Droplet generation for thermal transient stimulation of pyroelectric PZT element
Introduction
The pyroelectric effect is the change in spontaneous polarization Ps that appears in ferroelectric materials when a time dependant temperature variation is applied to it. When a pyroelectric material is submitted to a temporal temperature change, the net dipole moment of the polar material is modified and hence the value of the spontaneous polarization. As consequence the quantity of surface charge will adapt to compensate for the change in electric field. If the material is associated with electrodes and an electrical charge, a current will appear as long as the temperature changes. Pyroelectric materials also show in addition a piezoelectric behavior that is the variation of the permanent polarization with an applied mechanical strain ɛ.
The total pyroelectric coefficient p can be written as:
where Ps is the spontaneous polarisation, T the temperature, σ the mechanical stress and E the electric field. This total pyroelectric coefficient is for a material under constant stress and electric field. If we consider the coupled thermal and mechanical effects, the total pyroelectric effect can be splitted in two coefficients namely primary and secondary pyroelectric coefficients.
The first term pɛ in eq. (2) represents the primary piezoelectric coefficient and is the change in spontaneous polarization with temperature at constant strain, i.e. without mechanical deformation.
The secondary effect [1], [2] to account in the total pyroelectricity coefficient is the change in spontatenous polarization as a result of a strain in the material caused by its thermal elongation. It is the second term in eq. (2) and it is composed of the product of the piezoelectric coefficient d, the stiffness coefficient c and the thermal elongation coefficient α of the material. In some materials, the secondary effect can be in the same order of magnitude as the primary [2].
Considering a pyroelectric material including charge collection electrodes, the pyroelectric current that flows from these electrodes in an electric load can be written as:
Whereby, Ip is the pyroelectric current, p the total pyroelectric coefficient of the material, S the active surface area and dT/dt the time derivative of the temperature. Typical pyroelectric constants range from −27 μC m−2 K−1 for PVDF(polyvinylidene difluoride) [3], to 230 for PZT (Lead Zirconate Titanate Pb(Zrx,Ti1−x)O3) and 200 for BaTiO3 (Barium Titanate) [4].
With the direct dependency of the produced current with temperature derivative, currents can be created with fast heat exchanges that can happen in microdevices. Pyroelectric materials are then used for dynamic temperature measurements, infrared or terahertz detection but also waste heat energy harvesting [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. In the case of micro power generation with pyroelectricity, it is possible to produce energy from a pyroelectric element (PE) with the application of fast temperature changes in the material, in a cyclic way. It imposes the presence of different temperature sources but also of a system that loads the PE to temperature sources in a sequential way, like it is done with optical shutters for example. PE thermal loading can be done by direct conduction in solid through mechanical contact [8], application of air jet [13] or by dipping in liquids [14]. The standard pyroelectric energy conversion technique is to collect charges created in the PE during thermal cycles without biasing. There is another conversion technique that is the Olsen cycle [13], [14], [15]. It consists of circulating a cycle made of ramps of electric field at constant temperature and change in temperature at constant electric field. More recently, some works have been carried out on pyroelectric ZnO nanowires [16] in replacement of bulk material.
This paper investigates a new way of thermal loading of pyroelectric element. We combine microfluidic circuits and PZT pyroelectric elements to produce liquid driven fast thermal transients in order to exploit the dynamic effect of pyroelectric devices. In particular, we used two liquids at different temperatures that were brought in contact with a PE using a microfluidic microchannel aligned on top of the latter. The microfluidic channel is fed with a succession of hot and cold water streams coming from synchronized pumps in a first experiment and we used on chip generated droplets from non-miscible fluids in a second experiment. The water streams or droplets flow at the surface of the PE, combined with heat losses in the substrate, will produce a transient temperature rise. After this, the successive cold water stream or droplet absence will cool down the element producing an analogue temperature decrease. This paper will then show some first results of pyroelectric current generated by a new microfluidic thermal loading principle. It is the first step toward a waste heat pyroelectric energy harvester [19], [20].
The paper will show the complete fabrication procedure including the screen-printing of PZT material on glass substrate. Next the poling procedure will be shown, it allows aligning ferroelectric domains in the material and improving the pyroelectric coefficient. The paper will then show pyroelectric characterization of the material, and an evaluation of the piezoelectric coefficient. The last section will present the measurement of pyroelectric response produced by microfluidics; hot/cold water streams applied in a linear channel but also the use of a T-junction microfluidic system that generates on-chip a stream of hot water droplets flowing in cold oil.
Section snippets
Device fabrication
The device presented here is fabricated by screen-printing. This technique allows building microstructures over large surfaces but also the possibility to build thick layers in the range of tens of microns. Screen-printing, also known as serigraphy, is an ink patterns deposition technique that uses the localized transfers of these inks through a stencil in a woven mesh. The ink is generally spread with a blade on the surface of the screen being in contact with the substrate. In this work we
Pyroelectric characterization
After proper poling, pyroelectric devices have been tested under thermal load in order to extract the pyroelectric coefficient. Samples are placed on a Peltier device that creates transient heating and cooling steps. Electrical contacts are made with test probes on the bottom and top electrodes through the connexion pads visible in Fig. 2. The current is monitored with a shielded Keithley 6514 electrometer. Temperature on the sample is measured with a thermocouple placed on the sample surface
Water Jet
In order to evaluate the pyroelectric device under a fast temperature load, we use a jet of hot water as a first experiment. We used a syringe to send a droplet of hot (80 °C) or cold water (20 °C) on the pyroelectric sensor. This can be done either on the front side, e.g. directly on top of the PE. The latter being insulated by the SiNx passivation and the bonding wires by a drop of UV curable resist. But this can be done also from the backside of the glass substrate. In the latter case the
Conclusions
We presented the microfabrication by means of a screen-printing technology of a pyroelectric device on glass substrate. The device has been characterized in pyroelectric effect. It shows relatively small pyroelectric coefficient probably due to a non-ideal material sintering temperature. However the device has proved to deliver a pyroelectric signal when submitted to sequential hot and cold water streams flowing above the sensitive element by means of a microfluidic channel. The paper also show
Benoît Charlot was born in Vichy, France, in 1972. He received M.Sc and PhD degrees in microelectronics from the Institut National Polytechnique de Grenoble (INPG), France in 1996 and 2001, respectively. He is currently a researcher in CNRS (the French National Centre of the Scientific Research) and university of Montpellier within the IES laboratory (Institute of Electronics and Systems) in Montpellier, France. He is involved in MEMS, microfluidics, bioMEMS and biophysics.
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Benoît Charlot was born in Vichy, France, in 1972. He received M.Sc and PhD degrees in microelectronics from the Institut National Polytechnique de Grenoble (INPG), France in 1996 and 2001, respectively. He is currently a researcher in CNRS (the French National Centre of the Scientific Research) and university of Montpellier within the IES laboratory (Institute of Electronics and Systems) in Montpellier, France. He is involved in MEMS, microfluidics, bioMEMS and biophysics.
Denis Coudouel was born in Paris, France, in 1985. He received M.Sc degree in Sensors and electronics in 2009 anda doctoral degree in microelectronics from the Montpellier University, France, in 2013. He is currentlyresponsible for research & development of a sensorized intelligent/smart tire system for theCaptelscompany in Montpellier, France.
Florian Very graduated from Montpellier University in 2012 with aM.Sc degree in Sensors and electronics. He joins the Microsensors, Material and Acoustics research group within IES, (Institute of Electronics and Systems) of University Montpellier as a Ph.D student in the field of piezoelectrics, pyroelectrics and screen-printing technologies.
Philippe Combette received M.Sc and PhD degrees in microelectronics from the University of Montpellier, France in 1996 and 2000, respectively. He is currently professor at the University of Montpellier within the IES laboratory (Institute of Electronics and Systems) in Montpellier, France. He is involved in piezoelectric and pyroelectric materials.
Alain Giani was born in Arles in 1965, France. He received his Ph.D. in electronics,optronics and systems from the University of Montpellier, France, in 1992. Since then, he has been working in the Institute of Electronics and Systemsin the Montpellier University, as a specialist ofmicrosensors and vacuum deposition techniques on materials for multiphysics detection and implementation in devices. Presently, he is involved in thermal microsensors for inertial measurements and pyro thin and thick films deposition.