The Si-Based Chip Calorimeters

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

Manufactured by XENSOR, the Liquid Micro Calorimeter LCM [2] is one of the available sensors on the market. Mounting the element in an appropriate box, the system works as a non-differential calorimetric sensor in, for instance, isothermal condition. We studied the Chip XENSOR LCM 2524 shown in Fig. 1b. This chip consists of an around 25 µm thick, 8x8 mm 2 large mono-crystalline silicon membrane in a thick silicon rim. The sensitive area in the middle of the membrane contains a diffused p-type silicon resistor heater integrated in the membrane. It is a square torus of 4x4 mm 2 size and about 50 µm wide. On the silicon surface, 164 thermocouples of deposited Al and doped Si are located in a square shaped frame. The detector direction, determined by the hot and cold junctions, is always practically orthogonal to the heat flow interchanged with the reacting substances. The extremely small detecting surface only a line of “warmed” thermocouple junctions is the origin of reproducible but inaccurate results. The flat “nano- calorimeter” works with excellent resolution and reproducibility but accurate results are difficult to be established. This is due to the calibration procedure in terms of Joule measurements using the manufacturer’s resistance. The quantification of isothermal measurements in micro-sized calorimeters deposed in a Si-surface involves an evaluation of their characteristics. First of 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 229 all, the link between the sensitivity and the location of the energy dissipation needs to be quantified. We realized the analysis of the position dependence on the working surface x-y coordinates using a laser beam impinging on the flat upper surface. Also, the dependence on the z coordinate by Joule dissipation using a Pt resistance at different distances was studied. The complete set of the observations needs the construction of models that represent with more correction the characteristics of the experimental systems. From the experimental measurements a spatial shape factor, modifying the standard Joule calibration suggested by the manufacturer, is established. IV.1. X-y Position Dependence of the Sensitivity The positional x-y analysis was done by means of a laser signal the cross section of the spot is close to 1 mm 2 that impinged normally to the flat silicon surface of the LCM 2524 XENSOR chip. The system is well described in references [12] and [13], and it is schematized in Fig. 4. Careful attention is required to parasitic effects between the laser light and the Si and Al-Si thermocouples. Fig. 4. Outline of the experimental set-up based on XENSOR “Liquid Micro Calorimeter LCM” device and warming via a red laser beam. The output signal st or thermogram is digitized using a DMM Keithley 2000 multimeter. a-b section of LMC; c laser beam positioning by x-y displacement on the Si-surface. External PC computer controls two gates [19] starting the laser on-off beam and the digitizing command in the oscilloscope Laser light furnishes only a relatively appropriate tool: the photoelectric effect induces highly relevant actions in the output signal. The photon-electron signal, positive or negative and position dependent, overcomes 150 mV. The thermal signal induced by the same laser spot remains under 2.5 mV. To avoid the photoelectric effect, the digitizing process started 1 ms after the shutdown of the laser beam permitting the disappearance of photoelectric parasitic signal. To facilitate the readability of the relatively slow measurements, the output signal was digitized using a DMM Keithley 2000 multimeter. The sampling was close to 88 Hz. To smooth the 50 Hz ripple, a mean of 12 signals was used. In Figs. 5a and b the sensitivity values in reduced units it means, divided by the sensitivity value of the central point because the power absorbed by the laser is unknown are presented along the chip surface [12], [20], [21]. The dissipated heat shows a clear dependence from the center to the external boundaries. The standard deviation for the value of each experimental point is about ± 1 . The sensitivity vanishes to zero in the boundaries of the working surface see Fig. 5a. The sensitivity decreases from the center of the working surface. Nearly 8 from the central point to the border of the centered square of 4 x 4 mm 2 square of thermocouple junctions and “only” a 5 in an inner square of only 2 x 2 mm 2 see Fig. 5b. A progressive displacement of the dissipated heat from the center to the external boundary acts firstly in the warm junctions, and progressively moves to the cold junctions. In fact, negative values can be also measured in the boundaries of the Si-surface laser impact near the cold junctions of thermopairs. Analyzing the x and y coordinates by heat transfer models, the sensitivity behavior shows some square shaped symmetry related to the particular arrangement of the detectors. The experimental scatter avoids the visualization of this symmetry. In Fig. 5b we can see the asymmetry along the diagonal DD’, which is stronger than along the CC’ diagonal. Also the eventual effects of transmission, absorption and reflection of laser light have been analyzed [21], but at the practical level the different absorption effects are irrelevant in comparison to the changes due to the location on the chip surface. For each dissipation, a shape factor can be defined [12], [20], [21] from the ratio between the measured sensitivity and the sensitivity furnished by the manufacturer. For each configuration, it is possible to find a shape factor F xyz such that the effective sensitivity can be calculated as S ef = F xyz ·S J , being S J the sensitivity obtained using Joule heating through the chip heater. It is interesting the observation of irregularities in the shape of the sensitivity values on the chip surface. The fluctuation in the inner space suggests that the mean sensitivity can be determined as: 1 ens ens urf Surf S S x, y dxdy S = ∫∫ 3 where S ens x,y is the sensitivity in each point of coordinates x,y, and S urf is the total surface of the chip. 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 230 Figs. 5. Sensitivity in reduced units against x-y position. a for points distributed on the Si surface. b useable surface inside the “warm” junctions expanded central part of the Si surface.The decrease of sensitivity from the centre to the extreme can be seen. The asymmetry along the DD’ diagonal is stronger that along the CC’ diagonal The use of a shape factor requires an hypothesis about the spatial distribution of heat sources or an equivalent experimental knowledge. IV.2. Z Position Dependence of the Sensitivity The sensitivity z-dependence was determined in the case of a miniaturized flow-through device mixture [22]. The device with three outlets input: two reactants, output: the mixture is built to evaluate the mixing enthalpy in continuous injection processes. It consists of a silicon chip as described above which is covered by a squared plastic chip with a cylindrical cavity 4 mm diameter, 1.6 mm thick: the reaction chamber. The position dependence of the heat sources is determined via small platinum coils 0.5 mm thickness used as auxiliary heaters. The Pt-coil was introduced into the reaction chamber at different distances from the silicon membrane 0.25, 0.75, 1.25 mm. The chip assembly was mounted into an aluminum block to smooth the room temperature fluctuations. As a sample material, ethanol was filled into the reaction chamber before starting the measurements. For data acquisition at 1000 Hz and heater control, it was used a plug-in PC interface board with ADC and DAC. The experimental systems were controlled via PC- computers. The fast sampling induced unfavorable signal to noise ratio. By using series of “equal” measurements, the noise could be reduced via appropriate averaging. The experimental observation relates the measured sensitivity with the effective surface that receives the heat and, obviously, with the actual dissipation distribution, in this case for static no flow measurements. In the three-dimensional device, the cover and the reactants i.e. each substance, as also the flow rates and the effective percent of mixtures, modify the heat path. In the case of no flow rate, the standard Joule resistance can be considered as a thin frame of square section side: 3.4 mm. Considering a circle with the same surface, its associate mean radius r J can be approached to 1.92 mm. Assuming a heat transfer from the liquid chamber radius: 2 mm plus a supplementary effect by the heating in the massive squared plastic cover 8x8 mm 2 an effective radius of 3.5 mm needs to be considered. The experimental sensitivity 2.26 VW, point “a” in Fig. 6, determined using the manufacturer resistance, needs to be modified to 1.86 VW. Fig. 6. Sensitivity changes with the position of heater in the z-axis. o : standard sensitivity related to free device. a: standard sensitivity 2.26 VW using cover and content.1, 2 and 3 experimental values 1.70, 1.44 and 1.21 VW established via the three platinum resistances. : extrapolated value 1.86 VW at z = 0. The arrows show the expected uncertainty on the extrapolated value This value is the extrapolated one, with an uncertainty close to ± 5 . The associated shape factor F r is 0.822 obtained via the ratio 1.862.26. The cover and contents induces relevant changes on the sensitivity and, obviously, on the heat transfer. Its value in a free device -without cover- is close to 2.58 VW. The series of measurements shows that the measurements with an empty of free device using the heater of the furnisher are incorrect. 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 231 Only an extrapolation of z-effects can furnish reasonable results. The effects requires supplementary analysis related to used inflow rates, the particular heat capacity of reactants and the percent of accomplished mixture, which is a function of the flow rates or the times spent for the reactants inside the cell. Drop by drop process: Another experimental aspect of the sensitivity changes related to 3-D effects is studied by a test on the enthalpy of mixing in a drop by drop reaction. The basic device is a non-differential conduction calorimeter based on the LCM 2524 manufactured by Xensor, and the working chamber is outlined [22], [23] in order to add a drop of reactant 2 by a syringe on a drop of reactant 1 previously placed on the silicon plate. We analyze the positioning effects of the dissipation at different levels of the drop, in comparison with the standard chip heater CH furnished by the manufactured. For this we placed a coil of platinum wire near 11 Ohm, length close to 3 mm, diameter near 0.4 mm inside the drop at different vertical positions. It is located in bottom b, middle m or top t position inside the drop only one in each experiment with a careful geometrical positioning. The thermal contact between the drop and the silicon surface is improved via an auxiliary disk of cellulose fiber, delimiting the surface covered by the reactants. The effects of vaporization have been determined with the time dependence of the mass [23]. Measurements were made using drops of glycerol volume 6 and 9 µl, water 9 µl and undecane 9 µl. Several Joule effects in the different liquids and resistance position were studied. The practical Joule analysis used 10 Hz sampling and series of fifteen on-off signals were made to increase the resolution by averaging. The available resolution was close to 0.5 µV or equivalently to 0.2 µW. Repeated measurements with new positioned drops of the same volume of liquid showed a mean reproducibility related to “refilling” close to 1 percent. The change in sensitivity depending on the position of the heater within the drop is shown in Fig. 7 for water, glycerol and undecane, respectively. As expected, the sensitivity depends on the liquid and the value decreases with the distance of the heater to the chip membrane. But it is noteworthy that the sensitivity associated with the water drop is remarkably lower than that of glycerol in the chip heater as well as in the drop heater. The lower value of the chip heater sensitivity in the case of water seems associated to higher latent heat of vaporization. Fig. 7. Sensitivity: z-coordinate effects. The CH chip heater relates the manufacturer’s resistance in Si shell at zero distance. Effects on sensitivity S in drops of 9 µl glycerol, water and undecane, respectively, measured using an auxiliary heater inside the drop at bottom b, mean m and top t position Particular evaporation effects associated to Joule dissipation remain under the detection level and further study seems convenient. In comparison with water, glycerol and undecane have very low vapor pressures but strongly different viscosity. IV.3. Sensitivity and Gas Flow in Gas-Solid Reactions Gas-solid reactions relate one of the most important research subjects in sensor-actuator systems. Several specific sensors are described in the recent literature for a general overview see for instance [24]. In fact, arrays of different sensors are in the basis of the chemical or electronic nose [25]. Mainly conductometric sensors metal oxide, ionic conductor or conductive polymer and quartz microbalances are used. It has been shown also, that integrated circuit micro-calorimeters coated with a gas sensitive film can be applied for gas detection [26], [27]. The calorimetric improvements are related to reactions between extremely small amounts of the masses [28], [29]. For instance, in analytical studies, the actual target is close to several µg of the reactants. In order to clarify the effects of the variations of local sensitivity with flow rate we analyzed the roughl effects of gas flow on the sensitivity, and we compared the results using two Xensor type chips. We studied the behaviour of chip response when a gas flow overcomes on the chip surface with different flow rates and for different power in the chip resistance. The procedure has been as follows: we start recording the zero chip signal, then start the gas injection holding a constant flow rate with normal incidence. Flow rates between 0 and 0.4 cm 3 s were used. When stationary state was achieved, different power values were dissipated, nearly between 0.1 and 1.1 mW. Then the system was let to return to the zero power value. The sensitivity obtained is referenced to the base line corresponding to zero power and constant gas flow rate. We can see in Fig. 8 that the sensitivity decreases as the 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 232 power increases, and only a lower decrease with increasing flow rate is detected. The experimental analysis suggests that in the LCM 2524 chip, a non- linearity exists, produced by the dissipated power in the heater. The measurements used battery power supply, completely isolated from the output, but eventually parasitic cross-talk effects between heater and thermopile could not be completely avoided. The behavior of the sensitivity against power dissipation for an incidence angle of roughly 45º is similar than for normal incidence. We repeated the same experiment with the Xensor NCM 9924 chip 40 µm thick. It has a diffused p-type silicon resistor heater integrated in the membrane in the same way than in the LCM 2524 chip, and two aluminum resistances, galvanically isolated from the thermopiles, on the rear part of the surface. One of them is a square torus of 3.8 x 3.8 mm 2 , and about 50 µm wide, underneath the chip heater, and the other is of 2.6 x 2.65 mm 2 square shape, nearly centred [2]. Fig. 8. Chip heater sensitivity of the LCM 2524 XENSOR against the dissipated power for constant gas flow rates. A and B are the linear fit for both series of experimental points The use of the central aluminum resistance for power dissipation means that the heat appears in a more extended and uniform part of the surface 6.89 mm 2 in comparison with 0.7 mm 2 . By the other hand, the heat transfer acts on a higher silicon thickness 40 µm, compared to the case of the LCM 2524 25 µm. Increased surface and thickness works in the appropriate direction. This fact implies a more realistic approach to the energetic dissipation that would take place in a reaction between an incident gas over a deposited substance on the surface. On the other hand, it is still not equivalent to a real situation because the reactive substance would be placed on the upper part of the chip surface when the aluminum resistance is placed on the rear part of it. We have measured the signal using on one hand the heater resistor, and on the other hand the aluminum resistance placed in the centre, following the same procedure as with the former chip. Fig. 9 shows the power dependence for the NCM 9924. It is possible to appreciate the disappearance of the previous non- constancy. The experimental analysis established relevant differences when the location of the heat dissipation is altered, related to a poor heat flux integration. Improved accuracy needs supplementary hypotheses and experimental determinations. For gas-solid measurements the relatively relevant non-constancy, related to the power concentration in the LCM 2524 chip heater, induces an increase of the uncertainties in comparison with chip NCM 9924. The introduction of the Al-heater increases the reliability of the device. Fig. 9. Sensitivity against the dissipated power for constant gas flow rates. A and B are the linear fit for both series of experimental points. Power is dissipated in the central aluminum resistance of the NCM 9924 XENSOR chip

V. Difficulties on Calorimetric Evaluation