this might be possible that at 150C your naterial are convert to crystal structure from amourpous. this behavior are similarly found in MgSO4.7H2O at 255C.

Ata Ur Rahman, this exothermic peak is not due to its crystallization phenomenon as this process start beyond 400 C in most cases of HAp. this question is not so simple because there is no particular strong evidence to support this phenomenon in the accessible literature, the people dealing with HAp or Ca-P powder may suggest some particular explanation. thank you for your response.

I think that there is not an exotermic peak at 150 oC for several reasons:

1. in TG there is not such transition

2. It is very broad

3. the beginning of this "peak" is at negative temperature

In my experience this behavior is typical of temperature desbalance between the two thermocouples when heating the furnace from ambient temperature. Observe that this effect is also observed in the TG curve. May be your heating rate is too large, or for your experience you have to heat the furnace at 100oC and stop at this temperature until the two thermocouples reach the same temperature (1 hour?) then heat again and you must observe that TG and DTA lines are horizontal. Use a low heating rate 5-10 oC/h.

Best regards,

Pedro

@P. Grima Gallardo, I appreciate your experienced base explanation. I have 6 samples with same behavior as mentioned above, the heating was carried out in inert environment with Ar@ 20K/min, the beginning of peak was 100.5 at starting temperature of 38 C. I am not sure weather this peak (150 C) is exo or not, but i found little bit explanation, (there is exothermic peaks at around 150 C signifying removal of ammonia ion (occurrence is at a low level because of large cation radius )in HAp prepared via sol-gel. Interestingly, there is no endo peak corresponding to evaporation of physically absorbed water.