Papers by Author: S.C. Silva

Abstract: In this paper, the effect of the grain size on sintered Nd-Fe-B based permanent magnets was investigated. In order, the magnets were produced by different milling times at 200 rpm and then vacuum sintered at 1373 K for 60 minutes followed by cooling outside the furnace. The magnets either produced by lower and higher milling times (30 and 75 minutes) exhibited lower remanence and coercivity, due the inhomogeneous distribution of the grain sizes. The magnet produced by intermediary milling time (45 minutes) exhibited the highest properties among all samples, with remanence of 1.06 T, coercivity of 891.3 KA.m^-1, maximum energy product of 211 KJ.m³ and a squareness factor equal 0.92.

Abstract: The addition of alloying elements on rare-earth permanent magnets is one of the methods used to improve the magnetic properties. This present work evaluates the influence of alloying elements such as Zr, Nb and Mo on the microstructure and magnetic properties of sintered Pr-FeCo-B based permanent magnets. The permanent magnets were produced by the conventional powder metallurgy route using powder obtained by hydrogen-decrepitation (HD) method from as cast alloys. In order to produce the magnet Pr₁₆Fe_66,9Co_10,7B_5,7Cu_0,7 without alloying elements the mixture of alloys method was employed, mixing two compositions: Pr₂₀Fe₇₃B₅Cu₂ (33% w.t) and Pr₁₄Fe₆₄Co₁₆B₆ (67% w.t). With the purpose of evaluating the influence of the alloying elements, the Pr₁₄Fe₆₄Co₁₆B₆X_0,1 (where X= Zr, Nb or Mo) (67% w.t) alloy was employed. The characterization of the alloys and the magnets was carried out using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDXS) and the magnetic properties were measured using a permeameter. The magnet without any additions presented the highest intrinsic coercivity (μ_0iH_c = 748 KA.m-1) while the magnet with Nb addition presented higher remanence (Br = 1,04 T). The magnet with Zr addition presented the highest maximum energy product (BH_máx = 144 KJ.m^-3), and the magnet with Mo addition showed the highest squareness factor (SF = 0,73).

Abstract: The first goal of this work involved the study of HDDR powders obtained from annealed alloys with the general formula: PrxFe77.9-xCo16B6Nb0.1 (x = 12; 12.5; 13; 13.5 and 14). The alloys were processed at desorption / recombination temperature of 840°C. The highest magnetic properties were obtained with 13.5 at. % Pr (Br= 1000mT and µ0iHc= 890mT). The alloy with a minimum praseodymium content (12 at. %) exhibited the lowest magnetic properties (Br= 350mT e iHc= 120mT). The second aim of the work involved the characterization of HDDR powders using X-ray diffraction for phase quantification and mean crystallite sizes determination of the hard magnetic phase. The processed powders were characterized by scanning electron microscopy (SEM).

Abstract: The first goal of this work involved the study of the effect of variables the HDDR processing, such as: the added pressure of H2 in the system, the time of heat treatment and recombination of Pr12Fe65.9Co16B6Nb0.1 alloy with the aim of improving the magnetic properties like the magnetic properties of the Pr14Fe63.9Co16B6Nb0.1 alloy (Br= 865mT and iHc= 790mT). The second aim of the work involved the characterization of HDDR powders that were analyzed by X-ray diffraction for identification and quantification of crystalline phases. These materials were analyzed by scanning electron microscopy (SEM).

Abstract: An evaluation of the effect of alloying elements on the microstructure and magnetic properties of Pr15FebalCo8B7Nb0.05Mx (M = Cu, P, Gd and Ga; 0 ≤ x ≤ 0.25) sintered magnets has been carried out. A mixture of alloys and the high-energy milling technique have been used to prepare the magnets. The alloying elements have influenced the remanence, intrinsic coercivity and particularly the squareness factor (SF). Phosphorus addition improved (BH)max (254 kJm-3 ) and SF around 10% (0.89). The same improvement addition on intrinsic coercivity was observed with Gallium (1100mT) compared to the standard composition Pr15FebalCo8B7Nb0.05 (1000mT) magnet. Comparisons between the squareness factors obtained using the J×μ0H curve profile (SF), the estimated (sf) using microstructural parameters and Sf using a (BH)max and Br correlation have also been carried out.

Abstract: Sintered magnets have been produced with powder obtained using the hydrogenation, disproportionation, desorption and recombination process (HDDR). The new processing procedure for the production of the sintered magnets has been adopted in an attempt to reduce the milling time. Commercial cast ingot alloys based on the compositions Pr14Fe75.9Co4B6Nb0.1 and Pr20.5FebalB5Cu2.0 have been employed in this investigation. The HDDR powder was used to produce sintered magnets using a mixture of these alloys, in very distinct proportions. Only a small amount (20 wt. %) of the copper-containing alloy has been added as a sintering aid. Standard hydrogen decrepitation (HD) magnets have also been included in this work for a comparison. The effect of a reduced milling time on the magnetic properties of the HDDR sintered magnets has been investigated. Sintering temperature and time of were kept constant for all magnets (1050°C for 60 minutes). The microstructures of the permanent magnets have been investigated by scanning electron microscopy and energy dispersive X-ray analysis.

Abstract: Pr14FebalCoxB6Nb0.1 magnets have been produced using the hidrogenation disproportionation desorption recombination (HDDR) process. The effect of the Co content (x= 0, 4, 8, 10, 12, 16) and the reaction temperature (800-900 °C) on the microstructure and magnetic properties of the HDDR material have been investigated. The processing temperature (desorption/recombination) affected the microstructure and magnetic properties of the bonded magnets. The alloy with low cobalt content (4 at.%) required the highest reaction temperature (880°C) to yield anisotropic bonded magnets. The optimum temperature for alloys with 8 at.% Co and 10 at.% Co were 840°C and 820°C, respectively. Alloys with high cobalt content (12 at.% and 16 at.%) were processed at 840°C. Each alloy required an optimum reaction temperature and exhibited a particular microstructure according to the composition. Pr14Fe80B6Nb0.1 magnets have been processed for comparison.

Abstract: Fine magnetic powder has been produced using the hydrogenation disproportionation desorption and recombination (HDDR) process. The first goal of this work involved an investigation of a range of disproportionation/desorption temperatures between 800 and 900°C with the purpose of optimizing the HDDR treatment for a Pr14Fe80B6 alloy. The cast alloy was annealed at 1100°C for 20 hours for homogenization. The optimum disproportionation temperature for achieving high anisotropy was 820°C. The influence of the reaction temperature on the microstructure and magnetic properties of Pr14Fe80B6 HDDR powders and magnets has been shown. A second stage of this study involved the characterization, for each temperature, of the HDDR processed powder using X-ray diffraction analysis. Samples of the HDDR material have been studied by synchrotron radiation powder diffraction using the Rietveld method for cell refinement, phase quantification and crystallite sizes determination. Scanning electron microscopy (SEM) has also been employed to reveal the morphology of the HDDR powder.