Brazing of metallic conductors onto ceramic plates in solid oxide fuel cells Part II Attaching conducting wires

Commercialization of the solid oxide fuel cell (SOFC) will be facilitated by use of conventional materials and fabrication processes. In this paper, we discuss the results of brazing metallic wires, made of conventional heat-resisting alloys, onto metallic CrFe5Y₂O₃1 conductor plates in SOFC. Such wires would be used for the current transport between individual SOFC stacks and to the power-consuming device. Aluminum-alloyed ferritic stainless steels (iron, 20 to 25 wt% chromium, 4.5 to 6 wt% aluminum) and Inconel 617 were found to be suitable materials for the wires. They can be brazed onto CrFe5Y₂O₃1 using L-Ni 5 as filler for Inconel 617 and the ferritic steels, and Cr50Ni or L-Ni 5 for the ferritic steels. The effectiveness of the brazed conductor/CrFe5Y₂O₃1 joints was verified by monitoring their resistance at 1000°C in air for 1000 h.     

The microstructure and mechanical properties of yttria-stabilized zirconia prepared by arc-melting

The microstructure of ZrO₂-Y₂O₃ alloys prepared by arc-melting was examined mainly by electron microscopy. It was found that the microstructure changed markedly with yttria content between 0 and 8·7 mol%. Pure zirconia was a single monoclinic phase, while ZrO₂-8·7 mol% Y₂O₃ alloy was single cubic phase as expected from ZrO₂-Y₂O₃ phase diagram. Tetragonal phase was found in alloys with 1 to 6 mol% Y₂O₃ together with monoclinic or cubic phase. The tetragonal phase found in present alloys normally had a lenticular shape with a length 1 to 5m and a width 0.1 to 0.3m, which is much larger than that formed by annealing. The phase with a herring-bone appearance was found in alloys with Y₂O₃ between 2 and 3 mol%, which was recognized to be a metastable rhombohedral phase. The structure of the present alloys is likely to be formed by martensitic or bainitic transformation during fairly rapid cooling from the melt temperature. The change in hardness and toughness with yttria content of the alloys is discussed on the basis of microstructural observations.     

Acoustic emission study of phase relations in low-Y2O3 portion of ZrO2-Y2O3 system

The (metastable) tetragonal phase in 3–4 mol% Y₂O₃-ZrO₂ alloys undergoes a transition to the monoclinic form in the 200–300 °C temperature range. Microcracking due to the volume change at this transition has been detected in these compositions by sharp acoustic emission during heating. The phase change was confirmed by X-ray diffraction, dilatometry and scanning electron microscopy. The monoclinic tetragonal transition in ZrO₂-1 mol% Y₂O₃ alloy at 850–750 °C and the same phase change in 2, 3, 4 and 6 mol% Y₂O₃ compositions at the eutectoid temperature of about 560 °C was also clearly signalled by the acoustic emission counts during heating and cooling. There was no significant acoustic emission activity on heating and cooling the 9 and 12 mol% Y₂O₃ compositions, which are cubic. The acoustic emission data thus confirm the phase relations in the 1–12 mol% Y₂O₃ region, established by conventional methods such as differential thermal analysis, dilatometry and X-ray diffraction.     

Development of ternary Mg based alloy for soldering of AZ31B magnesium alloy

Abstract

Two kinds of ternary Mg based alloys were designed to join the AZ31B magnesium alloy plates by high frequency induction soldering with argon shielding gas. The microstructures and properties of the filler metals and joints were investigated by SEM, X-ray diffraction, differential scanning calorimetry, spreading test and tensile test. The results have shown that the microstructures of Mg–31·5Al–10Sn filler metal mainly consist of Mg₁₇Al₁₂, Mg₂Sn and a trace amount of α-Mg phases, while the microstructures of Mg–29·5Zn–1Sn filler metal include α-Mg phase and Mg₇Zn₃ with a trace of α-Mg and Mg₂Sn phases. Both of the filler metals have narrow melting zones; however, the spreading area of the Mg–31·5Al–10Sn filler metal is much larger than that of the Mg–29·5Zn–1Sn filler metal on the AZ31B base metal. The average tensile strength of solder joints with Mg–31·5Al–10Sn filler metal is a little higher than that of the latter solder joints with Mg–29·5Zn–1Sn filler metal.     

Long range coupling in YBa2Cu3O7−δ/Y1−xPrxBa2Cu3O7−δ multilayers

YBa₂Cu₃O_7–/(Y_1–xPr_x)Ba₂Cu₃O_7– multilayers have been used to probe coupling through (Y_1–x:Pr_x)Ba₂Cu₃O_7– alloys. We observe that the coupling between ultrathin YBa₂Cu₃O_7– layers, 12 or 24 Å thick, survives through several hundred Å of (Y_1–xPr_x)Ba₂Cu₃O_7– with x=0.4 and 0.55. T_c versus the thickness of the spacer-alloy, and activation energies for flux motion, with fields parallel and perpendicular to the c-axis, have been used to probe this long range coupling. All these experiments point to an unusually large coupling length for these two alloy compositions. In the x=0.55 case this result is particularly surprising since the alloy material display a semiconducting behaviour for this composition. T_c measurements, activation energies, and a study of the vortex dynamics in these coupled multilayers is presented along with new results obtained on a series of multilayers built with a more insulating alloy, x=0.7.     

Phase Transformations in the Y2Cu2O5–BaCuO2 System

The sequence of phase transformations in the Y₂Cu₂O₅–BaCuO₂ pseudobinary system was studied during heating and cooling in the range 1170–1310 K. The results demonstrate that the reaction zone in BaCuO₂/Y₂Cu₂O₅ diffusion couples consists of BaCuO₂/YBa₂Cu₃O_{7 –} /Y₂BaCuO₅/Y₂O₃/Y₂Cu₂O₅ layers, corresponding to the sequence of chemical changes during YBa₂Cu₃O_{7 –} crystallization between 1170 and 1220 K. In the range 1260–1310 K, BaCu₂O₂ is formed. During cooling of a Y₂Cu₂O₅ + 4.3BaCuO₂ mixture, YBa₂Cu₃O_{7 –} crystallizes in a wide temperature range, between 1240 and 1190 K. The process depends on the presence of BaCu₂O₂ on the surface of Y₂BaCuO₅ grains in the high-temperature solution and the oxygen supply from the gas phase.     

X-ray diffraction characterization of dental gold alloy-ceramic interfaces

X-ray diffraction (XRD) was employed to study dental alloy-ceramic interfaces. A Au-Pd-In alloy, which requires oxidation before porcelain firing, and a Au-Pt-Pd-In alloy, which does not require oxidation before porcelain firing, were selected in this study. Alloy specimens were centrifugally cast. Specimen surfaces were metallographically polished through 0.05 m Al_2,O_3} slurries. A thin layer (<50 m) of a dental opaque porcelain was fired on the alloy surfaces with and without initial oxidation. XRD was conducted at room temperature on four types of alloy specimens: polished, oxidized, porcelain fired after alloy oxidation, and porcelain fired without initial alloy oxidation. XRD was also performed on fired opaque porcelain without an alloy substrate. The detection of prominent gold solid solution peaks from alloy-ceramic specimens indicated that the incident X-ray beam reached the alloy-ceramic interface. In₂O₃ and -Ga₂O₃ were identified on the oxidized Au-Pd-In alloy, while In₂O₃ and SnO₂ were detected on the oxidized Au-Pt-Pd-In alloy. Preferred orientation was observed for all the oxides formed on the alloys. Minimum lattice parameter changes (<1%) for the gold solid solutions were observed for both alloys before and after oxidation and porcelain firing. Leucite (KAlSi₂O₆), TiO₂, ZrO₂ and SnO₂ were detected on the fired opaque porcelain. For both alloys, no additional oxides were identified at the metal-ceramic interfaces beyond those present in the oxidized alloys and the opaque porcelain. Similar results were obtained from alloy-ceramic interfaces where there was no prior alloy oxidation. The results indicate the critical role of alloy surface oxides in metal-ceramic bonding and support the chemical bonding mechanism for porcelain adherence. © 2001 Kluwer Academic Publishers     

TEM study of Mg-Zn precipitates in Mg-Zn-Y alloys

The precipitates in Mg-Zn-Y alloys in the as-cast state with nominal atomic composition of Mg₉₆Zn₁Y₃, Mg₉₆Zn₂Y₂ and Mg₉₇Zn₁Y₂ were studied by means of TEM as well as XRD techniques. The results show that there is a phase separation of Zn on the nanometer scale in these alloys. Two kinds of nano-sized precipitates were found, namely MgZn and MgZn₂. TEM observation shows that MgZn precipitates are distributed in both the Mg matrix and in Mg₂₄Y₅ grains, which is the typical precipitate in Mg-Zn-Y alloys. There is an inherent crystallographic relationship between MgZn and Mg₂₄Y₅ precipitates: // , // . The size of MgZn₂ precipitates is much smaller than that of MgZn precipitates. They are distributed only in Mg₂₄Y₅ grains, not in the Mg matrix. It is suggested that the nano-sized precipitates, MgZn and MgZn₂, can improve the mechanical properties of the Mg-Zn-Y alloys studied.     

Investigations of metallic alloys for use as interconnects in solid oxide fuel cell stacks

Some high-temperature alloys have been investigated in order to determine whether they are suitable as metallic interconnect materials in solid oxide fuel cell stacks. The requirements for such alloys are formulated. Thermal dilatometry and oxidation tests, as well as theoretical calculations of the stresses that are induced by differences in thermal expansion of the individual materials, have been performed. The results show that a chromium-rich alloy, with dispersions of fine Y₂O₃ particles, perform best among the samples investigated. Improvements are still needed in order to make the alloy fully applicable in a solid oxide fuel cell stack. Some suggestions for improvements are put forward.     

New NTC thermistors based on LaCrO<Subscript>3</Subscript>–Mg(Al<Subscript>0.7</Subscript>Cr<Subscript>0.3</Subscript>)<Subscript>2</Subscript>O<Subscript>4</Subscript> composite ceramics

In this study, the xLaCrO₃–(1?x)Mg(Al_0.7Cr_0.3)₂O₄ (x?=?0.1, 0.2, 0.3, 0.4) negative temperature coefficient composite ceramics were fabricated through conventional solid state reaction at 1650?°C. X-ray diffraction analysis has revealed that the sintered ceramics are consisted of cubic spinel Mg(Al_0.7Cr_0.3)₂O₄ phase and orthorhombic perovskite LaCrO₃ phase. The obtained values of \({{\rho }_{\text{300}}}\) and \({{B}_{400/800}}\) and \({{E}_{\text{a}}}\) are in the range of 1.55?×?10²–1.41?×?10⁸ Ωcm, 756–11317 K, 0.065–0.976 eV, respectively. The electrical properties of these ceramics can be adjusted by the LaCrO₃ contents. Such ceramics could be suitable for high temperature NTC thermistor application.     

Microstructural investigation of a rapidly solidified Ti-6Al-4V-1B-0.5Y alloy

A Ti-6Al-4V-1B-0.5Y (wt%) alloy has been prepared by consolidation of the melt-spun alloy fibres. The microstructures of the melt-spun fibre and the consolidated alloy were examined by different techniques. It was found that in the consolidated alloy, titanium boride and yttrium oxide particles have a refined particle size and a uniform distribution in the (+) matrix compared with the microstructure of the same alloy obtained by conventional ingot metallurgy. The boride phase in the consolidated alloy mainly has a needle-shaped morphology and has been identified by electron diffraction to be orthorhombic TiB with a B27 structure, while the yttrium oxide has a cuboidal morphology and has been identified as bcc Y₂O₃. Detailed TEM examination also revealed that yttrium addition has a strong influence on the TiB morphology by comparing the microstructures of Ti-6Al-4V-1B alloys with and without yttrium addition. Under similar processing conditions, the TiB phase in the consolidated alloys without yttrium addition mainly has a nearly equiaxed morphology with a finer particle size, while the TiB phase in the consolidated alloy with yttrium addition will mainly have a needle-shaped morphology. This effect of yttrium addition on the TiB morphology has been discussed in terms of heterogeneous nucleation and the reduced undercooling.     

Brazing Al2O3 to sintered Fe-Ni-Co alloys

Active metal brazing has been widely used to join ceramics to metals, as sound joints are usually achieved in a single step process without special equipment. However, residual stresses may be a potential problem especially upon joining ceramics to alloys with relatively high thermal expansion coefficients. This work investigates the brazing behavior of Alumina (Al₂O₃) to a sintered Fe-Ni-Co alloy, specially designed to match its coefficient of thermal expansion to that of the ceramic counterpart. The results indicate the presence of an interfacial zone whose microstructure depends on the filler alloy employed. A relationship was established between the microstructure of the interface and the flexural strength of the joints.     

Microstructure and bond strength of Ni–Cr steel/Al2O3 joints brazed with Ag–Cu–Zr alloys containing Sn or Al

Abstract

Sintered Al₂O₃ was joined to Ni–Cr steel by the active metal brazing route with Ag–Cu–Zr brazing alloys containing Sn or Al. A single ZrO₂ layer with a monoclinic structure was formed at the Al₂O₃ /brazement interface by the migration of Zr in the molten brazing alloy to the Al₂O₃ surface, followed by a redox reaction between the Al₂O₃ and Zr. The remainder of the brazement formed a Cu–Ag eutectic alloy. Precipitates CuZr₂ and Cu–Zr–Al were formed in the brazements of the Ni–Cr steel/ Al₂O₃ joints brazed with Ag–Cu–Zr alloys and Al containing Ag–Cu–Zr alloys, respectively. On the other hand, no precipitates were formed in the brazement of the Ni–Cr steel/Al₂O₃ joints brazed with Sn containing Ag–Cu–Zr alloys. The Ni–Cr steel/ Al₂O₃ joints brazed with Sn containing Ag–Cu–Zr alloys showed much higher fracture shear strengths than those brazed with Ag–Cu–Zr alloys or Al containing Ag–Cu–Zr alloys.     

Microstructure of brazed joints between mechanically metallized Si<Subscript>3</Subscript>N<Subscript>4</Subscript> and stainless steel

Brazing has been increasingly used to join metals to advanced ceramics. Brazing covalent materials requires either the use of active filler alloys or the previous metallization of the surface. To that end, a new and simple mechanical technique has been applied to metallize advanced ceramics, thus avoiding the use of costly Ti-based active filler alloys. The mechanical metallization of Si₃N₄ with Ti was employed as an alternative route to deposit active metallic films prior to brazing with stainless steel using 72% Ag--28% Cu or 82% Au—18% Ni eutectic alloys. The brazing temperatures were set to 40°C or 75°C above the eutectic temperature of each filler alloy. Ti-films of average thickness 4 μm produced adequate spreading of both filler alloys onto Si₃N₄ substrates, which were subsequently brazed to stainless steel. The interface of Si₃N₄/310 stainless steel basically consisted of a reaction layer, a precipitation zone and an eutectic microconstituent. Mechanically sound and vacuum-tight joints were obtained, especially upon brazing at relatively lower temperatures. Increasing the brazing temperature resulted in thermal cracking of the Si₃N₄, possibly due to increased thermal stress.     

Mechanochemical Synthesis of Yttrium and Lanthanum Tantalates

The phase changes in the Y₂O₃–Ta₂O₅ and La₂O₃–Ta₂O₅ systems during mechanical activation and subsequent heat treatment are investigated. Y₂O₃ and Ta₂O₅ are shown to react during mechanical activation to form Y₃TaO₇. Calcination of activated mixtures leads to the formation of YTaO₄ and LaTaO₄. The observed processes are interpreted in terms of transformations of the fluorite structure. A new method is proposed for the synthesis of rare-earth tantalates for applications in diagnostic x-ray contrast media. The method has a number of important advantages over the conventional ceramic route.     

Effects of Y<Subscript>2</Subscript>O<Subscript>3</Subscript> on the microstructure and electrical properties of Pr-ZnO varistors

Effects of Y₂O₃ contents on the microstructure and electrical properties of Pr₆O₁₁-based ZnO varistor have been studied. The varistor voltage (V_1mA) and the nonlinear exponent () increased with increasing Y₂O₃ contents, whereas the dielectric constant decreased. Y-rich phase segregated at nodal points in grain boundaries. The average grain size decreased with increasing Y₂O₃ content. The specimen with 4.0 mol% Y₂O₃ sintered at 1285 C exhibited the highest nonlinear exponent ( = 77) with the dielectric constant, ~352 at 1 kHz.     

Debond coating requirements for brittle matrix composites

The cause of improved fracture toughness in Y₂O₃-coated niobium-toughened TiAl relative to either uncoated niobium or Al₂O₃-coated niobium was examined. Reactively sputtered Y₂O₃ coatings, 1–2 m thick, were deposited on to rock salt (NaCl), polished single-crystal (0001) Al₂O₃, and polished polycrystalline niobium. Sputtered niobium coatings, 1–2 m thick, were also deposited on to polished single-crystal Y₂O₃ substrates for comparison. The oxide coating was characterized and consisted of stoichiometric bcc Y₂O₃ witha ₀=1.0602 nm. Indentation tests were performed to correlate the fracture toughness and debond characteristics of as-deposited Y₂O₃ coatings on Al₂O₃ and polycrystalline niobium, and niobium coatings on single-crystal Y₂O₃, to that found in TiAl/Nb and Al₂O₃/Al₂O₃ laminates. The calculated fracture toughness of sputtered Y₂O₃ on sapphire was similar to reported values for bulk Y₂O₃. However, a wide variation in interfacial fracture toughness was obtained by indentation methods, and is attributed to the microstructure of as-deposited coatings and to weak bonding between as-deposited yttria and the sapphire substrate. These results are related to factors that affect debonding and fracture toughness of brittle matrix composites. Reactive and non-reactive metal/ceramic systems were reviewed in an effort to understand why Y₂O₃ coatings perform well. It is postulated that yttrium oxide coatings applied to niobium have an atomically sharp interface that has a lower fracture energy compared to Nb/Al₂O₃, resulting in improved interfacial debonding and composite fracture toughness.     

Effect of heat-treatment on phase stability and grain coarsening in a powder-processed Al–Ni–Co–Zr–Y alloy

Processing of Al alloys via metastable amorphous intermediates can give much higher volume fractions of dispersed strengthening phases than in conventional precipitation- or dispersion-hardened systems. Here, we report a study on an Al–Ni–Co–Zr–Y alloy processed by gas atomization and consolidated/devitrified by warm extrusion. X-ray diffraction and electron microscopy are used to reveal the effects of heat-treatments at 300–500 °C for up to 96 h on the phase stability and coarsening behavior of the alloy. In all samples, the microstructure contains 22 % by volume of Al₁₉(Ni,Co)₅Y₃ plates surrounded by grains of FCC Al. Samples heat-treated at 350 °C and above also contain fine Al₃Y and Al₃Zr particles as minority phases. The softening of the alloy is limited for heat-treatment temperatures of up to 400 °C, and the Al₁₉(Ni,Co)₅Y₃ plates coarsen slowly. At higher temperatures, abnormal coarsening is observed with the development of a secondary population of much larger Al₁₉(Ni,Co)₅Y₃ plates. An analysis of the coarsening kinetics gives a constant coarsening exponent of 3, but a distinct transition in the activation energies. These values suggest that the normal coarsening at lower temperatures occurs by short-circuit diffusion, whereas the abnormal coarsening at higher temperatures involves lattice diffusion. The Al grain size is dictated by the Al₁₉(Ni,Co)₅Y₃ inter-plate separation, and grain growth is limited by the extent of plate coarsening. Such systems could form the basis of new high-strength high-temperature Al alloys for structural applications.     

The Effect of Excess Aluminum on the Composition and Microstructure of Nb-Al Alloys Produced by Aluminothermic Reduction of Nb2O5

Intermetallic aluminides including those phases of the Nb-Al system are of interest for high-temperature structural applications. Through aluminothermic reduction (ATR) of Nb₂O₅ different alloys of the Nb-Al system can be produced by varying the amount of aluminum (excess aluminum) in the thermit charge. In this work, various Nb-Al alloys were produced by varying Nb₂O₅ and Al powder blends. The resulting alloys were characterized by chemical analysis (Al, O, and C), X-ray diffraction and scanning electron microscopy. The aluminum content of the alloys increased linearly from 14.5 to 50.4 at% as the excess Al was varied from 10 up to 60% over the stoichiometric amount to reduce the Nb₂O₅. The carbon content was lower than 300 wt-ppm. The oxygen content decreases with increasing excess Al, reaching 1300 wt-ppm for the alloy produced with 60% excess Al. The inclusion content (Al₂O₃) decreases significantly as the excess Al is increased. The following metallic phases were identified in the alloys: Nb_ss (niobium solid solution) and Nb₃Al (alloy produced with 10% excess Al); Nb₃Al (alloys produced with 15 and 20% excess Al); Nb₃Al, Nb₂Al, and NbAl₃ (alloy produced with 30% excess Al); and Nb₂Al and NbAl₃ (alloys produced with 40, 50, and 60% excess Al).     

Oxidation behaviour of titanium-containing brazing filler metals

Brazements on alumina or partially stabilized zirconia (PSZ) of four silver- or copper-based brazing filler metals that contain titanium to promote wetting of and adherence to structural ceramics, were exposed in a thermogravimetric analyser at temperatures up to 700°C to atmospheres of 100% O₂, Ar-20% O₂ and Ar-3 p.p.m. O₂. The alloys included Cu-41.1Ag-3.6Sn-7.2Ti, Ag-44.4Cu-8.4Sn-0.9Ti, Ag-41.6Cu-9.7Sn-5.0Ti and Ag-37.4Cu-10.8In-1,4Ti, at%. All formed external oxides that were more or less protective under all of the test conditions studied. The growth of the oxides followed a parabolic time law. The gains in weight due to oxidation observed were small, ranging (for 45 h exposure at 400 °C to Ar-20% O₂) from 0.20 mgcm⁻² for the Ag-37.4Cu-10.8In-1.4Ti alloy to 0.46 mgcm⁻² for Cu-41.1Ag-3.6Sn-7.2Ti. As expected, weight gain increased with increasing temperature or . Unexpectedly, the titanium played a minor role in the scale formed on any of the filler metals with a titanium oxide, TiO₂, being found on only one alloy — Ag-41.6Cu-9.7Sn-5.0Ti. The brazements on PSZ gained weight at a higher rate than comparative brazements on alumina. We attribute this behaviour to oxygen transport through the zirconia resulting in the growth of an interfacial layer of titanium oxide.