全文获取类型
收费全文 | 6544篇 |
免费 | 1201篇 |
国内免费 | 510篇 |
专业分类
化学 | 1900篇 |
晶体学 | 43篇 |
力学 | 519篇 |
综合类 | 47篇 |
数学 | 157篇 |
物理学 | 5589篇 |
出版年
2024年 | 2篇 |
2023年 | 23篇 |
2022年 | 124篇 |
2021年 | 126篇 |
2020年 | 147篇 |
2019年 | 90篇 |
2018年 | 133篇 |
2017年 | 242篇 |
2016年 | 277篇 |
2015年 | 244篇 |
2014年 | 472篇 |
2013年 | 525篇 |
2012年 | 375篇 |
2011年 | 457篇 |
2010年 | 321篇 |
2009年 | 435篇 |
2008年 | 517篇 |
2007年 | 453篇 |
2006年 | 405篇 |
2005年 | 409篇 |
2004年 | 375篇 |
2003年 | 331篇 |
2002年 | 277篇 |
2001年 | 241篇 |
2000年 | 232篇 |
1999年 | 188篇 |
1998年 | 161篇 |
1997年 | 154篇 |
1996年 | 115篇 |
1995年 | 104篇 |
1994年 | 72篇 |
1993年 | 44篇 |
1992年 | 50篇 |
1991年 | 43篇 |
1990年 | 24篇 |
1989年 | 20篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 1篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 5篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有8255条查询结果,搜索用时 15 毫秒
961.
In this paper, we develop a method to estimate the bridge deflection using fiber optic Bragg-grating (FBG) strain sensors.
For most structural evaluation of bridge integrity, it is very important to measure the geometric profile, which is a major
factor representing the global behavior of civil structures, especially bridges. In the past, because of the lack of appropriate
methods to measure the deflection curve of bridges on site, the measurement of deflection has been restricted to just a few
discrete points along the bridge, and the measuring points have been limited to the locations installed with displacement
transducers. However, by applying classical beam theory, a formula is rearranged to estimate the continuously deflected profile
by using strains measured directly from several points. In addition, FBG strain sensors, which are electromagnetic, noise-free
and multipoint measurable, are employed to obtain flexural strains more accurately and stably. The regression analysis is
performed to obtain a strain function from the measured strain data. Finally, the deflection curve can be estimated by applying
the strain function to the formula. An experimental test has also been carried out to verify the developed method. 相似文献
962.
氧化铝纤维含量对Al2O3f+Cf/ZL109混杂复合材料耐磨性能的影响 总被引:3,自引:4,他引:3
利用挤压铸造法制备了Al2O3f Cf/ZL09短纤维混杂金属基复合材料,并探讨了炭纤维体积分数为4%时,Al2O3纤维含量变化对该复合材料耐磨性能的影响。结果表明:随着Al2O3体积分数增加,复合材料的摩擦系数逐渐增大,复合材料从轻微磨损到急剧磨损的临界转变载荷大幅度提高,并随Al2O3含量的增加而逐渐增大;在临界载荷以下,影响复合材料磨损率的Al2O3含量临界值为12%,当Al2O3含量低于临界值时复合材料磨损表面无明显剥落,而当Al2O3含量超过临界值后,复合材料磨损表面存在大量的剥落坑,磨损率增大。 相似文献
963.
稀土处理玻璃纤维填充聚四氟乙烯复合材料在油润滑下的摩擦学性能 总被引:4,自引:3,他引:4
分别用偶联剂、稀土以及偶联剂 -稀土混合物处理玻璃纤维表面 ,以改善玻璃纤维与聚四氟乙烯之间的界面结合力 ,考察了玻璃纤维填充聚四氟乙烯复合材料在油润滑下的摩擦学性能 .结果表明 :在油润滑条件下 ,表面处理玻璃纤维填充聚四氟乙烯复合材料的摩擦系数比未经处理玻璃纤维填充聚四氟乙烯复合材料的低 ,耐磨性亦较优 ;而稀土处理玻璃纤维填充聚四氟乙烯复合材料具有最低的摩擦系数及最高的耐磨性和极限 pv值 ;未经处理玻璃纤维填充聚四氟乙烯复合材料的磨损形式主要为粘着转移 ,偶联剂处理玻璃纤维填充聚四氟乙烯复合材料和偶联剂与稀土处理玻璃纤维填充聚四氟乙烯复合材料均以磨粒磨损为主 ,而稀土处理玻璃纤维填充聚四氟乙烯复合材料的磨损机理主要为粘着磨损和轻微磨粒磨损 相似文献
964.
B. Hinkelmann 《Rheologica Acta》1982,21(4-5):491-493
From literature some representative equations have been compiled describing the influence of filler on the viscosity of polymer melts. By application of these on the experimental results obtained from GF-SAN it was found that the relative viscosity
R
, i.e. the ratio of the viscosities of the filled and unfilled melt, shows a pronounced dependence on the shear rate
but not on the shear stress. Defining
R
with constant and not with constant
(as it is usually done), an analytical approach is possible independent of
Further the influence of pressure, temperature and filler content on the zero-shear viscosity of filled polymer melts may be expressed by a modified Arrhenius equation. 相似文献
965.
Flow-induced fiber orientation and concentration distributions were measured in a concentrated fiber suspension (CFS) and a dilute one (DFS). The channel has a thin slit geometry containing a circular cylinder. In the previous work, many researchers have qualitatively studied fiber orientation and concentration distributions in injection-molded products of fiber-reinforced plastics. In the present work, however, they are quantitatively estimated by direct observation of fibers in the concentrated suspension flow. For the CFS, some fibers rotate in an expansion part between the channel wall and the circular cylinder, and the fiber orientation becomes almost random state. On the other hand, fibers are perfectly aligned along the flow direction owing to the elongational flow near the centerline downstream of the cylinder. The fiber concentration has a flat distribution except near the channel wall and the centerline. For the DFS a minimum in the fiber concentration distribution was clearly observed on the centerline, and two peaks beside the centerline and near the channel wall. This characteristic distribution is caused by the fiber-wall and fiber-cylinder interactions. It is found that the obstacle such as the circular cylinder in the channel significantly affects the fiber orientation downstream of the obstacle for the CFD, while it affects the fiber concentration distribution for the DFS. 相似文献
966.
The spectral interference of polarization modes in a highly birefringent (HB) fiber to measure temperature is analyzed theoretically and experimentally. A tandem configuration of a birefringent delay line and a sensing HB fiber is considered and the spectral interferograms are modelled for the known birefringence dispersion of the HB fiber under test. As the delay line, a birefringent quartz crystal of a suitable thickness is employed to resolve a channeled spectrum. The channeled spectra are recorded for different temperatures and the polarimetric sensitivity to temperature, determined in the spectral range from 500 to 850 nm, is decreasing with wavelength. It is demonstrated that the temperature sensing is possible using the wavelength interrogation, i.e., the position of a given interference maximum is temperature dependent. The temperature sensitivity of the HB fiber under test is −0.25 nm/K and the resolution is better than 0.5 K. 相似文献
967.
随着光钟研究的发展, 光钟的稳定度和不确定度均达到10-18量级. 通过光纤可以实现光钟频率信号的高精度传输, 有望用于未来“秒”定义的复现. 演示了百公里级实验室光纤上的光学频率传递. 对于在实验室70 km光纤盘上实现的光频传递, 光纤相位噪声抑制在1-250 Hz傅里叶频率范围内均接近于光纤延时极限, 对应传输稳定度(Allan偏差)为秒级稳定度1.2×10-15, 10000 s稳定度为1.4×10-18. 实验室100 km光纤的光频传递秒级稳定度也达到了5×10-15. 提出了光纤噪声用户端补偿的方案, 可以简化星形传递网络中心站的复杂度. 在25 km光纤上演示了该传递方案, 实现的传输稳定度接近传统前置补偿传递方案. 相似文献
968.
969.
光纤表面等离子体共振传感器在高灵敏度传感和在线实时监测等领域具有重要意义. 设计了一种六重准晶体结构环形通道光纤表面等离子体共振传感器, 基于有限元法对该传感器的传感特性进行了数值模拟. 研究了光纤各结构参量对传感器特性的影响规律. 研究结果表明: 待测液折射率的有效监测范围为1.25–1.331, 最高灵敏度可达26400 nm·RIU-1, 传感器具有损耗谱杂峰少、探测范围广、灵敏度高、设计灵活性高和光路可弯曲等特点, 在生化检测、公共安全、环境污染监测以及高灵敏度传感等领域具有广泛的应用前景. 相似文献
970.
Carrier-mediated three-phase hollow fiber microextraction combined with high-performance liquid chromatography-ultra violet detection (HPLC-UV) was applied for the extraction and determination of propylthiouracil in biological samples. Propylthiouracil (PTU) was extracted from 7.5 mL of the basic solution (the source phase) with pH 12 into an organic phase (n-octanol containing 6% (w/v) of Aliquat 336 as the carrier) impregnated in the pores of a hollow fiber, and finally was back extracted into 24 μL of the acidic solution located inside the lumen of the hollow fiber (the receiving phase). The extraction was performed through the gradient of counter ion from the source to the receiving phase. The effects of different variables on the extraction efficiency were studied simultaneously using an experimental design. A half-fractional factorial design was employed for screening to determine the variables significantly affecting the extraction efficiency. Then, the factors with significant effect were optimized using a central composite design (CCD) and the response surface equations were developed. The optimal experimental conditions obtained from this statistical evaluation included: source phase, pH 12; temperature, 25 °C; extraction time, 40 min; counter ion concentration, 2 mol L−1 of NaClO4; organic solvent 6% of Aliquat in octanol and without salt addition in the source phase. Under the optimized conditions, the preconcentration factors were between 125 and 198 and also the limit of detections (LODs) ranged from 0.1 μg L−1 to 0.4 μg L−1 in different biological samples. The calibration curve was linear (r2 = 0.998) in the concentration range of 0.5-1000 μg L−1. Finally, the feasibility of the proposed method was successfully confirmed by extraction and determination of PTU in human plasma and urine as well as the bovine milk and meat samples in microgram per liter, and suitable results were obtained (RSDs < 6.3%). 相似文献