Droplet generation for thermal transient stimulation of pyroelectric PZT element

Highlights

• New unpublished technique for thermal stimulation of pyroelectric sensors with a flow of droplets in a microfluidic channel.

• PZT screen printing technique microfabrication technique.

• Use of glass substrate in screen printing deposition for higher thermal insulation.

• Complete pyroelectric characterization of the fabricated device.

• Integration of thermal sensor in a microfluidic system.

• First steps toward a pyroelectric waste heat energy harvesting device.

Abstract

This paper investigates a new way of thermal loading of pyroelectric element. Pyroelectric devices produce a current when submitted to a temperature change over time. In order to stimulate a pyroelectric device with fast thermal transients we used a stream of droplets from non-miscible fluids having different temperature created in situ by a T-junction microfluidic circuit. The succession of droplets flowing over the pyroelectric element creates temperature transients, heating up and cooling down steps that induce a pyroelectric current. This scheme has demonstrated the generation of AC signal from non-miscible fluids at two different temperatures without any synchronized pumping. The device is made of a stack of PZT and silver electrodes obtained by screen-printing on microscope glass blade. This patterning technique allows the deposition of a large number of devices on large surface substrates. It is a first step toward pyroelectric power generation harvesting energy from waste heat.

Introduction

The pyroelectric effect is the change in spontaneous polarization P_s 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:

$$p={\left(\frac{\partial P_{\text{s}}}{\partial T}\right)}_{\sigma ,E}$$

where P_s 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.

$$p_{\sigma ,E}=p_{\epsilon }+d_T{c}_{T,E}{\alpha }_{\sigma }$$

The first term p_ɛ 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:

$$I_{\text{p}}=pS\frac{\text{d}T}{\text{d}t}$$

Whereby, I_p 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(Zr_x,Ti_1−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.