I find it difficult to interpret the TGA/DTA data of my combustion deposits sample (collected from the exhaust valve of a Caterpillar Natural gas engine). Can anyone help me to interpret the TGA/TDA curve?
The sample have the chemical composition of : O (35.99%); C (28.12%); Ca (17.05); S (7.71%); Zn (6.29%); P (3.70%) while the major phases present in the sample are:
(i) Ca5(PO4)3.(OH)
(ii) ZnO
(iii) CaSO4 anhydrite
TGA/TDA curve is attached.
Hi Bao,
The XRD results only show only the three compounds listed below: (these results are confirmed from two different XRD machines)
(i) Ca5(PO4)3.(OH)
(ii) ZnO
(iii) CaSO4 anhydrite
Could you please give me some info about my TGA/TDA curve
Yi Bao's comments are valid. Up to 200℃ you will have the evaporation of water and condensation of physically adsorbed water. This is followed by condensation of hydroxyl groups up to about 400℃.
Oxidation of carbon (burnout) should have continued beyond 400℃. Since your sample is a cumbustion product and no peak is observed beyond 400℃, the absence of a peak for carbon oxidation is not surprizing.
Regards.
Agreed. The big weight loss is due to dehydration of the Ca5(PO4)3.(OH). If you have free carbon in that amount, there should be a large exothermic peak in the DTA curve, along with a big weight loss in the TGA curve due to combustion. You only lost a bit over 10 Wt% of the sample during the run. Are you sure there is free carbon in the sample?
Assuming that your sample contains 3.70% of P, Ca5(PO4) should be 20% of the total sample. Therefore, the dehydration of this compound does not explain a 10% weight loss in the TGA curve (only 0.67%). Did you consider the possibility of having amorphous phases? For example amorphous Zn carbonate decomposes within the temperature range of 200-400 degrees. However this still does not explain such a big amount of carbon.
I have conducted the compositional analysis my combustion product sample (natural gas engine) used for TGA/DTA analysis. Following phases (with their percentage) were identified during compositional analysis.
Anhydrite – CaSO4 (82%)
Hydroxylapatile – Ca5(PO4)(OH) (6%)
Zincite – ZnO (7%)
Calcite – CaCO3 (5%)
In the light of these new information could you please let me know about your expert opinion. TGA/DTA curve is attached.
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