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261.
The tensile deformation of nanocrystalline α-Fe2O3+fcc-Al composites at room temperature is analyzed using molecular dynamics (MD) simulations. The analyses focus on the effects of variations in grain size and phase volume fraction on strength. For comparison purposes, nanostructures of different phase volume fractions at each grain size are given the same grain morphologies and the same grain orientation distribution. Calculations show that the effects of the fraction of grain boundary (GB) atoms and the electrostatic forces between atoms on deformation are strongly correlated with the volume fractions of the Al and Fe2O3 phases. In the case of nanocrystalline Al where electrostatic forces are absent, dislocation emission initiates primarily from high-angle GBs. For the composites, dislocations emits from both low-angle and high-angle GBs due to the electrostatic effect of Al-Fe2O3 interfaces. The effect of the interfaces is stronger in structures with smaller average grain sizes primarily because of the higher fractions of atoms in interfaces at smaller grain sizes. At all grain sizes, the strength of the composite lies between those of the corresponding nanocrystalline Al and Fe2O3 structures. Inverse Hall-Petch (H-P) relations are observed for all structures analyzed due to the fact that GB sliding is the dominant deformation mechanism. The slopes of the inverse H-P relations are strongly influenced by the fraction of GB atoms, atoms associated with defects, and the volume fractions of the Al and Fe2O3 phases. 相似文献
262.
《Comptes Rendus Chimie》2015,18(10):1094-1105
Nanocrystalline tetragonal zirconia powders have been synthesized by aqueous combustion using glycine (Gly) as a fuel and zirconyl nitrate (ZN) as an oxidizer. The effect of the fuel-to-oxidant molar ratio on the structural and morphological properties of nanocrystalline zirconia powders was studied. Thermodynamic modeling of the combustion reaction showed that the increase in the Gly:ZN molar ratio leads to the increase in theoretical combustion temperature, heat of combustion and amount of produced gases. Powder properties were correlated with the nature of combustion and results of thermodynamic modelling. The increase in the Gly:ZN molar ratio produces more agglomerated powders characterized by a lower degree of uniformity, a lower specific surface area and a slightly bigger crystallite size. On the other hand, the presence of hard agglomerates suppresses the volume expansion, stabilizing tetragonal zirconia, as confirmed by Rietveld refinement. The absence of cubic zirconia was confirmed by FTIR and Raman Spectroscopy. The increase in the calcination temperature led to more agglomerated, compact and less uniform powders. The nanocrystalline nature of zirconia is the reason for the formation of bigger crystallites, the increase in the relative amount of monoclinic phase and sample sintering after calcination at high temperature. The highest measured specific surface area of zirconia was 45.8 m2·g−1, obtained using a fuel-lean precursor. 相似文献
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264.
To develop gas sensing materials with high performance,high sensitivity,excellent selectivity and quick response & recovery behavior,nanocrystalline material of rare-earth composite oxide HoFeO3 with the structure of perovskite type was synthesized by sol-gel method in the system of citric acid with the Ho2O3,Fe(NO3)3·9H2O,nitric acid(1: 1 vloume fraction)as the starting materials. The structure and crystal state of the powder were determined on an X-ray diffractometer(Germany Bluker D8-Advance)with a Cu K" radiation(wavelength λ = 0. 15406 nm)operating at 20 mA and 40 kV. The shape and size were analyzed with the help of JEM-100SX Transimission electron microscopy. The results show this perovskite-type oxide is spherical with the mean grain size of 25 nm and the dispersity of it is good. The influence of temperature on the sensitivity of sensors,gas sensors's selectivity and the response and recovery characteristics are tested at the optimum working temperature 310℃. The study of sensor's gas sensing characteristic shows that the sensitivities of HoFeO3 to 0. 5! C2H5OH is 103,which is 5 times of other tested gases,such as H2S、H2、SO2、gasoline and acetone. So the sensors based on HoFeO3 show good sensitivity and selectivity to C2H5OH. The response and reversion characteristic of sensor to 0. 5! ethanol at 310℃ is good too. The response time and recovery time are 12 and 7s,respectively. 相似文献
265.
Many important processing techniques for nanocrystalline solids, such as ball milling and compaction, are frequently accompanied by the presence of voids in the end products. These voids can apparently lower the yield strength of the material. In order to address the issue of competition between grain size and porosity, we develop an explicit, analytical composite model that allows us to determine the viscoplastic response of a porous, nanocrystalline solid. The development made use of the concept of a three-phase composite comprising of the plastically harder grain interior, plastically softer grain-boundary affected zone (GBAZ), and porosity. A homogenization theory that accounts for the evolution of porosity during plastic flow is established. This establishment is built upon the extension of a linear viscoelastic composite to a non-linear viscoplastic one, in which the viscoplastic behavior of the constituent phases is represented by a unified constitutive law. Then by means of a field fluctuation method, the local strain rates are linked to the applied total strain rate. Such a linkage in turn provides the secant viscosity of the constituent phases at every stage of deformation. In order to test the applicability of the developed theory, we have applied it to model the viscoplastic response of an iron and an iron–copper mixture tested by Khan et al. [Khan, A.S., Zhang, H., Takacs, L., 2000. Mechanical response and modeling of fully compacted nanocrystalline iron and copper. Int. J. Plasticity 16, 1459–1476] and Khan and Zhang [Khan, A.S., Zhang, H., 2000. Mechanically alloyed nanocrystalline iron and copper mixture: behavior and constitutive modeling over a wide range of strain rates. Int. J. Plasticity 16, 1477–1492]. It is demonstrated that the theory is capable of capturing the major features of the tested results at various grain sizes and porosities. Our calculations further point to the change of yield strength in the Hall–Petch plot from an initial increase to level off, and then to decline, at various porosities under a constant strain-rate loading. This in turn brings about the existence of a critical grain size in the nano-meter range at which the material exhibits maximum yield strength. Moreover, this critical grain size tends to move to the left in the Hall–Petch plot as the GBAZ becomes softer. 相似文献
266.
267.
Sol-gel法制备La1-xSrxMnO3巨磁电阻薄膜材料 总被引:8,自引:3,他引:5
利用sol-gel法在单晶硅Si(100)衬底上,制备了钙钛矿La1-xSrxMnO3(LSMO)低织构纳米晶薄膜.考察了制备条件对LSMO成相、晶粒的粒度和表面形貌的影响,研究了其磁特性和磁电阻性能.结果表明,La0.7Sr0.3MnO3薄膜磁电阻效应在相当宽的温度范围内不随温度改变,在6T磁场中室温下磁电阻比MR值可达-30%. 相似文献
268.
纳米ZnO薄膜可见发射机制研究 总被引:12,自引:5,他引:7
利用溶胶-凝胶法 (Sol-Gel)制备了纳米ZnO薄膜,获得了高强的近紫外发射室温下测量了样品的光致发光谱(PL )、吸收谱(ABS)、X射线衍射谱(XRD).X射线衍射(XRD)的结果表明:纳米ZnO薄膜呈多晶态,具有六角纤锌矿结构和良好的C轴取向;发现随退火温度升高,(002)衍射峰强度显著增强,衍射峰的半高宽(FWHM)减小、纳米颗粒的粒径增大.由吸收谱(ABS)给出了样品室温下带隙宽度为3.30 eV.在PL谱中观察到二个荧光发射带,一个是中心波长位于392 nm附近强而尖的紫带,另一个是519 nm附近弱而宽的绿带研究了不同退火温度样品的光致发光峰值强度的变化关系,发现随退火温度升高,紫带峰值强度增强、绿带峰值强度减弱,均近似呈线性变化.证实了纳米ZnO薄膜绿光发射主要来自氧空位(Vo)形成的浅施主能级与锌空位(VZn)形成的浅受主能级之间的复合,或氧空位(Vo)形成的深施主能级上的电子至价带顶的跃迁;紫带来自于导带中的电子与价带中的空位形成的激子复合. 相似文献
269.
270.
The nanocrystalline cubic, tetragonal, and submicron monoclinic phases of pure zirconia were prepared by thermal decomposition of carbonate and hydroxide precursors. The crystallization and isothermal phase transformations of the oxide were studied using high temperature X‐ray diffraction, X‐ray diffraction and Raman spectra of quenched samples. Cubic zirconia formed first, and then progressively transformed to tetragonal and monoclinic phases at temperatures as low as 320°C. The cubic, tetragonal, and monoclinic phases for ZrO2 were found to be distinct functions of crystallite size, indicating the nanocrystalline nature of these phases. They were found to exist within critical size ranges of 50 to 140 Å, 100 to 220 Å and 190 to 420 Å (±5 Å), respectively. Thus, as the crystallites grow during annealing, they first transform from cubic to tetragonal and then from tetragonal to monoclinic at critical sizes. The classical Avrami equation for nucleation and growth was applied to the tetragonal to monoclinic phase transition. 相似文献