全文获取类型
收费全文 | 2463篇 |
免费 | 492篇 |
国内免费 | 803篇 |
专业分类
化学 | 1700篇 |
晶体学 | 39篇 |
力学 | 432篇 |
综合类 | 47篇 |
数学 | 32篇 |
物理学 | 1508篇 |
出版年
2024年 | 8篇 |
2023年 | 19篇 |
2022年 | 87篇 |
2021年 | 84篇 |
2020年 | 88篇 |
2019年 | 92篇 |
2018年 | 77篇 |
2017年 | 106篇 |
2016年 | 121篇 |
2015年 | 96篇 |
2014年 | 111篇 |
2013年 | 253篇 |
2012年 | 201篇 |
2011年 | 245篇 |
2010年 | 184篇 |
2009年 | 177篇 |
2008年 | 152篇 |
2007年 | 192篇 |
2006年 | 216篇 |
2005年 | 163篇 |
2004年 | 146篇 |
2003年 | 142篇 |
2002年 | 116篇 |
2001年 | 112篇 |
2000年 | 98篇 |
1999年 | 85篇 |
1998年 | 68篇 |
1997年 | 56篇 |
1996年 | 53篇 |
1995年 | 54篇 |
1994年 | 27篇 |
1993年 | 29篇 |
1992年 | 14篇 |
1991年 | 22篇 |
1990年 | 16篇 |
1989年 | 13篇 |
1988年 | 9篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1982年 | 3篇 |
1980年 | 2篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1974年 | 1篇 |
1971年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有3758条查询结果,搜索用时 15 毫秒
31.
The influence of substitutionally dissolved Al in ferritic Fe–4.75 at.% Al alloy on the nucleation and growth of γ′ iron nitride (Fe4N1? x ) was investigated upon nitriding in NH3/H2 gas mixtures. The nitrided specimens were characterised employing optical microscopy, scanning electron microscopy, transmission electron microscopy, electron probe microanalysis and X-ray diffraction. As compared to the nitriding of pure ferrite (α-Fe), where a layer of γ′ develops at the surface, upon nitriding ferritic Fe–4.75 at.% Al an unusual morphology of γ′ plates develops at the surface, which plates deeply penetrate the substrate. In the diffusion zone, nano-sized precipitates of γ′ and of metastable, cubic (NaCl-type) AlN occur, having, with the ferrite matrix, a Nishiyama–Wassermann orientation relationship and a Bain orientation relationship, respectively. The γ′ plates contain a high density of stacking faults and fine ε iron nitride (Fe2N1? z ) precipitates, although the formation of ε iron nitride is not expected for the employed nitriding parameters. On the basis of dedicated nitriding experiments it is shown that the unusual microstructural development is a consequence of the negligible solubility of Al in γ′ and the obstructed precipitation of the thermodynamically stable, hexagonal (wurtzite-type) AlN in ferrite. 相似文献
32.
The reverse α–α′ phase separation in Fe-20Cr-6Al alloys has been monitored by differential scanning calorimetry in as-hot rolled and recrystallized samples previously aged at 435?°C. The splitting in the position of the minima, and the increasing enthalpy change involved, might be explained in terms of dissolution of α′ particles with different compositions and volume fractions. These results are fully consistent with the α–α′ phase separation kinetics determined by atom probe tomography during ageing at 435?°C. The monitoring of activation energy for α′ dissolution indicates that it is faster in as-hot rolled samples. Finally, the activation energy evolution indicates that there is first a dissolution of β′ Fe(AlTi) phase followed by α′ dissolution. 相似文献
33.
Shibayan Roy 《哲学杂志》2013,93(5):447-463
Deformation instabilities, such as shear cracking and grain boundary cavitation, which are observed in the secondary tensile region of Ti–6Al–4V alloy during compressive deformation in the (α?+?β)-phase field, do not form in Ti–6Al–4V–0.1B alloy when processed under the same conditions. This has been attributed to the microstructural modifications, e.g. the absence of grain boundary α and adjacent grain boundary retained β layers and a lower proportion of 90o-misoriented α-colonies that occur with boron addition. 相似文献
34.
35.
Abstract The kinetics of radiation defect accumulation and subsequent recovery during/after electron irradiation below 273 K, at 323 K and 373 K were investigated for the Fe-15.7 at.% Cr using positron annihilation measurements at room temperature. Formation of vacancy clusters was observed at all of the irradiation temperatures. The formation of clusters and kinetics of their accumulation point to mobility of vacancies at least at room temperature. The cluster rearrangement and variations in the cluster configuration take place during annealing. 相似文献
36.
为了使激光冲击强化技术能较好地应用于TC6钛合金的发动机叶片,对TC6钛合金进行试验研究。通过X射线衍射仪、透射电子显微镜等测试技术分析了不同参数下TC6钛合金的微观组织变化,用显微硬度计和残余应力测试仪分别表征表层硬度和残余应力变化,并测试材料冲击后的振动高周疲劳性能。试验结果表明:激光冲击材料后表面组织得到明显细化,随着冲击次数的增加,先后出现了高密度位错、位错胞、亚晶和纳米晶。性能方面,表面硬度在冲击一次即可提高19%,硬度影响深度达到700 m;与此同时表面残余应力最高达到-608.5 MPa,在500 m深度上仍具有-100 MPa左右的应力存在。经三次冲击后,标准疲劳试片的疲劳极限提高近20%。 相似文献
37.
Structural analyses of Fe-Cr and Fe-Cr-Ni low‐alloyed steels before and after annealing at 600○C using Mössbauer spectrometry, X‐ray diffraction and scanning electron microscopy are reported. At the received state, the two steels present a fine bainitic microstructure with equal hardness. From Mössbauer analysis, two iron sites are identified as substitutional by Cr, Mo, Ni atoms and as insertional by carbon in bainitic ferrite. Both sites are locally deformed by residual stresses. The presence of small quantities of retained austenite and ε carbide has been observed. Annealing for one hour at 600○C causes a decrease in hardness for both steels with a decrease of retained austenite. After longer time of annealing, precipitation of (Fe,Ni)23C6 occurs in the Fe-Cr-Ni steel and increases hardness. 相似文献
38.
C. Papatriantafillou C. E. Paraskevaidis C. Krikos P. Panagopoulos 《Journal of statistical physics》1989,54(1-2):191-200
We study a generalized Ising system consisting of a Bethe lattice on every site of which two spin-1/2 and two isospin-1/2 (or atomic species A) states can be realized, the spin-species interacting with appropriate nearest neighbor couplings. The system is equivalent to that of four states per site and we obtain its exact thermodynamic behavior. The case of a fixed concentration of species is the annealed or liquid magnetic binary alloy. The temperature dependence of the short-range-order (SRO) parameter of such systems is obtained as an application of the theory and discussed in connection with relevant material from the literature. When fixing both the concentration and the nearest neighbor spatial correlation of the species, we obtain Eggarter's formulas for the frozen-in species problem, which therefore are only approximately valid even on the Bethe lattice. 相似文献
39.
40.
Active surfaces of plastic injection moulds are nowadays textured using classical techniques (chemical etching or EDM). Replacement of these technologies by a laser technology introduces a big flexibility: absence of mechanical contact with the tool, decrease of the effluent's volume and a big machining precision, even in the case of the complex forms as injection moulds for example. This paper reports the experimental study of the surface laser texturing of TA6V alloy. The influence of the operating factors on the laser texturing process has been studied using two experimental approaches: Taguchi methodology and response surface methodology (RSM). Empirical models have been developed. They allowed us to determine a correlation between process operating factors and performance indicators, such as surface roughness and material removal rate. Results analysis shows that the laser pulse energy and frequency are the most important operating factors. Mathematical models, that have been developed, can be used for the selection of operating factors’ proper values in order to obtain the desired values of the objective functions. 相似文献