全文获取类型
收费全文 | 1580篇 |
免费 | 132篇 |
国内免费 | 46篇 |
专业分类
化学 | 778篇 |
晶体学 | 86篇 |
力学 | 414篇 |
综合类 | 22篇 |
数学 | 93篇 |
物理学 | 365篇 |
出版年
2024年 | 5篇 |
2023年 | 13篇 |
2022年 | 25篇 |
2021年 | 20篇 |
2020年 | 36篇 |
2019年 | 28篇 |
2018年 | 27篇 |
2017年 | 37篇 |
2016年 | 54篇 |
2015年 | 41篇 |
2014年 | 62篇 |
2013年 | 89篇 |
2012年 | 71篇 |
2011年 | 76篇 |
2010年 | 68篇 |
2009年 | 89篇 |
2008年 | 92篇 |
2007年 | 82篇 |
2006年 | 95篇 |
2005年 | 104篇 |
2004年 | 85篇 |
2003年 | 75篇 |
2002年 | 62篇 |
2001年 | 61篇 |
2000年 | 47篇 |
1999年 | 38篇 |
1998年 | 47篇 |
1997年 | 37篇 |
1996年 | 25篇 |
1995年 | 36篇 |
1994年 | 29篇 |
1993年 | 15篇 |
1992年 | 15篇 |
1991年 | 17篇 |
1990年 | 13篇 |
1989年 | 7篇 |
1988年 | 13篇 |
1987年 | 7篇 |
1986年 | 4篇 |
1984年 | 2篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有1758条查询结果,搜索用时 15 毫秒
101.
Liang Liu Dujuan Zeng Xianlong Wei Qing Chen Xide Li 《Acta Mechanica Solida Sinica》2009,22(6):584-592
Two kinds of clamping in micro/nano scale experiments are investigated in this paper, one based on electron-beam-induced deposition, and the other on the van der Waals interaction. The clamping strength and mechanism are analyzed both theoretically and experimentally. The influence of relative humidity on the micro/nano clamping and the method of electrostatic clamping are discussed. The clamping strength and performance of different clamping methods are compared considering the size and material of the clamped objects, and the application environments. 相似文献
102.
The Drivers, Pressures, State, Impact and Response or DPSIR framework has been with us for over a decade now and it is widely used as a means to assess and measure and, eventually provide a guide to managing the environment. With its repertoire of diagnostic and analytical components the DPSIR can be argued to be a Problem Structuring Method or PSM. Criticisms of the framework abound but it has a resilience which is noteworthy. Some argue that DPSIR, by its nature, is a narrowly formulated, engineering device, incompatible with the multiple perspectives which human interaction in global ecology requires. Is there a value in DPSIR being more flexible in expression and experience of users? In this article it is shown how the DPSIR framework was applied within a multi-methodology approach called Imagine in a number of coastal management projects around the Mediterranean and in other contexts. The article argues that DPSIR, whilst admittedly limited in its scope and approach can, if applied in a participatory and systemic multi-methodology, combine with other tools and help to create outcomes of value to local populations. 相似文献
103.
104.
东海区海岸带景观格局变化对生态系统服务价值的影响 总被引:2,自引:0,他引:2
对大尺度、长时序的海岸带景观格局和生态系统服务价值(ecosystem services value,ESV)的研究,有利于更好地促进区域生态系统保护工作的开展。以1990,1995,2000,2005,2010,2015年6期遥感影像解译结果为基础,对东海区海岸带景观格局变化特征和ESV时空演变进行了分析,并探讨了ESV与景观格局变化二者间的关系。结果表明,1990—2015年的25 a间,受人类活动的影响,东海区海岸带农田和海域面积大量减少,建设用地面积大幅增加;在所研究的5类地貌类型中,除中起伏山地外,其余地貌类型景观面积转移都较为剧烈。25 a间东海区海岸带斑块数量、斑块密度、边界密度、shannon多样性指数等不断增加,景观破碎化、复杂化、多样化增强。农田景观的最大斑块指数、形态指数、边界密度等变化均较大。东海区海岸带ESV呈先增加后减小的趋势,其中森林景观对ESV总量贡献最大;水文调节、生物多样性维持、气候调节、土壤保持、废物处理和气体调节是东海区海岸带主要的生态系统服务功能;在空间上表现出以单位面积中、低ESV为主导的空间分布,高、极高和极低ESV分布较少。研究区ESV总量与农田和海域以及水域和建设用地面积显著相关;与斑块密度、边界密度和形态指数呈显著负相关。同时,食物生产、气体调节、气候调节、土壤保持和生物多样性维持的服务价值与景观指数间均显著相关。 相似文献
105.
提出分析中心裂纹板剪切断裂的断裂过程区D-B模型的叠加原理,考虑了压剪断裂的摩擦阻力和体现有限权宽影响的载荷修正,以非常简便的方法推导出计算断裂过程区长度和位移的公式。 相似文献
106.
C.W. Tan Y.C. Chan Bernard N.W. Leung John Tsun Alex C.K. So 《Optics and Lasers in Engineering》2005,43(2):374
For the packaging of a pump laser in butterfly package, the most crucial assembly step is the fiber-to-laser diode coupling and attachment. The use of laser welding as the joining method offers several advantages if compared with the adhesive joints: strong joining strength, short process time and less contamination. This paper reports on laser welding process characteristics; weld strength and its fracture mode. The penetration depth and melt area of laser spot welds were found to be complicated functions of laser pulse energy, intensity, and beam diameter. Effects of pulse width, input power and size of the focal spot on the rate of energy input to the workpieces and consequently, the weld strength were reported. The weld strength was found to be dependent on the overlapping area between the two joining materials. Surface roughness, Ra, has influence on the fraction of energy absorbed, A, and therefore, affecting the penetration depth. Thermal analysis was carried out on the laser-welded joints and its heat-affected zone (HAZ) induced by various power densities was examined. These data are important in order to optimize and utilize the laser welding process as an effective manufacturing tool for fabrication of reliable pump laser. 相似文献
107.
A. Alsaad A. Ahmad 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,54(2):151-156
First-principles calculations are performed to compute the e33
piezoelectric coefficients of GaN, ScN and (ScxGa1-xN) alloys
exhibiting an alternation of hexagonal GaN, with hexagonal ScN along the c-axis. For Sc compositions larger than 50%, each atom has nearly five nearest
neighbors (i.e., the ground state exhibits a phase that is five-fold
coordinated). On the other hand, Sc-deficient (Sc, Ga) N alloys adopt a
ground-state that is four-fold coordinated). The magnitude of e33 in
the Sc-deficient ideally ordered (Sc0.25Ga0.75N) is found to be larger
than the magnitude of the corresponding e33 coefficients resulting from
the compositional weighted average over the hexagonal (h-ScN) and the wurtzite (w-GaN) parent compounds. On the other hand, the e33 coefficients of the Sc-rich
ordered (Sc0.75Ga0.25N) is found to be negligibly small. In addition, e33
piezoelectric coefficients in ordered (Sc0.5Ga0.5N) exhibit quite
large magnitudes, due to the nonpolar to polar transition occurs at Sc composition x = 0.5, and thus can bridge the corresponding coefficients of (Ga, In) N and
ferroelectric alloys. The microscopic origins for this huge enhancement in
the piezoelectric behavior in Sc-low and Sc-intermediate alloys and the role
of each atom are revealed and discussed. 相似文献
108.
Crystal growth,structure and optical properties of Pr~(3+) -doped yttria-stabilized zirconia single crystals 下载免费PDF全文
《中国物理 B》2021,30(7):78103-078103
The development of blue semiconductor light-emitting diodes(LEDs) has produced potential applications for Prdoped materials that can absorb blue light, especially crystals, and we now report structure and optical properties for high-quality Pr-doped single crystals of yttria-stabilized zirconia(YSZ) grown by the optical floating zone(FZ) method.X-ray diffraction(XRD) and Raman spectroscopy showed that all of the single crystal samples were in the cubic phase,whereas the corresponding ceramic samples contained a mixture of monoclinic and cubic phases. X-ray photoelectron spectroscopy(XPS) and electron paramagnetic resonance(EPR) spectroscopy showed that Pr was present as the Pr~(3+) ion in ceramic rods and single crystals after heating to high temperatures. The absorption and photoluminescence excitation(PLE) spectra of the Pr-doped YSZ crystals measured at room temperature showed strong absorption of blue light, while their photoluminescence(PL) spectra showed five emission peaks at 565 nm, 588 nm, 614 nm, 638 nm, and 716 nm under450 nm excitation. The optimum luminescence properties were obtained with the crystal prepared using 0.15 mol% Pr_6O_(11),and those with higher concentrations showed evidence of quenching of the luminescence properties. In addition, the color purity of Pr-doped YSZ single crystal reached 98.9% in the orange–red region. 相似文献
109.
Results of an investigation of in situ measurements of laser‐beam intensity Is and Im transmitted through aqueous ammonium dihydrogen phosphate (ADP) solutions saturated at 30 °C and water, respectively, and temperature Ts of solution and Tm of water during feeding of antisolvent acetone at different rates RA, using an indigenously designed experimental setup, are presented and discussed. It was found that: (1) for the measurement of MSZW, defined as the maximum volume fraction of acetone content Δxmax in the solution, obtained from temperature measurements are more reliable than transmitted laser‐beam intensity measurements for solutions, (2) two minima ΔTmin1 and ΔTmin2 associated with endothermic reactions, separated by a maximum ΔTmax due to exothermic reactions appear in the plots of temperature difference ΔT = Ts−Tm against acetone feeding time t, and (3) in the ΔT(t) plots there are time intervals Δt of constant rates RT of increase in ΔT of aqueous ADP solutions, and these values of RT increase linearly with acetone feeding time rate RA. The experimental data on the observed dependence of MSZW on antisolvent feeding rate RA, the appearance of minima ΔTmin1 and ΔTmin2 and maximum ΔTmax and their dependence on RA, and the relationship between RT and RA are discussed from consideration of processes of nucleation and growth of crystallites. 相似文献
110.
A simple method is introduced in this study to better understand the growth process of hemihydrate phosphoric acid crystals. Using the proposed approach, large particles (>2 mm) can be produced reflecting the true state of the phosphoric acid particulate system. In contrast to previous studies, the principles for the crystallization curves were divided into two regions, based on the consumption of the phosphoric acid in solution. To maintain a constant crystal growth rate, a programmed cooling requirement was needed. In this reported study, the influence of a complexing agent (EDTA) on the crystallization process of phosphoric acid was also investigated. The results of this study showed that the presence of EDTA affected the metastable zone widths, as well as the kinetics of crystal growth. 相似文献