# Thermal energy conversion by coupled shape memory and piezoelectric effects

Dimitri Zakharov, Gor Lebedev, Orphée Cugat, Jérôme Delamare, Bernard

Viala, Thomas Lafont, Leticia Gimeno, Alexander Shelyakov

► To cite this version:

Dimitri Zakharov, Gor Lebedev, Orphée Cugat, Jérôme Delamare, Bernard Viala, et al.. Thermal energy conversion by coupled shape memory and piezoelectric effects. Journal of Micromechanics and Microengineering, 2012, 10.1088/0960-1317/22/9/094005. [hal-00814609](https://hal.science/hal-00814609)

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# Thermal energy conversion by coupled shape memory and piezoelectric effects

Dmitry Zakharov<fnref n="1" />, Gor Lebedev<fnref n="1" />,<fnref n="2" />, Orphee Cugat<fnref n="1" />, Jerome Delamare<fnref n="1" />, Bernard Viala<fnref n="2" />, Thomas Lafont<fnref n="1" />, Leticia Gimeno<fnref n="1" /> and Alexander Shelyakov<fnref n="3" />

E-mail: dmitry.zakharov@g2elab.grenoble-inp.fr

## Abstract

This work gives experimental evidence of a promising method of thermal-to-electric energy conversion by coupling shape memory effect (SME) and direct piezoelectric effect (DPE) for harvesting quasi-static ambient temperature variations. Two original prototypes of thermal energy harvesters have been fabricated and tested experimentally. The first is a hybrid laminated composite consisting of TiNiCu shape memory alloy (SMA) and macro fiber composite piezoelectric. This composite comprises 0.1 cm<fnref n="3" /> of active materials and harvests 75 µJ of energy for each temperature variation of 60 °C. The second prototype is a SME/DPE ‘machine’ which uses the thermally induced linear strains of the SMA to bend a bulk PZT ceramic plate through a specially designed mechanical structure. The SME/DPE ‘machine’ with 0.2 cm<fnref n="3" /> of active material harvests 90 µJ over a temperature increase of 35 °C (60 µJ when cooling). In contrast to pyroelectric materials, such harvesters are also compatible with both small and slow temperature variations.

## 1. Introduction

The increasing demand for alternative energy sources for low-power electronics and autonomous wireless sensors has given rise to substantial research activities in the field of energy harvesting. Among the different energy sources, thermal sources are widely available. Usually, thermal energy can be directly converted into electricity by means of thermoelectric [1] (Seebeck effect) or pyroelectric [2] materials. Thermoelectric power generators have already been demonstrated. However, such devices require large spatial temperature gradients with cold source management in order to be efficient. Another scheme is to indirectly convert heat into electricity through mechanical transformations [3]. This concept requires two materials being mechanically coupled. The first material (i.e. sensor) must exhibit a large thermally induced strain. The second material (i.e. convertor) must exhibit a large stress-induced electric field. In this way, the thermally induced strain is converted into voltage. This

paper investigates the coupling between a shape memory alloy (SMA) and a piezoelectric material. In the vicinity of thermoelastic martensitic phase transition temperatures, SMAs exhibit the largest thermally induced strains (up to 10% [4]) compared to any other inorganic material [5]. Together with a large stress generated at transformation, it results in the largest actuation energy density for SMAs (up to 100 J cm⁻<fnref n="3" />) compared to any other known material [5]. The best candidates for the SMA are NiTi and TiNiCu alloys which are used for industrial applications. Regarding polycrystalline piezoelectric materials, PZT has the highest piezoelectric constants. Thus, by coupling TiNiCu or NiTi with PZT, one may expect efficient indirect thermal-to-electrical energy conversion and harvesting of temporal thermal variations.

Although the concept described above was already proposed in 2005 [6], neither experimental nor theoretical studies have been reported to our knowledge. Our first results on harvesting low-frequency thermal variations were reported in [7]. Very recently, both analytical model and experimental

<footnote n="1">¹ G2Elab, Grenoble Electrical Engineering Lab, CNRS-UJF-INPG, St Martin d'Hères, France</footnote>

<footnote n="2">² CEA, LETI, MINATEC Campus, Grenoble, France</footnote>

<footnote n="3">³ National Research Nuclear University ‘MEPhI’, Moscow, Russia</footnote>
characterization of a piezo/SMA structure clamped in a rigid frame were given in [8]. The authors have reported generated power at various heating frequencies for temperature amplitude of about 100 °C. In our work, we consider two different original bending-type harvesters and present the results for lower temperature variations (10–60 °C). Our experimental value of energy generated through thermal cycle lies within the same order of magnitude (~100 µJ). However, since no information on the harvester dimensions has been provided, accurate comparisons cannot be made here.

As most of the published work on energy harvesting using piezoelectrics concerns vibration energy, the harvesting systems are usually described in terms of frequency and power. We would like to point out that even though thermal hysteresis of SMA can be decreased by the adjustment of composition [9] (with inevitable decrease of its mechanical work output), a variation of at least several degrees is required for producing useful work. Such thermal fluctuations are hardly rapid in natural systems. For this reason here we think in terms of energy per cycle instead of power and we do not mention frequency dependence since we harvest thermal energy in the quasi-static regime. In contrast, in the case where the heat is presented or somehow converted in the form of temperature gradient, frequency dependence for an SMA/piezo harvester would indeed be an important characteristic [6].

## 2. Theoretical aspects

In this section, we aim at giving some basic ideas about the efficiency of thermal energy conversion into electricity using the SMA/piezo harvester, exploiting commercially available TiNi-based SMAs and PZT piezoceramics.

First, we will briefly introduce the phenomenon of shape memory. During cooling, the SMA undergoes a martensitic transformation which is a diffusionless first-order phase transition. As a result, the crystal lattice of the SMA deforms by a shear-like mechanism. The product of this transformation (cold) is called martensite and the parent phase (hot) is austenite. Not all alloys undergoing martensite transformation exhibit shape memory (e.g. martensitic steels), but only those where this transformation is thermoelastic, i.e. the coherence between two phases is conserved. This conservation allows reverse martensite transformation. Both direct and reverse transformations occur within some temperature interval and are characterized by start and finish temperatures. In addition, thermodynamically conditioned thermal hysteresis prevents these temperature intervals from coinciding. Large stress and strain can be developed by the SMA on heating (up to 600 MPa and 10% for NiTi). Generally, the larger the strain and stress, the larger the temperature intervals and hysteresis. This imposes restrictions on the energy that can be harvested from a given system. Moreover, all these characteristics are significantly dependent on the SMA's composition. That means that the choice of the optimal alloy and adjustment of its composition for specific energy harvesting applications are required.

We do not provide here a thorough calculation of the efficiency of heat conversion into mechanical energy by the

SMA since so-called martensitic engines have already been intensively studied [10–12]. The input thermal energy required by the SMA body to perform mechanical work consists of the heat for increasing its temperature (by typically 20 °C [13]) and the latent heat of transformation (~20 J g⁻¹ for NiTi [12]). The typical value of input energy is about 30 J g⁻¹.

Specific mechanical work that can be produced by the SMA can be calculated as a product of the mechanical stress generated (~500 MPa) by its relative deformation (~3%). In this way, the extractable work can be estimated as 15 J cm⁻³ or 2.3 J g⁻¹. The efficiency, which is the ratio of work to input heat, can then be estimated at 8%. While the published values vary from 3% to 30% [12], in the majority of theoretical and experimental publications the reported efficiency is ~10%.

Then the mechanical energy can be converted into electricity by PZT ceramics with the efficiency characterized by the coupling coefficient $k^2$. For a given material, $k^2$ depends on the reciprocal orientation of the applied mechanical stress and the induced electrical field. When the force is applied in the same direction as the poling direction, the efficiency can be as high as 50% in the case of PZT [14].

Thereby total efficiency of the SMA/piezo-harvester can be estimated at about 5%, which is higher than typical pyroelectric conversion efficiency (~1%) [2, 15].

It is worth noting that the SMA can only produce work during heating, and some bias element (spring, etc) is needed to return it to its cold state shape. This means that part of the work is diverted into elastic energy within the bias element. To minimize losses, the harvester should be designed so that the piezoelectric element acts as a bias element for the SMA. In the case of a laminated composite, we propose a promising pre-strained configuration [16].

## 3. Materials and methods

### 3.1. *Hybrid laminated composite*

In this work, a SMA/piezoelectric laminated composite was fabricated for energy harvesting from small temperature variations (figure 1). The prototype harvester was realized by assembling SMA and piezoelectric materials into a bilayer structure using cyanoacrylate glue. The piezoelectric layer was a commercial P1-type macro fiber composite (MFC) from 'Smart Materials Inc.' [17]. It consists of 100 µm diameter PZT fibers aligned horizontally between top and bottom electrodes and packed within thin polymer insulating films (Kapton). Packaging in polymer matrix prevents PZT fibers from mechanical damage. P1-type uses the superior $d_{33}$ piezoelectric effect of PZT combined with interdigitated electrodes. When being elongated or shortened, the PZT fibers generate electrical charges which can be collected at the bonding pads.

The SMA layer was a 40 µm thick $Ti_{50}Ni_{25}Cu_{25}$ ribbon prepared by the melt spinning technique as described in [18]. The ribbon was vacuum annealed for 5 min at 500 °C to obtain a suitable microcrystalline structure. For the fabricated composite, the measured phase transition temperature region was from 50 to 80 °C, which is in agreement with previous
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Photo and schematic view of the thermal energy composite harvester demonstrator. The TiNiCu ribbon is glued onto the MFC with cyanoacrylate glue.</figcaption></figure>

work [16]. Before assembling, the TiNiCu ribbon was pseudo-plastically pre-stressed in order to define a preferential deformation direction of the SMA during thermoelastic transformation as described in [16].

To induce the thermoelastic deformation of TiNiCu, the ribbon was heated by applying a direct electrical current. The resistance of the TiNiCu ribbon was 1.6 Ω, and a Joule power of 2 W was used to heat it up from ambient to 80 °C in about 1 min. The temperature was measured using a commercial thermal camera SC500 (FLIR Systems) with 0.1 °C thermal resolution. An additional magnifying lens allowed a spatial resolution of ~0.1 mm.

Different schemes of energy harvesting were investigated by our group. Previously, we have shown the results when harvesting periodic temperature variations [7]. In this paper, we present the results for single temperature variations. Two harvesting modes were tested, with the piezoelectric element in a closed circuit (figure 2(a)) and an open circuit (figure 2(b)).

In the first case, the electrodes of MFC are directly connected to the 1 MΩ oscilloscope input during temperature change of the composite. Therefore, the voltage being generated is continuously discharged into the oscilloscope.

In the second case, the electrodes of MFC are connected to the oscilloscope input with an electrical switch via an additional 10 MΩ load resistance. The composite was heated up from the ambient to various temperatures during various Joule heating times, and then it was cooled naturally from the reached temperature down to the ambient.

The circuit was closed twice for discharging the generated voltage instantly: (1) after Joule heating and (2) after natural cooling. The difference between these two configurations is discussed further.

## 3.2. Hybrid shape memory effect/direct piezoelectric effect (SME/DPE) 'machine'

The second approach tested in this work was a hybrid SME/DPE 'machine' using localized bonding between the two materials. The SMA in the form of ribbon or wire is connected at its ends to a PZT plate through some intermediate elastic body (figure 3). Thereby, the SME/DPE 'machine' uses the thermally induced linear strains of the SMA (up to 5%) which bends a PZT cantilever-type plate through a specially designed, flexible mechanical structure (www.youtube.com/watch?v = uS0z4ZEBeYw).

The spring in this 'machine' provides shape recovery of the SMA during cooling. We remind that normal SME is a one-way effect, since only the high-temperature shape is memorized. It should be noted that an optimized SME/DPE 'machine' will not need any spring since the piezoelectric material will be thicker and stiffer, thus acting as a spring itself. The optimized PZT thickness will (1) allow use of all the strain generated by the SMA and (2) be stiff enough to return the SMA to its initial shape upon cooling.

In this paper, we present the results for the following case: (1) NiTi wire as SMA, (2) continuous heating mode (no cycling), (3) instant electrical discharge using electrical switch, (4) PZT double plate consisting of two PZT ceramic plates with a serial electrical connection and an elastic spacer in between. The results for other configurations will be published elsewhere.

Commercial 'PIC 255'-type PZT plate from 'PI Ceramic GmbH' was used [14]. A 0.5 mm thick plate was cut into 50 × 6 mm² pieces. Two pieces were glued together using non-conductive commercial epoxy adhesive '3M Scotch-Weld DP460 Off-White' locally substituted with silver lacquer to provide electrical connection between the electrodes. The sides of connection were chosen in such a way as to provide charges of opposite signs on double-plate sides when bending. After the application of epoxy on the PZT plates and their connection, the double plate was pressed at room temperature for 24 h. The calculation of such a double-plate capacitance using the permittivity value taken from the supplier (ε<sub>33</sub>/ε<sub>0</sub> = 1750) gives the value of 4.7 nF.

Commercial NiTi wire from 'Memry Corporation' was 13 mm long with a diameter of 250 µm. The wire was heated

<figure><img src="image_3.png" /><figcaption>**Figure 2.** Closed circuit (a) and open circuit (b) schemes for piezoelectric element, for corresponding modes of thermal energy harvesting.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 3. Schematic views (a) and photo (b) of the SME/DPE harvester demonstrator. The NiTi wire is drawn around three sides of the Perspex bending system (to save space). A PZT double plate is inserted in the bending frame. Electrical contacts on the left and on the right are used for applying the heating current to the NiTi wire.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4. Output voltage generated by the TiNiCu/MFC composite harvester and measured SMA temperature, with direct Joule current heating (0–120 s) and subsequent natural air cooling (120–240 s), as a function of time.</figcaption></figure>

up for about 3 s through the thermoelastic transformation using 1.7 W of Joule heating thus provoking its shortening by 5 mm. When the wire shortens, the PZT double-plate becomes constrained in the Perspex bending frame which is shaped as an arc with a constant curvature radius of 0.5 m. Thereby the piezoelectric is subjected to a limited, homogenous bending deformation. With an assumption of pure bending the maximal strain in the piezoelectric εmax can be found as

$$ \epsilon_{\max} = \frac{h}{R}, \qquad (1) $$

where h is a single-plate thickness and R is the curvature radius. In our case, εmax = 0.1%. This value is close to the elastic strain limit in PZT (0.15%).

# 4. Results and discussion

## 4.1. Hybrid laminated composite

Figure 4 shows the thermoelectric response of the composite versus time for a single heating/cooling cycle, in a continuous

discharge mode. The measured temperature of the SMA is also reported in the graph (20–80 °C).

The positive and negative voltage peaks can be interpreted as the result of the superposition of two competing processes: (1) continuous electrical charging of the piezoelectric layer during thermoelastic strain of the SMA and (2) continuous discharging of the piezoelectric layer in the moderate oscilloscope input impedance (1 MΩ).

As observed, a positive voltage is generated (up to +60 mV) on heating and a negative voltage (up to −100 mV) is generated on cooling. As expected, the voltage is maximal when crossing the temperature transition of the SMA. The charge generation is observed on heating up to 100 s which corresponds to 80 °C, which in turn corresponds to the end of the thermoelastic transition at this composition of the SMA.

Without any discharging, the voltage should be constant when temperature stabilizes under or below the transition temperature (without taking into account any charge leakage). In our case the voltage drops after roughly 10 s. This indicates that the continuous discharge mode is not appropriate for slow energy harvesting.

The corresponding harvested energy was calculated for the two voltage peaks (using numerical integration of voltage squared as a function of time). An average value of 0.1 μJ is found representative of the heating and cooling processes. One notes the peak on heating is less sharp than that on cooling. This can be explained by the rate of crossing the thermoelastic transformation region: at constant power the time to heat from 50 to 80 °C is longer (typically 60 s) than subsequent cooling from 80 to 50 °C (typically 10 s).

Figure 5 shows the results of a series of experiments using the instant discharge mode (with a switch). This configuration is the most appropriate to evaluate the maximum useful harvested electrical energy from the SMA/piezoelectric composite [7]. Two parameters influence the useful harvested energy value: (1) the temperature which the composite was heated to and (2) the time needed to reach that temperature.

The voltage is generated by the system both on heating and cooling, but is of opposite sign. In order to characterize the
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Useful energy harvested by the SMA/piezoelectric composite, in switching mode, as a function of heating time by direct Joule current heating (filled circles) and subsequent natural air cooling to room temperature (empty circles). The corresponding measured SMA temperature is also shown (filled rhombs).</figcaption></figure>

energy generated by both processes, the switch was turned on for a short period (about 1 s) twice: (1) just after heating; (2) after cooling from the reached temperature down to 23 °C. The energy value was calculated for each experimental point by numerical integration of voltage squared divided by the value of the local resistance. The discharge time through the 10 MΩ probe on the oscilloscope was about 100 ms; thus the switching on period (1 s) was enough to collect all the charges.

An increase of the useful harvested energy both for heating and cooling cases is observed when increasing heating time up to 30 s, corresponding to 70 °C. This harvested energy increase is due to the thermoelastic strain increase. When increasing the heating time up to 100 s (80 °C), the energy harvested after cooling increases, but not that harvested after heating. This may be due to different charge leakage kinetics during heating and cooling [19]. To date in the literature there is no information on the influence of temperature on the properties of MFC.

Additionally, a significant voltage was observed even at temperatures below 40 °C, which are beyond the thermoelastic transformation region. This can be explained by inhomogeneous heating. Indeed, thermal imaging revealed temperature inhomogeneities over the surface of the ribbon with hot points localized at the extremities (figure 6). In fact,

the average temperature of the ribbon reported in figures 4 and 5 was taken in the central part (figure 6 (II), region 'A') calculated by the thermal camera software. Temperature variation inside this region did not exceed 0.5 °C. The inhomogeneities could be the result of imperfect contact between the TiNiCu ribbon and the MFC substrate. In contrast, on cooling the center of the ribbon cools down with a delay due to the large surface of contact with the MFC. Some contribution of pyroelectricity is also possible.

It is worth noting that this scheme provides good mechanical energy transfer from the shape memory material to the piezoelectric and makes the system compact. On the other hand, its disadvantage is that the piezoelectric participates in the heat exchange. Thereby the system is more thermally inert and requires more heat energy from the environment. Moreover, some pyroelectricity could contribute to our results, which needs to be investigated further.

Therefore, in this work another approach was also explored with a hybrid SME/DPE 'machine' using localized bonding between the two materials.

## 4.2. Hybrid SME/DPE 'machine'

Figure 7 shows the thermoelectric responses of the SME/DPE 'machine' when heating and cooling the NiTi wire by 35 °C, through its thermoelastic transition temperature region.

Firstly, the voltage, when heating, is generated by the PZT double plate, which is bent due to the NiTi wire shortening. During bending, the electrical circuit remains open (figure 2(b)). Just after the bending was complete, the electrical circuit was closed with the switch, and the voltage discharged in the 10 MΩ oscilloscope input. The harvested energy was calculated as the surface under the curve of output voltage squared versus time, and was about 90 µJ in the case of a double plate with intermediate spacer.

Then the electrical circuit was opened again and the NiTi wire was cooled down naturally to the ambient. Then, just after the PZT double plate recovered its straight shape, the electrical switch closed the circuit again. The harvested energy was similarly calculated at 60 µJ.

The capacitance of the PZT double plate is 4 nF, determined from these curves using the values of the peak voltage and electrical energy. This is in agreement with the value calculated from material constants (4.7 nF). This

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Thermal contrast images of the SMA/piezoelectric composite: (I)–(III) correspond to the same heating intervals related to figure 5. Average temperature in the region 'A' was taken as the value of the ribbon temperature. It should be noted that the ends of the SMA ribbon get hotter with Joule heating because they are not connected to the MFC layer.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Output voltage generated by the SME/DPE 'machine' harvested from a single temperature change of 35 °C of the NiTi wire through its thermoelastic transition temperature region. The voltage is discharged in the 10 MΩ oscilloscope input immediately after temperature change (with a switch). (a) Heating; (b) cooling.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Figure 8.** Voltage generated by the hybrid SME/DPE 'machine' measured using a contactless voltmeter as a function of time. The voltage was generated on direct Joule current heating (0–200 s) of the NiTi wire and subsequent natural air cooling (200–400 s). The observed voltage drop is due to charge leakage.</figcaption></figure>

means that no damage occurred to the PZT plates during the tests.

When the electrical circuit is open, one could expect that the electrical charges will not dissipate at the PZT electrodes. However, it was found that the charges vanish rather quickly: typically 15 s was enough for complete vanishing. This may be due to some parasitic electrical connections between the PZT electrodes, either externally or within the PZT itself, but no clear evidence has been found. It also means that some harvested energy was lost before closing the circuit, and that the achieved values were not maximal.

In order to determine the time constant of charge leakage in the PZT plates used, the thermoelectric response of the SME/DPE 'machine' was measured using a contactless dc-stable electrostatic voltmeter. The result is shown in figure 8. The charges vanished completely after about 3 min. This value is one order of magnitude higher than that observed in the experiments with the electrical switch. This may be explained by electrical arcs in the switch air gap when closing the circuit. The value of the maximal output voltage of the 'machine' in this experiment was less than that in the previous ones

(compare with figure 7). This may be due to some degradation of PZT ceramic plate.

## 5. Conclusion

We report new results on thermal-to-electric energy conversion by coupling SME and DPE. Two approaches were studied in this work.

First is a hybrid laminated composite consisting of TiNiCu SMA and MFC piezoelectric material. This composite had 0.1 cm³ of active material and harvested 75 µJ of energy during a single temperature variation of 60 °C (the minimum required energy for a standard wireless sensor is 30 µJ). In previous work, we also reported significant harvested thermal energy at medium-to-low frequencies which is unique to our knowledge [7].

The second prototype was a hybrid SME/DPE 'machine' which uses the thermally induced linear strains of the SMA to bend a PZT plate through a specially designed mechanical structure. A non-optimized SME/DPE 'machine' using 0.2 cm³ of active materials can harvest 90 µJ on heating and 60 µJ on cooling by 35 °C.

In these demonstrators, we used easily available SMA material: their phase transition temperature range was too high for harvesting ambient temperature fluctuations, so we heated them artificially via Joule effect. The chemical composition of the SMA can be optimized in order to reduce the phase transition temperatures to room temperature, according to [9].

MEMS harvester fabrication is now in progress using sub-micron multilayers. Since SME and DPE do not lose their efficiency at the micron and submicron scale [20], the proposed concept is suitable for MEMS. When reducing the volume of the harvester down to microscale, the output energy will also reduce. However, recent achievements in ultra-wide band data transmission allowed transmitting a bit of information using less than 1 pJ of energy [21].

Different from usual pyroelectric materials, the proposed harvesters are compatible with small and slow temperature variations. The proposed method of conversion of the thermal energy to electrical energy is not direct. Thus, the intimate
coupling between SMA and piezoelectric is a critical point and must be optimized. The proposed concept opens a new opportunity to harvest thermal energy from natural sources.

## Acknowledgments

This work was supported by the French National Research Agency (ANR) under the Carnot Institute Program and by the Nanosciences Foundation of Grenoble under the RTRA program.

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