We have characterized the microstructures of as-sintered and optimally post-sinter annealed Nd-rich Ga-doped Nd–Fe–B magnets by scanning electron microscopy (SEM) and aberration-corrected scanning ...transmission electron microscopy (STEM). While the Nd2Fe14B grains in the as-sintered sample with a coercivity of 0.99T are in direct contact with each other, those in the optimally annealed sample with a coercivity of 1.8T are completely enveloped by typically 10-nm-thick Nd-rich phase that contains little Fe. This strongly suggests that the Nd2Fe14B grains in the optimally annealed Nd-rich Ga-doped Nd–Fe–B magnets are exchange decoupled in contrast to those in the commercial sintered magnets.
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We have investigated the effect of Ga on the microstructure and coercivity in Nd-rich Ga-doped Nd-Fe-B sintered magnets with different amount of Ga additions using focused ion beam scanning electron ...microscope (FIB/SEM), aberration corrected scanning transmission electron microscope (STEM), three dimensional atom probe (3DAP) and synchrotron X-ray diffraction. While a ferromagnetic Fe-rich amorphous phase is a dominant grain boundary phase in the Ga-free magnet, the trace addition of Ga resulted in the formation of the Nd6Fe13Ga antiferromagnetic phase at grain boundaries as well as in triple junctions after post-sinter annealing above 480 °C. In addition, non-ferromagnetic Nd-rich phases with the Ia3¯ structure and the amorphous structure were formed along the grain boundaries. The structures and chemical compositions of these three types of grain boundary phases were identified. The high coercivity is closely related with the formation of the three types of non-ferromagnetic grain boundary phase rather than the amount of the Nd6Fe13Ga triple junction phase; hence the coercivity enhancement after the optimal heat-treatment is attributed to the magnetic isolation of the Nd2Fe14B grains through the formation of the non-ferromagnetic grain boundary phases. The underlying mechanism for the formation of these grain boundary phases is discussed based on the experimental results.
•Through microstructure analysis to clarify the role of Ga in Nd-Fe-B magnet.•Formation of Nd6Fe13Ga phase instead of the Fe-rich ferromagnetic amorphous phase.•Formation of non-ferromagnetic Nd-rich phase along grain boundaries.•Magnetic isolation of the Nd2Fe14B grains leading to the high coercivity.
A recessed-gate structure has been studied with a view to realizing normally off operation of high-voltage AlGaN/GaN high-electron mobility transistors (HEMTs) for power electronics applications. The ...recessed-gate structure is very attractive for realizing normally off high-voltage AlGaN/GaN HEMTs because the gate threshold voltage can be controlled by the etching depth of the recess without significant increase in on-resistance characteristics. With this structure the threshold voltage can be increased with the reduction of two-dimensional electron gas (2DEG) density only under the gate electrode without reduction of 2DEG density in the other channel regions such as the channel between drain and gate. The threshold-voltage increase was experimentally demonstrated. The threshold voltage of fabricated recessed-gate device increased to -0.14 V while the threshold voltage without the recessed-gate structure was about -4 V. The specific on-resistance of the device was maintained as low as 4 m/spl Omega//spl middot/cm/sup 2/ and the breakdown voltage was 435 V. The on-resistance and the breakdown voltage tradeoff characteristics were the same as those of normally on devices. From the viewpoint of device design, the on-resistance for the normally off device was modeled using the relationship between the AlGaN layer thickness under the gate electrode and the 2DEG density. It is found that the MIS gate structure and the recess etching without the offset region between recess edge and gate electrode will further improve the on-resistance. The simulation results show the possibility of the on-resistance below 1 m/spl Omega//spl middot/cm/sup 2/ for normally off AlGaN/GaN HEMTs operating at several hundred volts with threshold voltage up to +1 V.
The magnetism of grain boundary (GB) phases in the Nd-rich Ga-doped Nd-Fe-B sintered magnet that shows unusually high improvement in coercivity by post-sinter optimal heat treatment was studied using ...electron holography. Transmission electron microscopy observations identified three types of GB phases with distinct structures, i.e., Ia3¯-type, amorphous-type, and Nd6(Fe,Ga)14-type. The magnetic flux densities of these three GB phases were systematically analyzed by measuring the phase shifts of incident electrons, and all of these GB phases are concluded to be nonferromagnetic. Unlike the standard commercial sintered magnets, this study has shown that the Nd2Fe14B grains in the optimally heat treated Nd-rich Ga-doped sintered magnet are exchange decoupled by the formation of the three types of nonferromagnetic GB phases. The observations provide useful information for GB engineering of Nd-Fe-B sintered magnets, which is key for further improvements of their coercivities without relying on grain size refinement.
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•Magnetization of the grain boundary phases in Ga-doped Nd-Fe-B magnet was studied.•Three types of grain boundary phases were observed in this sintered magnet.•Electron holography revealed that the grain boundary phases were nonferromagnetic.•The result provides useful information for the grain boundary engineering of sintered magnets.
We observed the magnetization reversals of exchange-coupled and exchange-decoupled Nd-Fe-B sintered magnets with coercivities of 1.16 and 1.80 T, respectively, by magneto-optical Kerr effect (MOKE) ...microscopy. The cascade propagation of reversed magnetic domain throughout multiple grains was observed in the standard sintered magnet, in which Nd2Fe14B grains are believed to be exchange-coupled through ferromagnetic grain boundary phase. In the exchange-decoupled Nd-rich Ga-doped sintered magnet, the magnetization reversal is suppressed by thick nonmagnetic grain boundary phase. The EBSD revealed 11° misalignment of 2:14:1 grains in the standard sintered magnet, while higher misalignment of 18° was found in the exchange-decoupled Nd-rich Ga-doped magnet. Micromagnetic simulations incorporating such microstructural features reproduced experimentally observed demagnetization curves fairly well including the enhancement of the coercivity and the deterioration of the squareness in the exchange decoupled sintered magnet.
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Des‐gamma‐carboxy prothrombin (DCP) levels reportedly correlate with histological features of hepatocellular carcinoma (HCC). We examined serum DCP as a predictor of HCC recurrence in 144 patients ...who underwent living donor liver transplantation. Receiver operating characteristics (ROC) analysis revealed superiority of DCP and AFP over preoperative tumor size or number for predicting recurrence. Multivariate analysis revealed tumor size >5 cm, ≥11 nodules, and DCP >400 mAU/mL as significant independent risk factors for recurrence. Incidence of microvascular invasion (62% vs. 27%, p = 0.0003) and poor differentiation (38% vs. 16%, p = 0.0087) were significantly higher for patients with DCP >400 mAU/mL than for patients with DCP ≤400 mAU/mL. In ROC analysis for patients with ≤10 nodules all ≤5 cm to predict recurrence, area under the curve was much higher for DCP than for AFP (0.84 vs. 0.69). Kyoto criteria were thus defined as ≤10 nodules all ≤5 cm, and DCP ≤400 mAU/mL. The 5‐year recurrence rate for 28 patients beyond‐Milan but within‐Kyoto criteria was as excellent as that for 78 patients within‐Milan criteria (3% vs. 7%). The preoperative DCP level offers additional information regarding histological features, and thus can greatly improve patient selection criteria when used with tumor bulk information.
Des‐gamma‐carboxy prothrombin (DCP), not AFP, used with tumor bulk information provided additional ability to predict HCC recurrence compared with criteria based on tumor bulk alone after living donor liver transplantation due to a correlation of DCP with microvascular invasion and poor differentiation.
Recent developments in imaging technology have enabled CT and MR cholangiopancreatography (MRCP) to provide minimally invasive alternatives to endoscopic retrograde cholangiopancreatography for the ...pre- and post-operative assessment of biliary disease. This article describes anatomical variants of the biliary tree with surgical significance, followed by comparison of CT and MR cholangiographies. Drip infusion cholangiography with CT (DIC-CT) enables high-resolution three-dimensional anatomical representation of very small bile ducts (e.g. aberrant branches, the caudate branch and the cystic duct), which are potential causes of surgical complications. The disadvantages of DIC-CT include the possibility of adverse reactions to biliary contrast media and insufficient depiction of bile ducts caused by liver dysfunction or obstructive jaundice. Conventional MRCP is a standard, non-invasive method for evaluating the biliary tree. MRCP provides useful information, especially regarding the extrahepatic bile ducts and dilated intrahepatic bile ducts. Gadolinium ethoxybenzyl diethylenetriamine pentaacetic acid-enhanced MRCP may facilitate the evaluation of biliary structure and excretory function. Understanding the characteristics of each type of cholangiography is important to ensure sufficient perioperative evaluation of the biliary system.
AlGaN-GaN power high-electron mobility transistors (HEMTs) with 600-V breakdown voltage are fabricated and demonstrated as switching power devices for motor drive and power supply applications. The ...fabricated power HEMT realized the high breakdown voltage by optimized field plate technique and the low on-state resistance of 3.3 m/spl Omega/cm/sup 2/, which is 20 times lower than that or silicon MOSFETs, thanks to the high critical field of GaN material and the high mobility in 2DEG channel. The fabricated devices also demonstrated the high current density switching of 850 A/cm/sup 2/ turn-off. These results show that AlGaN-GaN power-HEMTs are one of the most promising candidates for future switching power device for power electronics applications.
We developed an algorithm of graft selection in which left lobe donation is considered primarily if the graft‐to‐recipient weight ratio (GRWR) is estimated to be greater than 0.6% in preoperative ...volumetry with utilization of a hemi‐portocaval shunt (HPCS) based on portal vein pressure (PVP) more than 20 mmHg at the time of laparotomy. A total of 11 consecutive adult living donor liver transplantations with small‐for‐size graft according to our graft selection algorithm were performed between December 2005 and August 2007. Ten patients required HPCS using a vein graft all survived without small‐for‐size syndrome (SFSS) and shunt complications with a median follow‐up of 296 days. One patient without HPCS died of chronic vascular rejection. In all cases, PVP were regulated successfully under 20 mmHg by HPCS. Graft volume reached in mean 84.3% of standard liver volume in right lobe grafts and mean 95.4% in left lobe grafts at 3 months after liver transplantation. Actuarial rate of shunt patency at 1, 3, 6 months and 1 year were 80%, 55%, 26% and 20%, respectively. Selective HPCS based on PVP is an effective procedure and results in excellent patient and graft survival with avoidance of SFSS in grafts greater than 0.6% of GRWR.
In this study, selective hemi‐portocaval shunt based on portal vein pressure resulted in excellent patient and graft survival with avoidance of small‐for‐size syndrome in grafts greater than 0.6% of GRWR in adult living donor liver transplantation.
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