1. Based on the Hume-Rothery Rules, speculate on the possible driving forces beh
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Question
1.Based on the Hume-Rothery Rules, speculate on the possible driving forces behind the tendency for ordering in this system.
2.Speculate on whether you would expect a mixture of AuAg3 to show a greater or lesser tendency toward ordering in this system and explain your rationale (use physical data available in the textbook).
3. Represent the defect chemistry for the introduction of Y2O3 as a solute in a ZrO2 host crystal using Kroger-Vink notation.
Explanation / Answer
THIS WILL HELP YOU Pure zirconia (ZrO2) has a monoclinic structure up to a temperature of about 1446 K, where it changes to the tetragonal modification. For temperatures higher than 2643 K zirconia adopts a cubic fluorite structure [1]. However, the high temperature phases can be partially or completely stabilized at room temperature by doping with aliovalent oxides such as yttria (Y2O3), calcia (CaO), or magnesia (MgO). Yttria is the most commonly used dopant for stabilizing the cubic phase of zirconia; a fully (cubic) stabilized zirconia is obtained with a Y2O3-content of >7 mol% [2], while a Y2O3-content of about 2-6 mol% gives a partially stabilized zirconia. Cubic stabilized zirconia has improved mechanical and thermal properties such as high strength, toughness, and thermal-shock resistance. Furthermore the addition of substitutional cation (e.g., Ca2+, Mg2+, Y3+), which have lower valency than zirconium ion (Zr4+), induces the generation of oxygen vacancies for charge compensation. For example, the substitution of Zr4+ with Y3+ causes the negative net charge in the lattice; for every mole of yttria incorporated into the zirconia lattice, the charge neutrality condition is kept by forming an oxygen vacancy. In a Kroger-Vink notation, it is written as follows (1) Where means Y in the Zr site with the apparent negative charge, and is the vacancy in the oxygen site with double positive charge. is the lattice oxygen, i.e., oxygen in the oxygen site with net charge of zero. The existence of vacancies on the oxygen site gives rise to the high ionic conductivity of YSZ. Oxygen is transported by hopping through its vacancy sites (vacancy diffusion mechanism). The concentration of oxygen vacancy is determined by the concentration of the dopant. Areas of Application of YSZ One major application of YSZ is in solid fuel cell (SOFC) technology where it is used as electrolyte material due to its oxygen ion conductivity, high chemical and cryastallographic stability, and low electronic conductivity [3]. The SOFC comprises of two porous electrodes, an anode and a cathode, separated by a dense electrolyte and it is usually operated at high temperatures (~1000oC). The function of the electrolyte is to transport the oxygen ions from the cathode to the anode where oxidation of the fuel by the ions occurs (as depicted figure 1), and also to block the electrons produced at the anode from passing through the cell to the cathode [4]. The typical thickness of the electrolyte material is 100 -150 micron. For an effective operation of a fuel cell, with the electrolyte within this range of thickness, the ionic conductivity of the electrolyte material must be higher than 0.1 Scm-1. The ionic conductivity determines, to a large extent, the available power and operating temperature of a SOFC. The ionic conductivity of 8 mol% YSZ at 1000oC is 0.18 Scm-1 [5] Figure 1. (a) Scanning electron microscopy (SEM) image of a typical SOFC showing the anode, electrolyte, and cathode layers; (b) Schematics of a SOFC showing the mode of operation. YSZ is also widely used as thermal barrier coating (TBC) layer material for gas turbine engine (see figure 2) because of its low thermal conductivity and relatively high strength, toughness, thermal shock resistanc [6]. The low thermal conductivity of bulk YSZ results from the low intrinsic thermal conductivity of zirconia and phonon scattering defects (vacancies) introduced by the addition of yttria. Both the yttrium cations and the oxygen vacancies are effective phonon scattering sites the thermal conductivity is decreased as the yttria content is increased. In practice, a yttria concentration in the range of 6 to 8 wt.% is generally used.
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