Determination of gibbs energy of formation of Ni-B-O system by electromotive force measurement using solid electrolyte
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EL-BASED alloys containing B, such as Ni-Si-B, Ni-Cr-B, and Ni-Cr-Si-B-Fe alloys, are widely used as nickel brazing filler metals.[1] Also, Ni-based alloys containing B are expected to have a possibility to be corrosionresistant and wear-resistant alloys. We have therefore investigated the corrosion resistance and wear resistance of Ni-Mo-B alloys in detail.[2–9] In order to obtain information on the high-temperature oxidation characteristics of these alloys, the thermodynamic investigation of Ni-Si-B-O, Ni-Cr-B-O, Ni-Cr-Si-B-Fe-O, and Ni-Mo-B-O systems is needed. The Ni-B-O ternary system is a fundamental system common to these Ni-based multicomponent alloys. Thus, we selected the Ni-B-O system and undertook determination of the Gibbs energy of formation of the Ni-B-O HIROAKI YAMAMOTO, Research Associate, MASAO MORISHITA, Professor, AKIO MIYATA, Graduate Student, and KOICHIRO KOYAMA, Professor, are with the Department of Materials Science and Chemistry, Graduate School of Engineering, University of Hyogo, Himeji, 671-2201, Japan. Contact e-mail: [email protected] Manuscript submitted February 6, 2006. METALLURGICAL AND MATERIALS TRANSACTIONS B
system including the binary Ni-B system. The Ni-B binary system has five compounds of Ni3B, Ni2B, o-Ni4B3, m-Ni4B3, and NiB, as shown in Figure 1,[10,11] the Ni-O binary system has one compound of NiO above 673 K,[12] and the B-O binary system also has one compound of B2O3. On the other hand, the Ni-B-O ternary system has two compounds of Ni3B2O6 and Ni2B2O5.[13] The former is an incongruent compound and present below 1612 6 5 K, while the latter is present over 1576 6 5 K.[13] Of the compounds described previously, the standard Gibbs energies of formation of NiO and B2O3 are known.[14] Thus, we undertook determination of the standard Gibbs energies of formation of six compounds, Ni3B, Ni2B, o-Ni4B3, m-Ni4B3, NiB, and Ni3B2O6, in the temperature range from 1173 to 1393 K by measuring electromotive forces of galvanic cells having an Y2O3-stabilized ZrO2 solid oxide electrolyte. II.
EXPERIMENTAL
A. Materials Used and Preparation of Electrodes The compositions of the electrodes used for the electromotive force measurements are shown by round marks I VOLUME 37B, AUGUST 2006—607
through VI in Figure 1, which shows the phase relationships of the Ni-B-O ternary system with the phase diagram of the Ni-B binary system. These phase relationships can be easily determined because the Ni-Ni3B-B2O3 and Ni3BNi2B-B2O3 three-phase equilibria have already been confirmed by Omori in our laboratory.[15] The compositions of the specimens, starting substances, and equilibrium phases identified by X-ray diffraction are shown in Table I. Ni-Ni3B(Ni-24.5 mol pct B), Ni3B-Ni2B(Ni-30 mol pct B), Ni2B-o-Ni4B3(Ni-39.5 mol pct B), o-Ni4B3-mNi4B3(Ni-42.5 mol pct B), and m-Ni4B3-NiB(Ni-45.3 mol pct B) two-phase alloys were prepared by arc melting the
compressed compacts of the mixtures of Ni and B powders with a tungsten electrode in an argon atmosphere, crushed in an iron mortar, and then finely powd
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