Surface tension of intergranular regions of NdFeB nanocomposite magnets

Materials Science and Engineering A 375–377 (2004) 1032–1035

Surface tension of intergranular regions of NdFeB nanocomposite magnets H. Chiriac∗ , M. Marinescu National Institute of R&D for Technical Physics, 47 Mangeron Blvd., 6600 Iasi 3, Romania

Abstract The surface tension and contact angle of some Nd-rich compounds specific to the stoichiometry of intergranular regions of NdFeB permanent magnets were investigated using the sessile drop method. The surface tension of the molten Nd90.9 Fe9.1 , Nd88 Co12 , and Nd77.5 Ga22.5 wt.% alloy drops resting on BN substrate was calculated under the Dorsey formalism. As an overall trend, the contact angle of the three investigated alloys, corresponding, respectively to the Nd7 Fe3 , Nd3 Co, and Nd5 Ga3 phases, in liquid state, decreases with increasing temperature, whereas, the surface tension generally increases with temperature raise above the melting point indicating a capillarity effect. From the experimental data, it can be inferred that the highest value of the surface tension at temperatures just above the melting point corresponds to Nd5 Ga3 , γ∼ = 400 mN/m, whereas Nd7 Fe3 and Nd3 Co presents a surface tension of about γ ∼ = 200 mN/m. Wetting angles around 150–160◦ are specific to all investigated compositions near the melting point. © 2003 Elsevier B.V. All rights reserved. Keywords: Surface tension; Nd-rich intergrain phase; NdFeB magnets

1. Introduction Currently, NdFeB sintered permanent magnets present the highest available maximum energy product. The hot-pressed and deformed by die-upsetting nanocomposite NdFeB magnets are comparable in terms of maximum energy product with classical sintered magnets due to nanostructure peculiarities and relation structure-properties [1]. Either liquid phase sintering of microcrystalline sintered NdFeB magnets and hot-compaction and deformation in nanocrystalline NdFeB magnets take place due to the existence of an intergrain Nd-rich phase that is liquid at working temperature (Tsinter = 1080 ◦ C, Thot-working = 700–850 ◦ C). The Nd-rich intergrain phase is vital in nanocomposite NdFeB magnets for coercivity mechanism (pinning of the domain walls), deformation behavior (appearance of internal cracks in case of a non-homogeneous distribution) and also for texturing mechanism (through liquid phase diffusion and preferential orientation of the main Nd2 Fe14 B grains within the liquid Nd-rich intergranular region [2,3]). Although, a high volume ratio of intergrain phase reduces the remanent

∗

Corresponding author. Tel.: +40-32-13-06-80; fax: +40-32-23-11-32. E-mail address: [email protected] (H. Chiriac).

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.008

magnetization directly through magnetic ‘dilution’ and indirectly, reducing the contribution of the short range exchange and long range dipolar coupling between the 2:14:1 grains [1]. Therefore, one must balance and carefully consider the composition of Nd-rich intergranular phase for achieving a high-performance magnet. It must be relatively reduced as volume fraction, but have a homogeneous distribution and a proper wetting behavior in liquid phase. That is the reason for which this is an important subject of studies. From SEM investigations it was derived that this volume fraction follows a linear increase with total Nd content. For example, 17 at.% Nd determines 15 vol.% of intergrain phase [4], amount enough to give rise to rheological flow of the grains during the temperature-assisted deformation process. As concerns the composition of the Nd-rich phase, this is determined by the total composition of the magnet alloy and it has been found by EDX at eutectic Nd7 Fe3 stoichiometry [5] (Nd90.9 Fe9.1 in wt.%). Other intergrain phases which may form at addition of various alloying elements to the NdFeB base composition are Nd3 (CoCu), (Nd,Dy)5 (CoCu Ga)3 , and Nd6 (Fe,Co)13 Ga [6]. This work is aimed to studying the surface tension and wetting behavior of some Nd-rich phases, in the stoichiometry suggested in the literature [5,6] but simplified to the corresponding main binary system: Nd7 Fe3 , Nd3 Co, and

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Nd5 Ga3 , for the intergrain phase in NdFeB permanent magnets. Importance of the surface tension of Nd-rich intergranular region in liquid phase as concerns the deformation and texturing mechanism at hot-pressed nanocomposite Nd2 Fe14 B magnets is particularly highlighted.

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45o h45 r45

2. Experimental procedure

θ

re

2.1. Sessile drop method The master alloys with the following compositions in wt.%: Nd90.9 Fe9.1 , Nd88 Co12 , and Nd77.5 Ga22.5 corresponding to Nd7 Fe3 , Nd3 Co, and Nd5 Ga3 phases were cast by arc melting the pure constituents in pure argon after vacuum of 10−3 Pa. The ingots were melted several times within the furnace for homogenization. Small pieces of 1 g (approximately 150 ␮l) were cut and subjected to surface tension investigation. For simultaneously measuring the surface tension and contact angle we have chosen the sessile drop method in which the liquid under investigation (molten sample ) rests freely on a substrate. The image of the drop contour is captured using a charge coupled device (CCD) camera and magnified by an external optical system corrected of optical aberrations. The magnifying power has been calculated using an etalon sample and a reference tungsten wire fixed vertically under the substrate in the same plane with the sample. The experimental set up is presented in detail in [7]. The image of the drop is captured in real time and recorded through a video acquisition board simultaneously with the value of the temperature measured with a digital multimeter. The image of the drop saved in a standard graphic format is processed through a contrast method. The measurements were performed in 10−3 Pa vacuum. The wetting behavior is quantitatively measured by the contact angle θ that is determined by the interfacial energies between solid, liquid, and vapor as the angle between the baseline of the drop and the tangent at the drop boundary (Fig. 1).

Fig. 1. Contour of the meridional section of the drop and geometrical parameters of interest.

For calculating the surface tension γ, we have chosen the Dorsey formalism that gives the following relation, deduced from fundamental Laplace equilibrium equation [8], and that expresses the dependence of the surface tension on some geometrical parameters of the drop:
 0.052 γ= − 0.1227 + 0.0481f re2 gρ f f =

r45 − h45 − 0.4142 re

where 2re is the maximum diameter of the drop, r45 and h45 are the geometrical parameters described within the Fig. 1 and ρ is the density of the liquid.

3. Results and discussion The accuracy of Dorsey method was checked by measuring the surface tension of molten Nd. Since the method is based on some empirical parameters related to the volume of the sample under investigation, surface tension determinations were done first with different masses of Nd specimen: 0.8, 1, and 1.5 g. The determined value near the melting point, closest to the value reported in the literature

Fig. 2. The shape of Nd77.5 Ga22.5 drop-sample during heating above the melting point. (a) 1080 ◦ C, (b) 1120 ◦ C, (c) 1155 ◦ C, and (d) 1170 ◦ C.

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[9] was for 1 g and fell within 10% error (γNd = 689 mN/m at melting temperature TmNd = 1024 ◦ C). For comparison, 2 g Cu sample (equivalent volume is 250 ␮l) was investigated and the determined surface tension fell within 4% error. The error in Nd surface tension determination is attributed to the oxidization process when keeping the sample at high temperatures with only 10−3 Pa vacuum as the furnace system used works. On the other hand, these are the same conditions like during the hot-pressing process and a certain degree of oxidization is promoted in order to limit the so called abnormal grain growth [10]. Several substrate materials were tested: alumina Al2 O3 , boron nitride BN, tantalum Ta and NdFeB commercial available magnet. Finally, BN substrate was chosen for the experiments for the following practical reason: the investigated alloys interact chemically with Al2 O3 substrate, and Ta and NdFeB substrates reveal a low surface tension themselves and the drops spread so much that the Dorsey formalism cannot be applied. The samples didn’t chemically interact with the BN substrate. All the measurements on BN substrate were performed with a heating rate of about 50 ◦ C/min read by the thermocouple. The temperature could not be kept constant longer than a few seconds since the samples rapidly oxidize and surface tension dramatically decreases (spreading drop effect). The profile of Nd77.5 Ga22.5 drop during heating above the melting point is successively shown in Fig. 2. The variation of surface tension and the contact angle of liquid Nd90.9 Fe9.1 , Nd88 Co12 , and Nd77.5 Ga22.5 drops on BN support is presented in Fig. 3. For surface tension calculus we considered the density of pure Nd at melting point ρ0 = 6.688 g/cm3 and the variation of density with temperature above the melting point dρ/dt = −0.528 × 10−3 g/cm3 K. For all compositions it was obtained that with increasing the temperature, the contact angle decreases and the surface tension generally increases (enhanced capillarity effect) in comparison with the values close to the melting point. The non-monotonous trend of the values of the contact angle and surface tension with temperature raise may be attributed to some thermal non-equilibrium conditions. However, under a qualitative approach, from our measurements it can be inferred that the highest value of the surface tension at temperature just above the melting point corresponds to Nd5 Ga3 , estimated at γ ∼ = 400 mN/m, whereas Nd7 Fe3 and Nd3 Co presents a surface tension estimated at about γ ∼ = 200 mN/m. If a high wetting behavior is favorable to a homogeneous morphology of the nanocomposite magnets enhancing the coercivity and mechanical strength, a low surface tension has favorable effect on the diffusion process. In this way, Nd, Fe and B atoms originating from missoriented crystallites diffuse to the oriented ones through a low energetic barrier of the Nd-rich liquid layer. One must mention that after the last indicated value of the surface tension at highest temperature in Fig. 3, on each composition, there occurred a dramatic fall of its value cor-

Fig. 3. Variation of surface tension and contact angle of molten Nd90.9 Fe9.1 , Nd88 Co12 , and Nd77.5 Ga22.5 samples (corresponding to Nd7 Fe3 , Nd3 Co, and Nd5 Ga3 phases) on BN substrate.

responding to a contact angle decreased below 90◦ , in which conditions the Dorsey formalism cannot be applied. All the investigated sample drops spread under contact angles of 150–160◦ at temperatures close to melting point. Several main sources of error were considered: contamination by surface-active impurities, reaction with the sub-

H. Chiriac, M. Marinescu / Materials Science and Engineering A 375–377 (2004) 1032–1035

strate and thermal equilibrium of the system. We estimate the relative errors in calculating the surface tension ranging between 20 (at high values of f parameter) and 15% (at low values of f parameter). As for the contact angle, we considered a reading error of 5◦ .

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and Nd5 Ga3 phases in molten state, it can be inferred that the highest value of the surface tension at temperature just above the melting point corresponds to Nd5 Ga3 , γ ∼ = 400 mN/m, whereas Nd7 Fe3 and Nd3 Co presents a surface tension of about γ ∼ = 200 mN/m. Wetting angles of about 150–160◦ are specific to all investigated compositions.

4. Conclusions The hot-compaction and deformation in nanocomposite NdFeB magnets take place due to the existence of an intergrain Nd-rich phase that is liquid at working temperature. Also the texturing mechanism within these magnets is realized through liquid phase diffusion. Therefore, in order to obtain high-performance NdFeB nanocomposite magnets, one must study and control the surface tension along with the wetting behavior of the Nd-rich intergranular region. From our studies performed on Nd90.9 Fe9.1 , Nd88 Co12 ,and Nd77.5 Ga22.5 alloys (corresponding to Nd7 Fe3 , Nd3 Co, and Nd5 Ga3 phases) in molten state, as the stoichimetry of the Nd-rich intergrain regions in NdFeB magnets, there was obtained that the contact angle decreases with increasing temperature whereas the surface tension generally increases with temperature raise above the melting point indicating a capillarity effect. The non-monotonous trend of the values of the contact angle and surface tension with temperature raise may be attributed to some thermal non-equilibrium conditions. Comparing the wetting behavior of Nd7 Fe3 , Nd3 Co,

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