Distribution Of Grain Boundary Phase Research Articles

Effects of grain boundary phase distributions on mechanical characteristics of the high thermal conductivity AlN ceramics

AlN powders with different O impurity contents were used to fabricate the high thermal conductivity (TC) AlN ceramics of different grain boundary (GB) phase distributions via pressureless sintering. Bending strength, fracture toughness, R-curve characteristic, and fracture mechanism of the ceramics were then investigated. The results showed that the GB phase distributions had great influences on mechanical characteristics of the ceramics. The ceramics with the continuous/semi-continuous GB phases possessed low strength and toughness. Comparatively, the ceramics with the isolated island-like GB phases, especially the fine GB phases at the triple AlN GB junctions, owned higher strength and toughness. The AlN ceramics principally followed the intergranular fracture mode, but the GB phase distribution had great effects on the fractions of transgranular fracture. The AlN ceramics with the continuous/semi-continuous GB phases had a lower fraction of the transgranular fracture, and thus lower crack propagation resistance and reliability than the others.

Study on Ce2Fe14B-based sintered magnets with Ce/RE> 80%

Up to now, the development of Ce/RE> 80% sintered permanent magnets still faces a challenge, owing to their seriously deteriorated microstructure. In this article, the Ce25.5∼31.5Nd0∼5Febal.B1.25M1.15 (wt.%) sintered magnets with Ce/RE > 80% were investigated, the roles of minor Nd substitution were explored. The typical island-like phase common existed in the grain-boundary (GB) regions of Ce2Fe14B-based SC alloys (which destroys the continuity of RE-rich GB phase), was confirmed as a tetragonal B-rich phase (fct-RE5Fe18B18), and probably generated by the peritectic reaction of “L + Ce2Fe14B → CeFe2 + B-rich” at 797 °C. It was found that: the deteriorated microstructures of high Ce sintered magnets were hardly improved (Nd element was not enriched in the main phase as it was expected), and the coercivity increments were far below expectations by directly adding 3–5% Nd in alloy designs. However, the GB phase distribution was more uniform and continuous, Nd-rich shells formed, and magnetic properties were remarkably promoted by blending 2% NdHx powders into the Ce27.5Nd3Febal.B1.25M1.15 (Ce27.5Nd3) magnet during JM milling, compared with the Ce25.5Nd5Febal.B1.25M1.15 (Ce25.5Nd5) magnet with similar composition. The good comprehensive magnetic properties of Hcj = 1.714 kOe, Br = 9.395 kG, and (BH)max = 11.16 MGOe have been reached in the Ce27.5Nd3+2% (NdHx) magnet (Ce accounted for 84.5 wt% of total rare earth). The present work deepens our understanding of metallurgical behavior, process/microstructure designing for Ce/RE> 80% sintered magnets, and shed a light on the large-scale utilization of Ce element in a permanent magnet.

Elevated corrosion resistance and improved magnetic properties of sintered Nd-Fe-B magnets via the infiltration of DyCr film

The low coercivity at high temperature and poor corrosion resistance limit the application field of Nd-Fe-B magnets. In this work, a DyCr film was prepared on the surface of Nd-Fe-B magnet through magnetron sputtering, and then subjected to grain boundary diffusion (GBD) to improve magnetic properties and enhance corrosion resistance. The results show that the residual Cr-rich film exists on the surface of NdFeB magnets, acting as a protective coating to enhance the corrosion resistance, which is evidenced by the weight loss experiment and scanning electron microscopy (SEM) image. Moreover, Cr element in the grain boundary phase increase the electrochemical potential from − 0.985 to − 0.733 V, which is also beneficial in improving corrosion resistance. Transmission electron microscope (TEM) analysis confirmed that the crystal structure of intergranular phase remained unchanged after diffusion, i.e. BCC-RE2O3 (RE=Rare Earth) phase. The coercivity was improved significantly, obtaining an increment of 19.4% comparing to that of initial magnet, which is attributed to the formed Dy-rich shell with high magnetocrystalline anisotropy field and the improved distribution of grain boundary phase. This suggests that it is feasible to fabricate high coercivity magnets and strong corrosion resistance only through grain boundary diffusion, which is expected to further shorten the processing flow of Nd-Fe-B magnets.

Engineering microstructure to improve coercivity of bulk MnBi magnet

MnBi is a candidate material for high-temperature magnets because of its increasing coercivity with increasing temperatures up to 255 °C. However, most efforts in fabricating bulk MnBi magnets have run into the problem of preserving the coercivity (Hcj) of its feedstock powders. About 70% of powder's Hcj would be lost during the densification process. Our micromagnetic modeling shows that the coercivity mechanism of the MnBi bulk magnet is controlled by nucleation of the reversal magnetization domains, and the large Hcj loss that occurred during the powder consolidation process can be attributed to the inter-grain magnetic coupling. To attain a high Hcj, the grains in the MnBi bulk magnet must be separated with a non-magnetic grain boundary phase (GBP). To validate this GBP hypothesis, we engineered MnBi bulk magnets with two different types of GBP. The first type of GBP was created in-situ by precipitating excessive Bi from the grains; the second type was created ex-situ by coating silicates on the feedstock powders before the consolidation. While both GBP work, the ex-situ approach resulted in a better Hcj due to a more uniform GBP distribution. The Hcj loss was reduced from 70% to 15%, and the (BH)max of a warm sintered bulk magnet reached 8.9 MGOe.

Study on the microstructure and properties of Nd-Fe-B regenerated magnet by adding Ho69.3Ni30.7 alloy at grain boundary

The Nd-Fe-B regenerated magnets are prepared by adding low melting point Ho69.3Ni30.7 alloy at the grain boundary, and the microstructure, structure and overall performance of the Nd-Fe-B regenerated magnets are studied. The results show that the addition of Ho69.3Ni30.7 alloy to grain boundary can optimize the distribution of grain boundary phase, accompanied by the formation of clear and continuous grain boundary phase, which weakens the exchange coupling effect between the adjacent main phases. Meanwhile, the coercivity of the regenerated magnet increases with the increase of Ho69.3Ni30.7 alloy content. The optimal coercivity (16.35 kOe) was obtained at 2 wt% Ho69.3Ni30.7 additions, and the coercivity increased by 1.24 kOe (8.2%) compared with the initial magnet (15.11 kOe). In addition, with the addition of Ho69.3Ni30.7 alloy, the curie temperature of Nd-Fe-B regenerated magnet increased from 303.2℃ to 305.6℃. The reflection loss of regenerated magnet reached −26.42 dB when the amount of 2 wt% Ho69.3Ni30.7 was added, which greatly improved the absorbing performance of the regenerated magnet. Finally, the secondary utilization of waste Nd-Fe-B magnets was realized.

Microstructure of core-shell NdY-Fe-B sintered magnets with a high coercivity and excellent thermal stability

We developed the NdY-Fe-B sintered magnet by effective microstructure modification, which exhibits an excellent thermal stability of coercivity with 15 at.% Nd substituted by Y. The feature of microstructure and its inherent link with the magnetic configurations were comprehensively investigated by experimental and micromagnetic simulation approaches. Microstructure analysis confirmed that the novel inhomogeneous distribution of Y in the core and shell regions of the grains was obtained in the whole magnets, contributing to the magnetic hardening of the grain surface areas. TEM observation indicates the tendency of Y depletion in grain boundary (GB) phases, which permits the significantly formation of Nd6Fe13(Ga,Cu)1 phase in the post-annealing magnet, accompanying with the optimization of intergranular GB phase distribution. The thick and Nd-riched intergranular GB phases play a significant role in magnetic isolating the adjacent grains and thus gives rise to a remarkable coercivity increase from 1.16 to 1.72 T. In addition, since the weakened temperature dependence of the magnetocrystalline anisotropy field, the substitution of Nd by Y effectively relieves the deterioration of the coercivity with temperature increasing. Relying on the combined effect of Y substitution on microstructure and intrinsic properties, a low absolute value (0.548%/ °C) of the temperature coefficient (20–120 °C) of the coercivity is achieved.

Sintering of Silicon Carbide Ceramics with Co-Addition of Gadrinium Oxide and Silica and their Mechanical Properties

Effects of simultaneous addition of SiO2 and gadrinium oxide on densification of SiC ceramics were examined, and relation between microstructure and their mechanical properties were discussed. Total 11wt% of Gd2O3 and SiO2 were mixed with fine -SiC powder. The weight of Gd2O3 in (Gd2O3 + SiO2) were set as 0, 20, 40, 60, 80 and 100%. The mixture was hot-pressed at 1950oC under 40 MPa applied pressure for 1 h. In the case of 40Gd2O3 and 80Gd2O3 compositions, the effect of sintering temperature from 1900 to 2000oC was also examined. The bulk density increased with increasing Gd2O3 content at the sintering temperature of 1950oC. Bending strength of the sintered bodies also improved with increasing Gd2O3 content generally, but at 40Gd2O3 composition, the maximum over ~800 MPa was observed. Young’s modulous, Vickers hardness and fracture toughness also increased with increasing Gd2O3 content. The distribution of grain boundary phase was not homogeneous. Evaporation of additives, mainly SiO2, caused non-homogeneous distribution of grain boundary phase between outside and inside of sintered bodies. High temperature bending strength of 80Gd2O3 specimen was superior than that of 40Gd2O3 specimen.

Effects of Li<SUB>2</SUB>O on Densification, Microstructure and Thermal Property of Spark Plasma Sintered AlN Ceramics

A newly developed rapid densify technique, spark plasma sintering (SPS), is used to prepare full-densified aluminum nitride (AlN) ceramics which are poorly sinterable. Some sintering aids are also used to promote the AlN ceramics' densification and improve its thermal conductivity. In this work, effects of sintering aid Li2O on densifica- tion, microstructure and thermal property of SPS sintered AlN ceramics were investigated. Results suggest that the ini- tial sintering temperature of AlN samples reduce from 1550℃ to lower than 1200℃ with 1.0wt% Li2O and 1.5wt% Sm2O3 (or Y2O3) adding as sintering aids . With Li2O addition, AlN compacts can be fully densified at 1650℃. The microstructure of AlN compacts indicates that Li2O is beneficial to generate aluminate liquid phase with better wet- tability and promote the densification of AlN ceramics, but it is unable to obtain higher relative density of AlN com- pacts because the escapement of gas phase though out liquid phase is very difficult in a rapid sintering process. Meanwhile, Li2O addition affects the growth of AlN grains, and the better wettability of aluminate with AlN grain in- duces the homogeneous distribution of grain boundary phase. The deterioration of thermal conductivity of AlN ceram- ics is caused by the fact that the scattering of phonon is enhanced by small grain size and the secondary phase spreading adequately along the AlN grain boundaries. The thermal conductivity of AlN samples with 1.0wt% Li2O and 1.5wt%Sm2O3 as sintering aids is lower than that of sample only with Sm2O3 as sintering aids.