Oxidation interconnect

Cells to be operated in the temperature region of 900-1,000°C. Usually oxide interconnects are used together with electrolyte made of YSZ. Since materials compatibility is severe at higher temperatures, stable lanthanum manganite-base cathode is adopted, whereas nickel anodes are used in a similar manner to other types. Electrolyte-self-support or cathode-support types are adopted. 

The technical issues associated with oxide interconnect can be summarized as follows ... 

The material cost of lanthanum is high so that the thickness of the oxide interconnects should be thin enough to reduce the amount used in cells [31]. On the other hand, the LaCrOs-based intercoimects should be thick enough to prevent the electrochemical oxygen permeation which takes place as the bipolar diffusion of oxide ions and holes inside the interconnect. For such a purpose, Ca-doped LaCrOs is not appropriate because those ceramics are highest in gas permeation among the alkali earth-doped LaCrOs [36, 37]. 

From the materials design point of view for those electrical conductive oxides which are stable under an oxygen potential gradient, oxide intercoimects and coating materials for alloys are quite similar. From the fabrication method, however, there are several differences. Inexpensive sintering process can be applied for oxide interconnects, whereas high-temperature heat treatment is limited for coating. The low-pressure plasma splay technique was successfully adopted to form dense films however, from the cost point of view, this is not appropriate. The atmospheric pressure plasma splay is more cost effective, but the quality of fabricated films is not excellent. 

Identification of high cost element. The most expensive elements are Y in YSZ, La in cathodes or oxide interconnects Co in LSCF, Ni in anode can be also... 

In the beginning, strong interest arose in utilization of metal interconnects instead of LaCrOs-based oxide interconnects [13]. Because of the severe corrosion of metals at high temperatures, operation temperature needs to be lowered. 

Recently efforts have been made on oxide anodes. The main reason for such investigations is to overcome the demerits of Ni cermet anodes as just described. Although the oxide anodes should be in service under a reducing atmosphere, the fabrication is usually performed in air so that oxide anodes should be stable at both oxidative and reductive atmospheres. This requirement is similar to those for oxide interconnects, implicitly indicating that material selection becomes severe to meet the chemical stability requirement. 

Doped ceria and doped lanthanum chromites were investigated a long time ago because ceria is a mixed conductor in a reducing atmosphere, whereas lanthanum chromites are typical candidates for oxide interconnects. Neither of the materials shows good performance as an anode. In recent years, other types of perovskite oxides have attracted attention, as is described in other chapters of this book. The basic trade-off relationship associated with oxide anodes is stability versus performance. 

The reasons to utilize metal interconnects [13] instead of oxide interconnects... 

Material cost La in the oxide interconnect is expensive, whereas ferritic alloys can be regarded as inexpensive. 

Fig. 2.8 Oxygen potential distributions in the oxide interconnects and the metal interconnects. Inside the oxide, the oxygen potential distribution is determined by the oxide ion and electron conductivity, whereas the surface oxide scale determines the main features of the metal interconnects...

Flattened tubes Anode-supported or cathode-supported tubes are flattened so that there is no need to seal the side of the tubes. When both ends are open, at least the entrances are sealed. When one end is closed, there is a need for an additional pipe to introduce air or fuel. Normally, interconnect materials are fabricated simultaneously. For this purpose, oxide interconnects are more appropriate. Figure 2.10 shows the flattened tube cells fabricated by Kyocera to be operated at 750°C. 

Since the A-site-deficient lanthanum manganite shows better compatibility with YSZ, its compatibility with oxide interconnect is of particular interest. Nishiyama et al. [62] found the following interesting behaviour of the A-site-deficient manganite at the interface with (La,Ca)Cr03 (LCC) that had excess CaO to enhance its air sinterabillty ... 

The alternative oxide interconnects have been proposed for the 10 kW class tubular SOFC stacks by Mitsubishi Heavy Industiy/Electric Power Development Co., Japan (Konishi et al., 2002). The material is based on strontium titanates with the substitution of alkaline earths and rare earths, (M, M )TiOs (M = Ca, Sr, Ba) (M = La, Sm, Pr, Gd, Nd, Y, Er, etc.) (Miyachi et al., 1999 Nishi et al., 1999). 

The flux of oxygen permeation ) through a dense plate of oxide interconnect with thickness L can be expressed by using the defect model described by van Van Hassel et al. (1993). 

An electrochemical vapor deposition (EVD) technique has been developed that produces thin layers of refractory oxides that are suitable for the electrolyte and cell interconnection in SOFCs (9). In this technique, the appropriate metal chloride (MeCl ) vapor is introduced on one side of a porous support tube, and H2/H2O gas is introduced on the other side. The gas environments on both sides of the support tube act to form two galvanic couples, ie. 

The result is the formation of a dense and uniform metal oxide layer in which the deposition rate is controlled by the diffusion rate of ionic species and the concentration of electronic charge carriers. This procedure is used to fabricate the thin layer of soHd electrolyte (yttria-stabilized 2irconia) and the interconnection (Mg-doped lanthanum chromite). 

The blue-black Hon oxide formed in this process fills some of the interconnecting porosity and much of the surface. Hence the density is increased, resulting in higher compressive strength. Furthermore, the oxide coating increases hardness (qv) and wear resistance. 

Plasma etching is widely used in semiconductor device manufacturing to etch patterns in thin layers of polycrystaUine siUcon often used for metal oxide semiconductor (MOS) device gates and interconnects (see Plasma TECHNOLOGY). 

Directed Oxidation of a Molten Metal. Directed oxidation of a molten metal or the Lanxide process (45,68,91) involves the reaction of a molten metal with a gaseous oxidant, eg, A1 with O2 in air, to form a porous three-dimensional oxide that grows outward from the metal/ceramic surface. The process proceeds via capillary action as the molten metal wicks into open pore channels in the oxide scale growth. Reinforced ceramic matrix composites can be formed by positioning inert filler materials, eg, fibers, whiskers, and/or particulates, in the path of the oxide scale growth. The resultant composite is comprised of both interconnected metal and ceramic. Typically 5—30 vol % metal remains after processing. The composite product maintains many of the desirable properties of a ceramic however, the presence of the metal serves to increase the fracture toughness of the composite. 

Microstructural examinations of the external surface revealed an interconnecting network of graphite flakes embedded in a matrix of iron oxide. 

The sulphide usually forms an interconnected network of particles within a matrix of oxide and thus provides paths for rapid diffusion of nickel to the interface with the gas. At high temperatures, when the liquid Ni-S phase is stable, a duplex scale forms with an inner region of sulphide and an outer porous NiO layer. The temperature dependence of the reaction is complex and is a function of gas pressure as indicated in Fig. 7.40 . A strong dependence on gas pressure is observed and, at the higher partial pressures, a maximum in the rate occurs at about 600°C corresponding to the point at which NiS04 becomes unstable. Further increases in temperature lead to the exclusive formation of NiO and a large decrease in the rate of the reaction, due to the fact that NijSj becomes unstable above about 806°C. 

In general, greatly reduced rates of attack are observed for impure or dilute nickel alloys compared with pure nickel when exposed to SO2 + O2 atmospheres. Haflan et al. have attributed this to the segregation of impurities at the sulphide/oxide interface causing breakup of the sulphide network. For example in the case of silicon additions, it has been shown that silicates form and it has been proposed that these alter the wetting characteristics of the sulphide and prevent the establishment of an interconnected sulphide network. 

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