The zirconiumhydrogen phase diagram

The Sieverts' law behavior illustrated in Fig. 9.9 does not increase the concentration of A indefinitely as pA increases. There is a limit that the metal can accept without precipitating a new phase. This limit is called the terminal solubility. At this limit additional gas in the solid ends up in forming a M-A compound called a hydride if A = H, a nitride if A = N, and an oxide of A = O. This process is shown by the zirconium-hydrogen phase diagram in Fig. 9.9.

The Zr-H system exhibits three single-phase regions separated by three two-phase zones. The narrow a-Zr phase next to the left-hand axis consists of Zr with a hexagonal crystal structure in which hydrogen is incorporated in the interstitial sites. The terminal solubility of hydrogen in this metal is indicated by the lines forming the right hand border of the a-Zr zone. These lines are known as the upper phase boundary of hydrogen in a-Zr or the solubility limit of hydrogen in this metal phase. At a constant temperature of 500oC, addition of hydrogen to the metal maintains the a-Zr structure until the solubility limit at an H/Zr ratio of 0.04. Added hydrogen precipitates the 5-hydride phase, which has the formula ZrH1.33 at its lower phase boundary. As hydrogen is added, the a-Zr phase is converted to the 5-hydride until the mixture is completely to the latter. Further addition of hydrogen increases the H/Zr ratio of the hydride from its lower phase boundary value of 1.33 to the upper limit corresponding to ZrH2.

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Solar Panel Basics

Solar Panel Basics

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