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排序方式: 共有132条查询结果,搜索用时 296 毫秒
1.
Co-precipitation of nano Mg–Y/ZrO2 ternary oxide eutectic system: Effects of calcination temperature
《Ceramics International》2022,48(16):23452-23459
In the family of inorganic nanomaterials, zirconia is a highly promising functional ceramic with a high refractive index, hardness, and dielectric constant, as well as excellent chemical inertness and thermal stability. These properties are enhanced in nano-zirconia ceramics, because nanopowders have a small particle size, good morphology, and uniform and dispersive distribution. In this study, a co-precipitation process was proposed to synthesise highly dispersed MgO–Y2O3 co-stabilized ZrO2 nanopowders. The effects of different calcination temperatures on the crystallisation degree and particle dispersion of zirconia nanopowders were characterised by X-ray diffraction (XRD), thermogravimetry-differential scanning calorimetry (TG-DSC), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), nitrogen adsorption using the Brunauer–Emmett–Teller (BET) theory, transmission electron microscopy (TEM), and field emission scanning electron microscopy (FESEM). The optimum synthesis conditions were obtained as follows: 6 h of high-energy planetary grinding and calcination at 800 °C in an electric furnace. Under these optimum conditions, the average particle size of the prepared powder was 28.7 nm. This process enriches the literature on the controllable preparation of Mg–Y/ZrO2 nanopowders obtained by the co-precipitation method. 相似文献
2.
L.Q. Xing P. Ochin M. Harmelin F. Faudot J. Bigot J.P. Chevalier 《Materials Science and Engineering: A》1996,220(1-2)
Cylindrical bulk amorphous samples with diameters up to 10 mm have been prepared by casting ZrTiAlCuNi alloys in a copper mould. In order to rank glass-forming ability as a function of alloy composition, alloys were also cast into wedge-shaped moulds; to a first approximation, the thickness of the amorphous region obtained can be taken as an indication of glass-forming ability. The compositions which lead to the production of bulk glasses all have reduced glass transition temperatures in excess of 0.65 and the extremely high glass-forming ability of these compositions is discussed. We suggest that both the Al and Ti contents are determining factors for the production of bulk amorphous samples and these are believed to reduce the driving force for, and hence the rate of, crystallisation. These amorphous alloys have been found to display high thermal stability and can be annealed for several minutes in the supercooled liquid region. They are ductile at room temperature and have a high value of yield stress. 相似文献
3.
The thermal stability of polymers is a main issue when used as friction elements under dry sliding. Cast polyamide grades processed with either natrium or magnesium catalysors are slid on a small-scale and a large-scale test configuration to reveal the effect of softening or degradation on the sliding stability and to investigate possibilities for extrapolation of friction and wear rates between both testing scales. The combination of softening and afterwards transition into the glassy state is detrimental for the sliding stability of natrium catalysed polyamides, characterised by heavy noise during sliding. A transfer film formed under continuous softening also provides high friction. Melting during initial sliding is necessary for stabilisation in both friction and wear, and eventual softening of a molten film near the end of the test then not deteriorates the sliding stability. Softening of magnesium catalysed polyamides is favourable for the formation of a coherent transfer film resulting in more stable sliding than natrium catalysed polyamides. The differences in softening mechanisms of both polyamide grades is correlated to structural changes investigated by thermal analysis and Raman spectroscopy: the γ crystalline structure prevails in magnesium catalysed samples and the α crystalline structure is predominant in natrium catalysed samples. For internal oil lubricated polyamides, a time dependent degradation of the polyamide bulk deteriorates the supply of internal oil lubricant to the sliding interface, resulting in high friction and wear under overload conditions. As the degradation mechanisms during sliding are strongly correlated to the test set-up, extrapolation is only possible for friction in a limited application range, while wear rates cannot be extrapolated. 相似文献
4.
Poly(l-lactic acid) with 4% d-lactic acid comonomer has been drawn in the amorphous state at 80, 90, 100, 110 and 120 °C at an extension rate of 4 s−1 while simultaneously recording WAXS and SAXS patterns at intervals of 0.12 s. At 80, 90 and 100 °C, crystallisation is very rapid (1-4 s−1) and follows a first order transformation process to give highly oriented crystals. SAXS patterns were barely detectable at these temperatures despite fractional crystallinity of ∼0.2. At 110 and 120 °C, crystallisation was very slow (∼0.01 s−1) and gave rise to crystals with a lower degree of orientation. After eventual crystallisation at 120 °C, a two-point SAXS pattern develops with narrow lateral spread, suggesting ‘shish kebab’ morphology. When the 80 °C drawn sample was annealed at 120 °C, a strong four point SAXS pattern develops. The change in drawing and crystallisation behaviour at higher draw temperature is attributed to the onset of chain retraction relaxation processes. The WAXS fibre pattern after annealing shows sampling on intermediate layer lines that is consistent with the α crystal form with a 103 helix. However, prior to annealing, sampling indicates a different, less defined helical configuration. 相似文献
5.
The phase segregation in P3HT:PCBM blend films has been investigated from an experimental and theoretical viewpoint. Optical microscopy, atomic force microscopy, scanning electron microscopy and X-ray diffraction show that thermal annealing of P3HT:PCBM blend films leads to the formation of PCBM aggregates. These aggregates are composed of dense randomly packed ∼50 nm PCBM crystallites with an overall aggregate density of ∼0.85 g cm−3. By applying the critical radius of nucleation for PCBM and the Stokes-Einstein equation for mobility of PCBM in a P3HT matrix, a model is developed which explains the formation of both crystallites and aggregates. 相似文献
6.
M.-W. Choi 《钢铁冶炼》2017,44(7):544-550
The current research attempted to investigate the crystallisation mechanism of iron oxide-devoid basic oxygen furnace (BOF) slag with adding SiO2. First, the glass sample was prepared by adding 29?wt-% of SiO2 to BOF slag, followed by eliminating iron oxide by reduction process. Non-isothermal DSC analysis together with confocal laser microscopy, XRD and EPMA mappings were carried out to observe the crystallisation process. The glass sample showed that the crystallisation process started from the surface where the main phases were identified to be akermanite, merwinite and wollastonite. In addition, the crystallisation process was affected by the nucleation temperature which was decided by the heating rate because of the difference in the nucleation rate between wollastonite and Mg-rich phases. The current results could be used to propose the feasibility of utilising BOF slag as glass-ceramics by chemical modification with heat treatment, which controls the crystallisation behaviour. 相似文献
7.
《粉末冶金学》2013,56(3):445-449
AbstractIn the present study, crystallisation behaviours of the Al80Fe10Ti10 amorphous phase (prepared by mechanical alloying) has been investigated using X-ray diffraction and differential thermal analysis techniques. It was found that Al80Fe10Ti10 amorphous phase exhibits one-stage crystallisation on heating [amorphous to Al13Fe4, Al(Fe,Ti) and Al5Fe2 intermetallic phases]. The activation energies for crystallisation of Al80Fe10Ti10 amorphous phase were determined as 326, 322, 351 and 301 kJ mol?1 by means of the Kissinger–Akahira–Sunose, Flynn–Wall–Ozawa, Friedman and Afify equations respectively. Kinetic parameters such as Avarmi exponent or reaction order n and frequency factor Ko were also discussed. The reaction orders obtained for the crystallisation was ~2·03, suggesting that a diffusion controlled crystallisation process with a decreasing nucleation rate was the crystallisation mechanism. The deduced Ko value for crystallisation peak was calculated to be 6·75×1010 s?1. 相似文献
8.
We investigate the effect of absorption of polar solvents (water and ethanol) on the structure of amorphous and semi-crystalline copolyamides containing aromatic entities. Different length scales of structural organisation are probed using a combination of Small angle X-ray scattering and neutron/X-rays diffraction. Crystallisation in the presence of solvent, characterised by very slow kinetics, is observed in the originally amorphous material, as a combined effect of a rearrangement of the hydrogen-bond distribution and plasticization. The steady-state crystalline structure is reached on a timescale much longer than the absorption kinetics, and is similar but not identical to that of processed semi-crystalline materials. In semi-crystalline polyamides water absorption induces a swelling of the spacing of the lamellar stacking but also modifications of the unit cell structure, which suggest a moderate but significant rearrangement of the hydrogen bond structure in the crystalline phase. 相似文献
9.
《Materials Science & Technology》2013,29(2):134-138
AbstractDifferential strain rate tests have been carried out at different temperatures and at various stages along the stress-strain curve at a given temperature for a series of Ni—Co alloys of varying stacking fault energy (SFE). The thermal stress component has been estimated and apparent activation enthalpies have been determined. The likely reasons for poor applicability of the Reed-Hill approach to modelling flow stress and activation parameters for a low SFE alloy, and its better applicability to the temperature range where thermal obstacles conform to a linear force-displacement relationship, are discussed. A mixed analysis including both the Reed-Hill approach and a conventional method is suggested for the determination of activation parameters using limited experimentation. 相似文献
10.
Effect of sugar, cocoa particles and lecithin on cocoa butter crystallisation in seeded and non-seeded chocolate model systems 总被引:1,自引:0,他引:1
The effect of major chocolate ingredients (sugar, cocoa particles and lecithin), in combination with the two pre-crystallization techniques, seeding and non-seeding, was investigated with respect to the kinetics of cocoa butter crystallisation and the resulting microstructure. Confocal laser scanning microscopy (CLSM) was used to monitor microstructural evolution under dynamic thermal conditions. DSC measurements and image analysis were also applied in order to quantify the impacts of processing and formulation on microstructure. All ingredients and pre-crystallisation techniques considered proved to have a large impact on fat crystallisation kinetics and the resulting microstructure. Seeded samples tended to form multiple nucleation sites, inducing rapid growth of a crystal network. The non-seeded samples showed an altering structure, with some domains developing large spherical crystals while in other domains a more heterogeneous microstructure resulted. Lecithin showed a remarkable impact on crystallisation kinetics in both the seeded and non-seeded samples. For the seeded samples, the effect was most noteworthy in samples containing cocoa butter and sugar, where lecithin significantly reduced the induction time. In the absence of seeds, lecithin itself acted as the nucleation site for fat crystallisation. 相似文献