Copyright © 2011 Praise Worthy Prize S.r.l. - All rights reserved International Review of Mechanical Engineering, Vol. 5, N. 2 Special Issue on Heat Transfer
228 The calorimetric output st can be written as:
s r
s rpairs
s t T
T
α
−
= −
∑
2 The calorimetric output st simulated by numerical
methods [13], [14] changes when the dissipation w
i
modifies its position. The heater furnished by Xensor is a thin squared “o-
ring” near the thermocouples connection. In a strictly bi- dimensional system each position inside the squared
inner part is equivalent from the sensitivity evaluation but in dynamic measurements the heater response is
faster when a heater is situated in the center of the chip. In the actual 3-D conditions, each position of the x-y
coordinates furnishes different sensitivities and dynamic behavior associated to different heat losses to the
surroundings.
III. Aging in NiTi
The calorimetric devices not only can be used for energetic measurements. As an alternative, the
temperature behavior of the transformation of materials can be analyzed. For instance, one appropriate
application of DSC calorimeter is the observation of the evolution of the maximum calorimetric outputs in the
case of aging of SMA samples. In Fig. 3 we can see the calorimetric outputs on cooling obtained from a TA
Instruments [15] Q1000 Differential Scanning Calorimeter using the Tzero technology TA
Instruments. Wires of NiTi, 0.5 mm in diameter were used as furnished or aged for different time intervals at
373 K [16]. These calorimetric measurements compare samples as furnished with samples aged for several
daysmonths in a furnace at 373 K. To optimize the temperature uniformity a purge of 50 mlmin of nitrogen
was used. In our measurements for the available cooler possibilities the thermal cycling was performed between
191 and 313 K, and the temperature rate was of 2 Kmin.
A peak, which is currently attributed to the formation of the martensitic phase [17], appears during the cooling
run. The obtained curves were quite reproducible for cooling and heating, for consecutive cycles and for
different samples. The output signal denoted in arbitrary units correspond, according to TA device, to mWmg.
In the present case the base line effects were not taken into account and therefore the quantitative value is of
less relevance. Fig. 3 shows the output signals cooling thermograms, two cycles for each sample, two samples
without aging A, two samples after 48 days of aging at 373 K B, two samples after 158 days at 373 K C, and
one sample after 270 days at 373 K D. It can be observed that the aging induces a shift in the peak
position towards higher temperatures it occurs also in heating; and some change in the shape of the peak also
takes place. The value is only mainly affected by the temperature rate.
Fig. 3. Calorimetric measurements in cooling NiTi wires of 0.5 mm diameter: A “as furnished; B, C, D: aged at 373 K for 48, 158 and 373
days, respectively
The expected temperature uncertainty can be estimated to be of the order of the temperature rate in K
min or ± 2 K. The analysis of the position of the calorimetric peaks
shows that the temperature of the peak evolves to an asymptotic value following an exponential behavior with
the aging time. Using the four points obtained via TA Instruments calorimeter, the time constant approaches
250 days [18].
IV. The Si-Based Chip Calorimeters
