# A Study of the Thermodynamic Properties of Nd-C and Ce-C TRISO Fission Product Binary Systems
## 4 Discussion  
All tests resulted in the samples adhering to the boron nitride crucibles. Alumina crucibles also chemically reacted with the samples and were unusable for this experimental campaign. Visual inspection of the sample and crucible after testing confirmed adhesion occurred, but degradation of the exterior of the crucible was not observed and thermogravimetric analysis during the DSC tests confirmed no mass changes over the course of each heating cycle. This also confirmed that oxidation due to air ingress during testing was negligible, validating the partial pressure of oxygen measurements taken by the Zirox probe for each test. Despite this observation, there was difficulty in analyzing the DSC responses of some of the specimens, and it is suspected that oxidation during transportation from UTSA to OTU may have occurred.  
In general, data reported from the second and third heating cycle correlated well within the same test, with an average variance of phase transition temperatures between cycles of 3.0 \({}^{\circ}\)C. However, phase transitions between tests of samples that were tested multiple times displayed a larger variation. Test Nd1-2-1 solidus onset temperature of the second cycle reported a value of 886.3 \({}^{\circ}\)C that correlated well with the onset temperature of the third cycle of 883.1 \({}^{\circ}\)C. In test Nd1-2-2, the second and third cycle onset temperatures for the solidus were found to be 876.7 \({}^{\circ}\)C and 878.0 \({}^{\circ}\)C, respectively, and in test Nd1-3-1, the second and third cycle onset temperatures were 807.5 \({}^{\circ}\)C and 850.1 \({}^{\circ}\)C, respectively. Results from Nd1-2-1 and Nd1-2-2 aligned fairly well with each other, but Nd1-3-1 found significantly lower onset temperatures. Therefore, a higher subjective weight was applied to tests Nd1-2-1 and Nd1-2-2 than Nd1-3-1 when averaging reported phase transition temperatures. This also accounted for the significant error in the reported data.  
Samples Nd3, Nd4, Ce1, and Ce2 all displayed dampened DSC responses or unexpected DSC responses captured by the STA thermocouple but lacking in clarity of the exact onset and/or offset temperatures of some phase transitions. Powder was present in the sample Ziploc bags used during transportation that appeared similar in appearance to neodymium oxide and cerium oxide, displaying a blue and black powdery color and yellow and grey powdery color, respectively. Discussions with UTSA confirmed that oxide formation of rare earths generates a substance similar to this observed material. Additionally, it is suspected that lowered heating rates would improve identification of lower-temperature phase transitions by allowing additional time for reaction kinetics to complete.  
The 40 mol%C, 60 mol%Nd sample and 67 mol%C, 33 mol%Ce samples compared well to experimental and predicted values in the open literature. Mixed solids experience non-congruent melting that can add difficulty to measuring phase transitions. After initial melting and during subsequent cooling of a mixed phase, the phase of each crystal will tend to separate from the mixture and fail to completely form the predicted low-temperature solid phase [54]. As a result, eutectic and liquidus transitions of neighboring compositions may appear in the measured sample, and therefore, annealing of the sample is necessary to return to equilibrium [54]. Practically, this would necessitate an isothermal hold for all DSC measurements in this campaign of potentially 48 h and at temperatures slightly below the first predicted phase transition to improve accuracy of onset temperatures [54]. However, this explains the lower temperature effect seen on some of the tests, such as Nd2.5-4-1 at approximately 680 \({}^{\circ}\)C. This effect is not a real transition and is instead suspected to be an artifact of non-congruent melting, and therefore, it is omitted in this work.