全文获取类型
收费全文 | 1818篇 |
免费 | 142篇 |
国内免费 | 247篇 |
专业分类
化学 | 470篇 |
晶体学 | 24篇 |
力学 | 79篇 |
综合类 | 6篇 |
数学 | 287篇 |
物理学 | 399篇 |
综合类 | 942篇 |
出版年
2024年 | 3篇 |
2023年 | 17篇 |
2022年 | 33篇 |
2021年 | 38篇 |
2020年 | 35篇 |
2019年 | 34篇 |
2018年 | 39篇 |
2017年 | 55篇 |
2016年 | 41篇 |
2015年 | 50篇 |
2014年 | 71篇 |
2013年 | 107篇 |
2012年 | 108篇 |
2011年 | 136篇 |
2010年 | 90篇 |
2009年 | 110篇 |
2008年 | 113篇 |
2007年 | 115篇 |
2006年 | 129篇 |
2005年 | 99篇 |
2004年 | 95篇 |
2003年 | 97篇 |
2002年 | 78篇 |
2001年 | 77篇 |
2000年 | 61篇 |
1999年 | 47篇 |
1998年 | 46篇 |
1997年 | 42篇 |
1996年 | 38篇 |
1995年 | 33篇 |
1994年 | 28篇 |
1993年 | 27篇 |
1992年 | 25篇 |
1991年 | 25篇 |
1990年 | 14篇 |
1989年 | 19篇 |
1988年 | 13篇 |
1987年 | 9篇 |
1986年 | 4篇 |
1985年 | 1篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有2207条查询结果,搜索用时 343 毫秒
1.
《Operations Research Letters》2022,50(3):274-280
The minimum k-enclosing ball problem seeks the ball with smallest radius that contains at least k of m given points. This problem is NP-hard. We present a branch-and-bound algorithm on the tree of the subsets of k points to solve this problem. Our method is able to solve the problem exactly in a short amount of time for small and medium sized datasets. 相似文献
2.
ZrO2基固体电解质氧传感器的研究现状及发展趋势 总被引:6,自引:0,他引:6
近年来,由于ZrO2基固体电解质氧传感器具有高的测氧灵敏度和在高温下的化学稳定性,因而在节约能源、环境保护等方面得到了广泛的应用。该文首先介绍了ZrO2基固体电解质氧传感器的测氧原理,然后阐述了ZrO2基固体电解质氧传感器的研究现状;重点介绍了氧传感器导电理论,及在高温和低温条件下工作的氧传感器研究状况和氧传感器的应用研究,并对氧传感器未来的发展进行了分析和展望,指出为了获得高性能的氧传感器应重点发展纳米材料氧传感器。 相似文献
3.
Proton conductivity of phosphoric acid derivative of fullerene 总被引:1,自引:0,他引:1
The proton conductive property of methano [60] fullerene diphosphoric acid has been investigated under various humidity conditions at the temperature range between 15 and 45 °C. It shows proton conductivity as high as 10−2 S cm−1 at 25 °C under relative humidity of 95%. Thermal analyses including TG–DTA and thermal desorption mass spectroscopy (TDS) confirm that the compound is thermally stable up to 200 °C. Proton conduction of the compound depends very much on humidity or water content. The logarithmic conductivity at 25 °C is increased linearly with increasing relative humidity. The activation energy (Ea) estimated from the slope of log(σT) vs. 1/T is decreased from 1.08 to 0.52 eV, as the relative humidity is increased from 40% to 75%. The humidity dependence of conductivity is discussed in the light of the observed hydration isotherm. 相似文献
4.
This paper presents the importance and usage of natural zeolites, which was recently recognized in the industry. Therefore, the grinding properties of natural zeolite were studied with the emphasis on a kinetic study in a ball mill. The experimental mill employed was laboratory sized, 200 mm diameter, 191 mm length, providing a total mill volume of 6000 cm3, with a total mass of 5.62 kg of steel balls of 25 mm diameter with a charge of 20% of the mill volume and with a rotation speed of 76 rpm. The breakage parameters were determined by using single sized feed fractions of ?850+600 μm, ?600+425 μm, and ?425+300 μm for the zeolite samples. The Si (specific rate of breakage) and Bi, j (primary breakage distribution) values were obtained for those feed size fractions in order to predict the product size distributions by simulation for comparison to the experimental data. From the experimental results, Si values increased as the feed sizes became coarser, i.e., the highest Si value was 0.85 min?1 for ?850+600 μm, while the lowest Si value was 0.65 min?1 for ?425+300 μm feed ground in the mill. The Bi, j values obtained for the zeolite sample were γ=0.84, ?=0.61, and β=4.25. The breakage parameters obtained showed that the zeolite is broken faster than quartz and slower than calcite and barite in terms of the aT values reported previously at the same experimental conditions. The simulations of the product size distributions of zeolite were in good agreement with the experimental data using a standard ball mill simulation program. The slowing‐down effect was also seen in the mill after 4 min. of grinding. 相似文献
5.
1 Results Oxy-apatite materials are thought as zirconia-substitutes in Solid Oxide Fuel Cells due to their fast ionic conduction. However, the well known difficulties related to their densification prevent them from being used as such. This study presents strategies to obtain oxy-apatite dense materials and the influence of elaboration route on transport properties. Particular emphasis is put on the microstructure effect on ion conduction. By the combined use of freeze-drying and conventional or spark p... 相似文献
6.
7.
在水热晶化合成一系列NASICON型化合物的基础上,应用固体高分辨31P和29Si MAS NMR,研究了几种NASICON化合物的结构,观察其骨架原子P和Si在结构中的状态及分布,并解释了由于在结构中发生取代作用而引起,Si或P的化学位移的变化. 相似文献
8.
This paper presents a method of measuring a light field of a light source with high directional resolution using a mirrored
ball and a pinhole camera. The light field describes a spatial and directional distribution of radiances from the light source.
The directional distribution is expanded by a reflection on the mirrored ball, and the radiances are measured by a charge-coupled
device (CCD) camera with a pinhole lens. The light source is laterally moved by a robot arm to measure the directionally expanded
light field, and each pixel on a CCD can obtain the radiances from the light source through the pinhole lens with high directional
resolution. The light field is estimated from the pixel value and the position of each pixel using a ray tracing technique.
The light field of a krypton lamp was experimentally measured by the proposed method, and the accuracy of the measurement
was evaluated against the irradiances measured by a spectro-radiometer at sample points. 相似文献
9.
不良导体导热系数测定实验中仪表风扇对实验结果的影响 总被引:1,自引:0,他引:1
根据对物体冷却规律的分析,并从实验结果比较,指出稳态法测不良导体导热系数实验中,设置散热盘的冷却过程应与其在稳态过程时的强迫对流散热方式保持一致. 相似文献
10.