Search results

But a large number of surface acidic sites of Pd/γ-Al2O3 are covered by highly fragmented Pd-grain, it causes DME selectivity reduced.
The internal heat of the catalysts was uneven, it casese a large number of cracks during the heating in muffle.
And more Pd loading makes larger Pd grains increase and worse Pd dispersion.
The author learns that when the loading is within 2%, the reduction of surface acid is mainly because of acidic sites covered by Pd metal; when the loading is between 2% and 3%, the dispersion of Pd drops severely, the grain particle size increases slightly and the number of grain nearly unchanged, the covered area just increases slightly, therefore the amount of surface acid reduces slightly; but when the loading is more than 3%, some γ-Al2O3 pores are blocked by the excessive metal Pd grains, it makes that the amount of acid in pore can not be detected, and the amount of surface acid reduces significantly again.
Besides, a large number of Pd grains have spread and mounted to the pore of γ-Al2O3 carrier, it leads to that the Pd activity surface and the acid activity surface of catalyst reduce simultaneously, and finally leads to the activity of catalyst reduced.

However, at larger number of revolution the deformation progressively reaches the sample centre and the final material exhibits an ultrafine grained composite structure.
Pure metals or commercial alloys processed by SPD typically exhibit a grain size in a range 100 to 500 nm.
However, it has been demonstrated that composite materials (for instance multi-phase metallic alloys) processed by SPD may exhibit a much smaller grain size down to 20nm [3, 4, 5, 6].
Thus interphase boundaries play a critical role in the grain size reduction mechanism.
For higher number of revolutions by HPT, the deformation progressively reaches the sample center and the original lamellar structure completely disappear (Fig. 3 and Fig. 4)).

Due to their dissimilar properties and different deformation mechanisms between grain interior (GI) and grain boundary affected zone (GBAZ) in the nanocrystalline (NC) materials, a two-phase composite model consisting of GI and GBAZ was developed and adopted to build strain gradient plasticity theory.
Introduction Compared with conventional coarse-grained polycrystalline metals, nanocrystalline (NC) materials are structurally characterized by a large volume fraction of grain boundary (GB), which may significantly affect their overall physical, mechanical and chemical properties.
For the sake of simplicity, these grains will be further idealized as spherical inclusions.
According to geometrical deformation condition, shear strain γG�Ds created by GNDs can be obtained / G�Ds GBAZ GI GSnb d γ γ γ φ = − = (1) where n is the number of GNDs around the GI in a single grain, dGS is grain size, φ is the geometrical factor and b is Burgers vector.
It can be seen that the predictions are in good agreement with the experimental results, especially for grain size 23 nm with higher plasticity hardening rate.

Repeated quenching is used as a grain refinement method.
Repeated quenching is used as a grain refinement method.
The number of measurements was three times for each.
The total number of cycles was 1.0×108.
The horizontal axis shows the total number of cycles to failure.

In order to simplify of calculations, the thickness of the film is expressed in number of monolayers.
The recent studies show that the excess chemical potential of the surface relative to grain boundaries produces a net flow of adatoms into the grain boundaries which generates compressive stress within the grains [8,9].
The nucleation of dislocations and glide in grains result in the plastic deformation and fragmentation of grains and in the formation of “slip-steps”.
If dislocation movement, which occurs at the grain boundaries, is prevented the formation of subgrains within the original grain structure takes place.
The oxidation rate changes as the number of grain boundaries available for the atomic transport across the oxide scale changes.

The 300-500 nm grain size has been reached (see Fig. 7).
As a result of TE, the grain refinement to submicron level has occurred (see Fig. 8).
This fact allows us to increase considerably the number of patients which can be operated in order to insert such implants.
Zhu: Producing Bulk Ultrafine-Grained Materials by Severe Plastic Deformation.
Symp. on Ultrafine Grained Materials, eds.

For 800 ℃, the equiaxial recrystallized grains were obtained.
The number of recrystallization nucleus increased for annealing at 700℃(Fig.4(e)).
In fact, recrystallization grain nucleated on the boundary of grains at 660℃.
Of course, long held time led to other grains recrystallize.
The number of recrystallization nucleus increased for annealing at 700℃.

Although this method could make error in the interface of different parts of the simulation area when the grains go through the interface, but the error has less effect on the grain growth.
Although the phase field simulation of 3D grain growth has been achieved, such as equal axial grain [7], dendrites [8], columnar crystals during directional solidification [9], but these are only limited to a relatively small computational space and a simple form.
The Fig. 5 is the simulation results which the grid number is 400×400×400.The area which the grid number is 400×400×400 is calculated by using four areas that the grid number is 400×400×100 due to the grid number 400×400×100 is a extremity limitation which personal laptop can calculates.
Nikolas, Characterizing solute segregation and grain boundary energy in binary alloy phase field crystal models, Comput.
Simulation of the structure and motion of grain boundary in pure substances by phase field crystal model, J.

The growth rate is limited as the number of available nuclei increases more quickly than their size until impingement occurs.
Once DRX has begun during preloading beyond wc, the number of SRX nuclei begins to decrease along with the volume fraction of the deformed matrix and grain boundary area available for the nucleation of SRX.
At the same time, the number of DRX grains growing during unloading by the MDRX mechanism increases with w.
The net number of nucleation sites available for SRX therefore progressively decreases with prestrain, thus weakening the strain dependence of softening.
Beyond this point, the nucleation rate for SRX changes and so does the growth rate that is limited by the number of nuclei.

Grain Size Wire specimens were produced using different deposition methods and measured under XRD for their grain size.
Variation of grain sizes with pulse plating duty cycle.
Herzer [2] analyzed that in ferromagnetic materials with grain size, larger than the exchange interaction length, the magnetization can follow the easy magnetic directions in the single grains and domains can be formed within the grains.
As a consequence, the effective anisotropy for the magnetic behavior is an average over several grains, N K Ke 1 = (4) where Ke is the effective anisotropy and N is the number of grains included by the exchange interaction length, 3 )/( DLN ex= (5) where D is grain size.
At duty cycle 50%, the grain size was found to be the smallest.