年 | 30篇 |
2019年 | 21篇 |
2018年 | 349篇 |
2017年 | 305篇 |
2016年 | 266篇 |
2015年 | 40篇 |
2014年 | 67篇 |
2013年 | 102篇 |
2012年 | 174篇 |
2011年 | 316篇 |
2010年 | 355篇 |
2009年 | 202篇 |
2008年 | 294篇 |
2007年 | 273篇 |
2006年 | 149篇 |
2005年 | 153篇 |
2004年 | 77篇 |
2003年 | 85篇 |
2002年 | 60篇 |
2001年 | 48篇 |
2000年 | 52篇 |
1999年 | 16篇 |
1998年 | 17篇 |
1997年 | 10篇 |
1996年 | 16篇 |
1995年 | 7篇 |
1994年 | 9篇 |
1993年 | 6篇 |
1992年 | 2篇 |
1991年 | 7篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 1篇 |
1985年 | 1篇 |
排序方式: 共有3590条查询结果,搜索用时 625 毫秒
881.
882.
883.
884.
一系垂向悬挂对重载货车轮轨动力作用的影响 总被引:1,自引:0,他引:1
为了实现机车车辆低动力作用,基于车辆/轨道耦合动力学原理,应用车辆与线路最佳匹配设计方法和车辆/轨道空间耦合动力学模型,仿真分析了重载货车一系垂向悬挂对轮轨动力作用的影响,优化了一系悬挂参数,降低了重载货车轮轨动力的相互作用.研究结果表明:一系垂向刚度对车辆轮轨动力作用影响甚微,一系垂向阻尼在高量值范围增加阻尼值,减轻轨道结构的振动,加剧车辆本身振动;重载货车一系垂向阻尼取50~500 kN.s/m为宜. 相似文献
885.
气象条件是影响三峡工程坝区船舶通航的重要因素之一,目前坝区船闸通航调度已实现了计算机自动编制,但现有的模型和算法对气象条件的处理能力不足.文中立足于三峡工程现有的气象服务信息化水平,以及常规条件下已稳定运行的滚动时域调度模型和算法,建立了针对气象预报信息的数学模型局部修正方法,实现了在现有通航调度优化框架内有效处理气象预报信息的目的. 相似文献
886.
The B2-L21 ordering transitions in Au-Cu-Al shape-memory alloys are studied by the Monte Carlo exchange simulations, where a set of
the first, the second and the third nearest-neighbor mixing potentials for Cu-Al in the Au-Cu-Al alloys are calculated from
first principals using the Connolly-Williams methods. To ensure the phase stability of the β-Au-Cu-Al, the investigation includes the range of compositions Au2Cu1−x
Al1+x
(−0.15 ⩽ x ⩽ 0.15). The B2-L21 transition temperatures are predicted, and are in agreement with the experimental results. The atomic ordering around vacancy
of the L21 structure is further discussed. 相似文献
887.
Multi-element ultrasound phased array applicator for the ablation of deep-seated tissue 总被引:1,自引:0,他引:1
High-intensity focused ultrasound (HIFU) has attracted increasing interests as a promising noninvasive modality for the treatment
of deep tumors in the thoracic and abdominal cavity. A 90-element HIFU spherical phased array applicator operated at 1MHz
has been developed for deep tissue ablation. The spherical array with a 5 cm wide central hole has a 21 cm diameter and an
18 cm radius of curvature. Annular element distribution with unequal element spacing is used to reduce the number of elements.
The array is constructed with piezoelectric lead zirconate titanate (PZT-8) circular elements that are 1.4 cm in diameter
and have a wall with thickness of 0.2 cm. The array offers an effective ablating depth of at least 8 cm in the tissue for
both simulations and ex vivo experiments. The simulations demonstrate that the developed array can steer the focus with good
quality of intensity distributions up to 6mm off center over ranges from 17 to 21 cm when the water depth is set at 11 cm.
We also present the beam focusing capability in deep tissue through a series of ex vivo experiments by measuring discoloration
areas after sonications. These results indicate that the developed array is ideal for the ablation of deep-seated tissue. 相似文献
888.
The objective of this paper is to optimize the parameters of pellet drying and preheating process in grate. A mathematical
model of drying and preheating is developed, and is verified by the experiments. Further, the operating parameters of wind
temperature, wind speed, grate speed and bed depth etc. are investigated through the orthogonal method using the simulation
results. And a relationship of drying, preheating effects and operating parameters is achieved. The results show that the
optimization effect can be achieved in the given range when air velocity is about 3.0 m/s and temperature is about 300, 500,
800, and 1 100 °C in the updraft drying zone, downdraft drying zone, preheating I zone, and preheating II zone respectively.
The work is help for saving energy and reducing emissions in pellet induration. 相似文献
889.
For many image classification tasks, color histogram is usually employed as an important “signature” to describe the color
distribution of the image and infer the image content. However, most traditional color histograms cannot achieve satisfactory
results in many image classification systems. In order to improve the accuracy and reduce the computational complexity of
the classification task, an information-based color feature representation is proposed in this paper. The mutual information
between the feature and the class label is adopted to evaluate the discriminative power of the feature. A novel quantization
scheme is presented, which removes the redundant color components and combines the adjacent components together to generate
a new feature to maximize the discriminative ability. An iterative algorithm is performed to derive the color space quantization
and color feature generation. In order to illustrate the effectiveness of the proposed color representation, a specific image
classification task, i.e., differentiating the adult images from benign ones, is employed. Experimental results show that
our color feature achieves better classification performance and better efficiency compared with the traditional color histogram. 相似文献
890.
Structure-borne sound attenuation at corner interface of two plates with dynamic vibration absorber attached is investigated
by wave approach. Equations governing transmission and reflection coefficients are deduced by introducing some non-dimensional
coefficients, which help to reveal the physical sense inside and to simplify the analysis. Numerical investigation on vibration
energy transmission of bending wave is carried out as well. The results from measurement and prediction show almost the same
trends in the simplified experiment. It is found that energy transmission at corner interface depends greatly on whether the
dynamic vibration absorber attached acts at resonance and is relatively lower right after the nature frequency of dynamic
vibration absorber. Furthermore, the dynamic vibration absorber attached provides less energy transmission of bending wave
than blocking mass at the end of “passing band”. 相似文献