(c)Indicate how the local quench rate,T,varies on the GL-R plot. (d)Describe how the secondary dendrite arm spacing varies with local solidification time,tr,and local quench rate,T. 4. The elements A and B are solids at 1200K and form two solid "stoichiometric"compounds A2B and AB2 at 1200K.B has very limited solubility in A.A has a negligible vapor pressure,but for the sublimation of pure B: B(s)=B(v)AG(joules)=187,000-lIOT R=8.314 J/mol-K The vapor pressure for B over the equilibrated AB2-A2B mixture is given as: 1ogP(atm=.100+6.5 T The vapor pressure for B over the equilibrated A-A2B mixture is given as: 10gP(atm-130o0+7.0 T (i)What are the enthalpy and entropy changes for the sublimation of pure A? (ii)Calculate the standard Gibbs energies of formation for A2B and AB2- (iii)Carefully plot the AG of mixing diagram for the system between pure A and AB2- (iv)What is the activity of B for the two-phase field AB2-A2B? 5. Analyze the thermodynamics and kinetics of reduction of a NiO coupon at 1000K under the following three sets of conditions: (a)Ultra-high vacuum(outer space) (b)Rowing Ar containing 1%H2(1 atm) (c)Flowing H2(1 atm) In each case,predict the rate-limiting-step,and set up some expressions to describe the kinetics.The following standard Gibbs energies of formation at 1300K may be useful: AG Nio =-148,680 joules/mole AG for H20 =-192,540 joules/mole R=8.314 joules/mole-K VNi 6.6cm/mole;VNio 9.24 cm/mole(c) Indicate how the local quench rate, T, varies on the GL-R plot. (d) Describe how the secondary dendrite arm spacing varies with local solidification time, tf, and local quench rate, T. 4. The elements A and B are solids at 1200K and form two solid "stoichiometric" compounds A2B and AB2 at 1200K. B has very limited solubility in A. A has a negligible vapor pressure, but for the sublimation of pure B: B(s) = B(v) (joules) =187,000-ll0T R = 8.314 J/mol-K 0 ΔG The vapor pressure for B over the equilibrated AB2-A2B mixture is given as: log P(atm) = - 11000 T + 6.5 The vapor pressure for B over the equilibrated A-A2B mixture is given as: l0g P(atm) - - 13000 T + 7.0 (i) What are the enthalpy and entropy changes for the sublimation of pure A? (ii) Calculate the standard Gibbs energies of formation for A2B and AB2- (iii) Carefully plot the AG of mixing diagram for the system between pure A and AB2- (iv) What is the activity of B for the two-phase field AB2-A2B? 5. Analyze the thermodynamics and kinetics of reduction of a NiO coupon at 1000K under the following three sets of conditions: (a) Ultra-high vacuum (outer space) (b) Rowing Ar containing 1% H2 (1 atm) (c) Flowing H2 (1 atm) In each case, predict the rate-limiting-step, and set up some expressions to describe the kinetics. The following standard Gibbs energies of formation at 1300K may be useful: AG°Ni0 = -148,680 joules/mole AG0 for H20 = -192,540 joules/mole R = 8.314 joules/mole-°K VNi = 6.6cm3 /mole; VNi0 = 9.24 cm3 /mole