scholarly journals A Combined TEM/STEM and Micromagnetic Study of the Anisotropic Nature of Grain Boundaries and Coercivity in Nd-Fe-B Magnets

2017 ◽  
Vol 2017 ◽  
pp. 1-12 ◽  
Author(s):  
Gregor A. Zickler ◽  
Josef Fidler ◽  
Johannes Bernardi ◽  
Thomas Schrefl ◽  
Ahmad Asali
Keyword(s):  
Domain Wall ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Coercive Field ◽  
Boundary Phase ◽  
Micromagnetic Simulations ◽  
Fe Content ◽  
Néel Domain Wall ◽  
High Resolution Tem ◽  
Numerical Finite Element

The nanoanalytical high resolution TEM/STEM investigation of the intergranular grain boundary phase of anisotropic sintered and rapidly quenched heavy rare earth-free Nd-Fe-B magnet materials revealed a difference in composition for grain boundaries parallel (large Fe-content) and perpendicular (low Fe content) to the alignment direction. This behaviour vanishes in magnets with a high degree of misorientation. The numerical finite element micromagnetic simulations are based on the anisotropic compositional behaviour of GBs and show a decrease of the coercive field with an increasing thickness of the grain boundary layer. The magnetization reversal and expansion of reversed magnetic domains primarily start as Bloch domain wall at grain boundaries parallel to thec-axis and secondly as Néel domain wall perpendicular to thec-axis into the adjacent hard magnetic grains. The increasing misalignment of grains leads to the loss of the anisotropic compositional behaviour and therefore to an averaged value of the grain boundary composition. In this case the simulations show an increase of the coercive field compared to the anisotropic magnet. The calculated coercive field values of the investigated magnet samples are in the order ofμ0HcJ=1.8 T–2.1 Tfor a mean grain boundary thickness of 4 nm, which agrees perfectly with the experimental data.

scholarly journals Nanocompositional Electron Microscopic Analysis and Role of Grain Boundary Phase of Isotropically Oriented Nd-Fe-B Magnets

2017 ◽  
Vol 2017 ◽  
pp. 1-17 ◽  
Author(s):  
Gregor A. Zickler ◽  
Josef Fidler
Keyword(s):  
Domain Wall ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Magnetization Reversal ◽  
Coercive Field ◽  
Microscopic Analysis ◽  
Realistic Model ◽  
Boundary Phase ◽  
Electron Microscopic ◽  
The Impact

Nanoanalytical TEM characterization in combination with finite element micromagnetic modelling clarifies the impact of the grain misalignment and grain boundary nanocomposition on the coercive field and gives guidelines how to improve coercivity in Nd-Fe-B based magnets. The nanoprobe electron energy loss spectroscopy measurements obtained an asymmetric composition profile of the Fe-content across the grain boundary phase in isotropically oriented melt-spun magnets and showed an enrichment of iron up to 60 at% in the Nd-containing grain boundaries close to Nd2Fe14B grain surfaces parallel to the c-axis and a reduced iron content up to 35% close to grain surfaces perpendicular to the c-axis. The numerical micromagnetic simulations on isotropically oriented magnets using realistic model structures from the TEM results reveal a complex magnetization reversal starting at the grain boundary phase and show that the coercive field increases compared to directly coupled grains with no grain boundary phase independently of the grain boundary thickness. This behaviour is contrary to the one in aligned anisotropic magnets, where the coercive field decreases compared to directly coupled grains with an increasing grain boundary thickness, if Js value is > 0.2 T, and the magnetization reversal and expansion of reversed magnetic domains primarily start as Bloch domain wall at grain boundaries at the prismatic planes parallel to the c-axis and secondly as Néel domain wall at the basal planes perpendicular to the c-axis. In summary our study shows an increase of coercive field in isotropically oriented Nd-Fe-B magnets for GB layer thickness > 5 nm and an average Js value of the GB layer < 0.8 T compared to the magnet with perfectly aligned grains.

GRAIN BOUNDARIES: PHASE TRANSITIONS AND CRITICAL PHENOMENA

1991 ◽  
Vol 05 (19) ◽  
pp. 2989-3028 ◽  
Author(s):  
E.I. RABKIN ◽  
L.S. SHVINDLERMAN ◽  
B.B. STRAUMAL
Keyword(s):  
Phase Transitions ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Critical Phenomena ◽  
External Surface ◽  
Theoretical Models ◽  
Boundary Structure ◽  
Boundary Phase ◽  
Recent Experimental Data ◽  
Grain Boundary Structure

Recent theories of grain boundary structure have been reviewed briefly. The possibility of existence of the same variety of phase transitions on grain boundaries as that on the crystal external surface has been demonstrated. Recent experimental data and theoretical models concerning grain boundary phase transitions are critically analysed. Grain boundary phase transitions connected with the formation of thin disordered layers on the boundary (prewetting, premelting) are particularly distinguished. Results of recent indirect experiments, which may be treated in terms of prewetting and premelting, have been reviewed. Experimentally observed critical phenomena in the vicinity of the prewetting transition on the tin-germanium interphase boundary have been discussed in terms of the critical exponents theory. Some ideas regarding directions of further research are presented.

Grain Boundary Phase Analysis of Silicon Nitride by a Newly Developed 300kv Field-Emission Electron Microscope

1994 ◽  
Vol 346 ◽  
Author(s):  
Y. Bando ◽  
H. Suematsu ◽  
M. Mitomo
Keyword(s):  
Electron Microscope ◽  
Silicon Nitride ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Phase Analysis ◽  
Field Emission ◽  
Boundary Phase ◽  
Emission Electron ◽  
Triple Points

ABSTRACTThe grain boundary phase of silicon nitride containing additives Y2O3 and Nd2O3 has been studied by means of a newly developed 300kV field emission ATEM. The composition of the two-grain boundary phase of about 1 nm thick is successfully determined. It is then found that the compositions among the grain boundaries are not the same and the additives of Y2O3-Nd2O3 are poor in the two-grain boundary, while they are rich in the triple points.

Mechanism of Surface Dissolution in Dense Hydroxyapatite

2007 ◽  
Vol 121-123 ◽  
pp. 1241-1244 ◽  
Author(s):  
Dong Seok Seo ◽  
Hwan Kim ◽  
Jong Kook Lee
Keyword(s):  
Mechanical Properties ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Xrd Analysis ◽  
Boundary Structure ◽  
Molar Ratio ◽  
Dissolution Mechanism ◽  
Grain Boundary Structure ◽  
High Resolution Tem ◽  
Moisture Protection

In this study, it was demonstrated how second phases with small amount, which are hardly detected by XRD analysis, affect grain boundary dissolution and related mechanical properties of HA. All HA disks sintered at 1200 oC for 2 h in air with under moisture protection were phase pure and had Ca/P molar ratio of 1.67. Following certain period of exposure to the distilled water, the surface dissolution initiated at grain boundaries and particle loosening, subsequently resulting in decrease in mechanical properties of HA. In order to understand the dissolution mechanism, grain boundary structure of HA was identified by transmission electron microscopy (TEM) and high resolution TEM observation. From the analysis, it was found that the non-stoichiometric phase as α-tricalcium phosphate (TCP) transformed from β-TCP was existed at grain boundaries and caused surface dissolution of HA. From the XRD analysis, it was found that (211) and (112) planes of hydroxyapatite were susceptible to dissolution, whereas (300) plane was relatively stable.

High-resolution TEM of grain boundaries in silicon

1978 ◽  
Vol 36 (1) ◽  
pp. 428-429
Author(s):  
H. FÖll ◽  
D.G. Ast
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Large Scale ◽  
Boundary Structure ◽  
Scale Production ◽  
Silicon Crystals ◽  
Grain Boundary Structure ◽  
Large Scale Production ◽  
High Resolution Tem ◽  
Boundary Properties

In contrast to grain boundaries in metals, little is known about the structure and the properties of grain boundaries in covalent crystals, especially in semiconductors. The reason for this lack of knowledge is that grain-boundary-free crystals of, e.g., silicon, are easy to grow; commercially used silicon crystals are free of dislocations and thus nearly perfect. It was not until after the use of polycrystalline silicon had been proposed for the large scale production of cheap solar cells that grain-boundary properties in silicon gained considerable interest. In particular their electronic properties and their influence on device performance is important in this case. Moreover, “grain boundary devices”, i.e., devices with a grain boundary as the active element and with rather interesting properties, are conceivable - provided the relationship between the grain boundary structure and their electronic behaviour can be understood (cf. /l/). In addition, the study of grain boundaries in silicon, with an electronic structure and a binding configuration very different from metals, may provide a valuable tool to test competing grain boundary models (see, e.g., /2,3/) and may lead to a deeper insight into the crystal parameters governing the grain boundary properties.

Degradation of high-Tc superconductors by annealing in dry and moist atmospheres

1990 ◽  
Vol 5 (9) ◽  
pp. 1855-1859 ◽  
Author(s):  
W-K. Lee ◽  
A. S. Nowick
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Meissner Effect ◽  
Treatment Temperature ◽  
The Other ◽  
Boundary Phase ◽  
Maximum Effect ◽  
High Tc Superconductors ◽  
High Tc ◽  
Anneal Temperature

A study is made of the effects of annealing both La2−xSrxCuO4 (for x = 0, 0.1, and 0.15) and YBa2Cu3O7 in wet and dry O2 atmospheres at various temperatures between 200 and 930 °C. In the case of La2−xSrxSrCuO4, substantial degradation of resistive properties occurs during annealing in moist O2, the degradation being highest at 200 °C and decreasing as the treatment temperature increases. Since the Meissner effect remains unaffected, it is concluded that degradation is due to the formation of a hydroxide species at grain boundaries, which decomposes as the anneal temperature is increased to 930 °C. In the case of YBa2Cu3O7, on the other hand, moisture does not produce degradation for anneals at 200 °C and above, but severe degradation of resistive behavior does occur for dry O2 anneals, with a maximum effect at 500 °C. It is found that this effect results from a contaminant gas, possibly CO2, absorbed by the furnace when it is open to air. Again, the degradation is due to formation of a grain-boundary phase which decomposes by annealing at 930 °C.

Observations of Interaction Between Magnetic Domain Wall and Grain Boundaries in Fe-3wt%Si Alloy by Kerr Microscopy

1999 ◽  
Vol 586 ◽  
Author(s):  
K. Kawahara ◽  
Y. Yagyu ◽  
S. Tsurekawa ◽  
T. Watanabe
Keyword(s):  
Domain Wall ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Domain Structure ◽  
Misorientation Angle ◽  
Magnetic Domain ◽  
Magnetic Domain Structure ◽  
Magnetic Domain Wall ◽  
Domain Structures ◽  
Grain Boundary Plane

ABSTRACTMagnetic domain structures in Fe-3wt%Si alloy have been observed by a Kerr microscopy to understand the interaction between the magnetic domain wall and grain boundaries. It was found that the domain structures in the vicinity of the grain boundary depend on the misorientation angle; the high angle random boundary disturbs the magnetic domain structure more than the low angle boundary. In addition to the misorientation angle, magnetic domain structures were affected by the inclination of the grain boundary plane. Moreover, dynamic observations of rearrangement of the magnetic domain structure during magnetization revealed that grain boundaries could act as the sink and/or the source for magnetic domains.

The Effect of Cu Alloying on Al Alloy Thin Films: Microstructural Mechanisms That Enhance Electromigration Resistance

1993 ◽  
Vol 309 ◽  
Author(s):  
D. R. Frear ◽  
J. R. Michael ◽  
A. D. Romig
Keyword(s):  
Thin Films ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Room Temperature ◽  
Boundary Diffusion ◽  
Boundary Phase ◽  
Al Alloy ◽  
Cu Films ◽  
Isothermal Heat Treatments ◽  
Lower Temperature

AbstractThe microstructural evolution of unpatterned Al-2wt.%Cu thin films has been examined to elucidate the mechanism by which copper improves electromigration resistance. After annealing at 425°C and cooling to room temperature at a rate of approximately 1°C/min., the microstructure of the Al-Cu films consisted of 1 μm aluminum grains with θ-phase Al2Cu precipitates at grain boundaries and triple points. The grain size and precipitation distribution did not change with subsequent isothermal heat treatments in the temperature range of 200° to 400°C. Al-Cu thin films annealed at 400°C, a temperature just below the Al/Al+θ solvus, exhibited microstructures in which the aluminum grain boundaries were depleted in copper except for the presence of the pre-existing large, widely dispersed AI2Cu precipitates. Al-Cu thin films annealed at 200° to 300°C were enriched with copper at the aluminum grain boundaries. The large, widely dispersed Al2Cu precipitates remained after the lower temperature anneals. From these results, it is proposed that the presence of copper in aluminum thin films improves electromigration resistance due to the precipitation of a thin film of Al2Cu, or a substoichiometric precursor, along the grain boundaries. The grain boundary phase retards grain boundary diffusion in the thin films, thereby reducing total mass transport and improving electromigration resistance.

Grain Boundary Structure as a Function of Aluminum Level in Ni3Al

1990 ◽  
Vol 213 ◽  
Author(s):  
J.A. Horton ◽  
C.T. Liu ◽  
S.J. Pennycook
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Boundary Structure ◽  
Boundary Phase ◽  
Second Phase ◽  
Contrast Imaging ◽  
Minimum Width ◽  
Undoped Material ◽  
Boron Doped ◽  
Aluminum Level

ABSTRACTUnderstanding the boron effect in Ni3Al currently centers on determining thestructure of the region near grain boundaries, especially the presence of a disordered γ phase. In this study, a series of alloys was examined by transmission electron microscopy (TEM) as a function of aluminum level, boron level, and cooling rate for thepresence of any grain boundary phases. The base alloy series all contained 0.5 at. % Hf. At 21.5 at. % Al,γ formed in the matrix as expected. However, in slowly cooled specimens with 22 and 22.5% Al, a second phase with thicknesses from 25 to 50 nm formed on some of the grain boundaries. At higher aluminum levels no evidence of any second phase was observed. The minimum width of grain boundary images formed by normal diffraction contrast imaging was generally 0.5 to 1 nm for grain boundaries that presumably had no second phase and were parallel to the electron beam. Therefore, a second phase with a thickness of 1 nm or less would not be discernable by TEM. High resolution Z-contrast imaging by scanning TEM of a very low angle boundary in directionally solidified (DS) material also showed no evidence of disordering, but did show a surprising amount of non-planarity of the boundary.Whereas slower cooling rates might be expected to aid the formation of any grain boundary phase and thereby increase the ductility, the ductility was slightly lower for the more slowly cooled alloys than for the furnace cooled alloys. From 21.5 to 24% Al, the tensile elongations remain fairly constant with values ranging from 45 to 52%. Above 24%, the ductility drops off rapidly but is still much higher for the boron-doped material than for the undoped material. At 25.2% Al, the ductility is still 4% as compared to essentially zero for undoped material. Therefore the ductility improvements in boron-doped Ni3Al do not require the presence of any grain boundary disordered phases discernable by TEM.

Identification of a Grain Boundary Nucleated Phase in 300m Steel

1981 ◽  
Vol 39 ◽  
pp. 294-295 ◽  
Author(s):  
R. Padmanabhan ◽  
W. E. Wood
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Prior Austenite ◽  
Lower Bainite ◽  
Sem Analysis ◽  
Boundary Phase ◽  
Austenite Grain ◽  
300M Steel ◽  
Strain Fracture ◽  
Prior Austenite Grain

One reason proposed for the improvement of plane strain fracture toughness (at similar yield strength levels) of UHSLA steels, austenitized at 1473°K, rather than the conventional 1143°K, is the elimination of non-martensitic phases, at prior austenite grain boundaries. However, contrary to expectations, optical microscopy results from early investigations suggested the existence of bainite at prior austenite grain boundaries in both 4340 and 300M steels when oil quenched from 1473°K and tempered at 180°C or 280°C (but not when quenched from 1143°K). The purpose of the present investigation was to identify through TEM the nature of grain boundary nucleated phase in 300M steel and to observe whether it is absent in the 1143°K heat treatment as reported earlier. A blocky grain boundary phase, resembling lower bainite was observed readily by SEM analysis of etched metallography samples (Fig. 1 a and b). This phase was studied further through TEM and the results are presented in Figs. 2 and 3 for the 1143°K and the 1473°K austenitized specimens respectively.

Sign in / Sign up

Export Citation Format

Share Document