Powder cermet

Dispersions of flake aluminum powders having surface oxide up to 14 wt % Al O have been pressed, sintered, and worked to a material known as sintered aluminum powder (SAP). This product exhibits high strength at elevated temperatures. Nickel containing small additions of thoria, known as TD-nickel, is also a high temperature cermet. 

Dining outgassing of scrap uranium-aluminium cermet reactor cores, powerful exotherms led to melting of 9 cores. It was found that the incident was initiated by reactions at 350°C between aluminium powder and sodium diuranate, which released enough heat to initiate subsequent exothermic reduction of ammonium uranyl hexafluoride, sodium nitrate, uranium oxide and vanadium trioxide by aluminium, leading to core melting. 

The electrical conductivity of a Ni-YSZ cermet anode depends on the composition (i.e., Ni to YSZ volume ratio), the microscopic features of the starting materials (e.g., particle size and distribution of NiO and YSZ powders), and the sintering and reduction conditions (e.g., temperature and atmosphere), as will be discussed in detail in the following sections. 

FIGURE 2.1 Change of electrical conductivity with respect to Ni content in the Ni-YSZ cermets at 1,000°C. Two types of YSZ were used to prepare the cermets one was Toyo Soda powder with a specific surface area of 23 m2/g and an agglomerate size of -0.3 pm, and the other was Zircar powder with a specific surface area of 47 m2/g and an agglomerate size of -0.1 pm the NiO used has a specific surface area of 3.5 m2/g. (From Dees, D.W. et al., J. Electrochem. Soc., 134 2141-2146, 1987. Reproduced by permission of ECS-The Electrochemical Society.)... 

Similar to the percolation threshold, the effective electrical conductivity of a porous Ni-YSZ cermet anode depends on the morphology, particle size, and distribution of the starting materials as well. In general, the effective conductivity increases as the NiO particle size is reduced when other parameters are kept constant. As shown in Figure 2.4 (samples 1 and 2), the cermet conductivity increased from -10 S/cm to 103 S/cm as the NiO particle size was decreased from 16 to 1.8 pm while using the same YSZ powder (primary particle size of -0.3 pm) and the same Ni to YSZ volume fraction [30],... 

FIGURE 2.2 (a) Particle size distribution for three different NiO powders used in the Ni-YSZ cermets the circle is for a commercial NiO powder with 0 h milling, the diamond is for the commercial NiO milled for 138 h, and the square is for the NiO powder prepared by the GNP process, and (b) resistivity versus Ni volume percent for the Ni-YSZ cermets made with the three different NiO powders at 1,000°C in reducing atmosphere. The YSZ powder has grain size of 0.1 to 0.2 pm, and the cermets were all sintered at 1,400°C for 2 h. (From Huebner, W. et al., Proceedings of the Sixth International Symposium on Solid Oxide Fuel Cells, 95(l) 696-705, 1995. Reproduced by permission of ECS-The Electrochemical Society.)... 

FIGURE 2.14 (a) Influence of NiO particle size on the anode overpotential r (at a constant current density of 250 m A/cm2), anode ohmic resistance Rn, and anode interfacial resistance, RE, for cermets made from YSZ and three different types of NiO NiO-1, NiO-2, and NiO-3, of which the particle size is 1, 5, and 10 pm, respectively, (b) Influence of TZ3Y particle size on the anode overpotential rj (at a constant current density of 250 mA/cm2) for cermets made from 600°C-calcinated NiO-1 and six different TZ3Y powders with different particle sizes. (From Jiang, S.P. et al., Solid State Ionics, 132 1-14, 2000. Copyright by Elsevier, reproduced with permission.)... 

Ni-YSZ cermet anodes satisfy most of the basic requirements for SOFC anodes. The effective conductivity of a Ni-YSZ cermet anode increases with the Ni to YSZ volume ratio, relative density, and decreasing the particle size ratio of NiO to YSZ. While coarse YSZ powders may result in poor mechanical strength and low stability, coarse NiO powders may lead to poor effective conductivity. The effective conductivity increases with the temperature at which the NiO is reduced to Ni metal in a reducing atmosphere. Further, very low reduction temperatures (e.g., below 400°C) may result in not only low electrical conductivity, but also poor mechanical strength. 

Misono T, Murat, K, Fukui T, Chaichanawong J, Sato K, Abe H et al. Ni-SDC cermet anode fabricated from NiO-SDC composite powder for intermediate temperature SOFC. J. Power Sources 2006 157 754—757. 

Unfortunately, it appears that this method cannot be applied to the fabrication of Cu cermets due to the low melting temperatures of CuO and CU2O. For lower sintering temperatures, it is likely that the electrolyte powder within the electrode does not become connected, either to itself or to the electrolyte. 

Another application concerning the liquid-metal-cooled fast breeder reactors (LMFBR) was studied as development of advanced control rod materials for FBR (22). Fabrication tests and out-of-pile measurements were made of B4C/Cu cermet to obtain high-performance neutron absorber materials for LMFBR. A coating layer of Cu was formed on the surface of B4C/Cu powder, and then the coated B4C... 

The term cermet derives from the combination of ceramic and melal. Cermets are produced by powder metallurgy techniques and represent Ihe bonding of two or more metals. They are particularly useful at high temperatures (850- I250"C). Chromium is used in several cermet combinations, including chromium-bonded aluminum oxide, metal-bonded chromium carbide, and metal-bonded chromium horide. 

The anode (fuel electrode) consists of an approximately 120/rm thick Ni/YSZ cermet layer. This is achieved simply by applying a slurry of nickel and YSZ powders to the electrolyte and sintering. A cross-section of the 3-layer microstructure is shown in Fig. 4.33. 

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