Relationship between flow direction and dendritic growth rate in NH 4 Cl-H 2 O
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Relationship between Flow Direction and Dendritic Growth Rate in NH4CI-H20
Bookey (1952)5
M.H. McCAY, J.A. HOPKINS, and T.D. McCAY -1/,0( I
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Fig. 1--Comparison of the standard Gibbs energy change for the reaction 3MgO (s) + P2 (g) + 5/202 (g) = Mg3P208 (s) in the temperature range 1200 to 1600 K reported in the literature with that assessed in this study.
evaluations of Barin et al. in 1973 t91 and 1977, u~ both of which were based on estimated values for the heat capacity of Mg3P208. Since experimentally determined low- and high-temperature heat capacities are used in the present assessment, the results obtained are more reliable. In summary, an error of 35 to 40 kJ. mol-~ exists in recent thermochemical compilations u,2] for the enthalpy and free energy of formation of Mg3P208. The error resulted from a change in standard state for phosphorus in the new compilations without appropriate corrections in the enthalpy of formation of the phosphate. These errors have been corrected in the new assessment presented in this communication.
REFERENCES 1. M.W. Chase, Jr., C.A. Davies, J.R. Downey, Jr., D.J. Frurip, R.A. McDonald, and A.N. Syverud: JANAF Thermochemical Tables, 3rd ed., J. Phys. Chem. Ref. Data, vol. 14 (1), 1985, p. 1499. 2. O. Knacke, O. Kubaschewski, and K. Hesselmann: Thermochemical Properties of Inorganic Substances, 2nd ed., Springer-Verlag, Berlin, 1991, p. 1146. 3. D.R. Stull and H. Prophet: Janaf Thermochemical Tables, 2nd ed., NSRDS-NBS-37, U.S. Department of Commerce, Washington, D.C., 1971. METALLURGICAL AND MATERIALS TRANSACTIONS A
Earlier studies l~l on single-component dendritic solidification have shown the influence of latent heat rejection on natural convection. When multicomponent alloys are solidified, solutal rejection also occurs and combines with the thermal effects to drive convection. Once created, these flows in turn impact the growth of the dendrites. The NHaC1-H20 system, which is often used for twocomponent dendritic solidification studies, rejects a solute lighter than the bulk fluid, and as a result, natural convection almost inevitably occurs. Attempts to stabilize it by the imposition of a positive temperature gradient simply creates a density inversion and contributes to thermosolutal convection. The flow that occurs is present both within the dendritic two-phase mushy zone and in the bulk liquid and can adapt several morphologies (cellular, tzJ pluming, TMand jetting), depending upon the stage of solidification. As a consequence of these flows, warm, bulk composition fluid is carried down to the dendrite front, while cold depleted fluid rises from within the mush. An earlier article TMdiscussed the effect of such flows on the overall dendritic front growth rate. The influence of the direction of the fluid flows on the growth rate of an individual
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