Patent Application
20240022186
The invention relates to a piezoelectric device and method of manufacturing the same. More specifically, the invention relates to a flexible piezoelectric device which with good impedance while ensuring stability and feasibility of industrial manufacturing.
Energy harvesting has the potential to achieve long-life stand-alone operations of wireless sensor networks, wearable devices, and medical implants. This has therefore attracted considerable interest from academia and industry. The piezoelectric effect has been widely adopted for converting mechanical energy into electricity, due to its high energy conversion efficiency, ease of operation, and miniaturization.
From applications viewpoint, an energy harvesting device has to be able to generate sufficient power under variable excitation. Therefore, academia and industry had concentrated their efforts on methodologies leading to high power output and broad operational bandwidth. Different designs, nonlinear methods, optimization techniques, and harvesting materials have been investigated.
Piezoelectric materials commonly used in energy harvesters include aluminum nitride (AlN), ZnO, BaTiO3, polyvinylidene fluoride (PVDF), PZT, PMN-PT (Pb[Mg1/3Nb2/3]O3—PbTiO3), PZN-PT (Pb[Zn1/3Nb2/3]O3—PbTiO3), and various piezoelectric composites. AlN and ZnO have a much weaker piezoelectric effect than the other commonly used materials. Usually piezoelectric coefficients are order so that d15>>d33>d31. For PZT, d31˜0.5d33. The single crystals PMN-PT and PZN-PT demonstrate the highest piezoelectric properties, but are more sensitive to temperate change, more susceptible to fatigue, and more difficult to manufacture than lead zirconate titanate ceramics (PZT). Therefore, PZT is still the most popular piezoelectric material in energy harvesters.
However, obtaining a large scale of global deployment for IoT (Internet of things) applications requires to consider the piezoelectric material abundance, recyclability and toxicity (RoHS compliance, REACH regulation). Therefore, PZT has to be replaced by lead-free materials. This is typically a necessity when the resulting piezoelectric device has to be used in health device which are implanted in patients, used for general public or used outdoor.
For achieving this objective of industrially produce lead free piezo electric devices, other piezoelectric materials have to be considered.
Lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) crystals are industrially produced piezoelectric materials easily accessible, rare-earth and toxic-element-free, cheap, available in form of wafers (with diameter up to 6 inches), widely exploited for developments of acoustics and optical devices.
In particular, LiNbO3 and LiTaO3 presents dielectric constant much lower than lead-based piezoelectric materials. In the case of energy applications, this impacts the electromechanical coupling under bending defined as:
Piezoelectric energy harvesting demonstration was investigated so far by using industrially available LiNbO3 wafers with thickness of 300 to 1000 μm. However, they cannot be used in real applications due to unmatched impedance with electronic circuit and their fragility.
There's thus a need for providing a technique for producing energy harvesting piezoelectric devices which can be industrially produced while respecting environmental constraints and while delivering predicable and constant impedance in operational conditions.
According to the present disclosure, a flexible piezoelectric device for energy harvesting is proposed. The flexible piezoelectric device comprises a flexible substrate layer of thickness ts which comprises an upper face and a lower face, and at least one LiNbO3 and/or LiTaO3 film, called LNT film bonded to one of the faces of the flexible substrate layer, wherein thickness tf of said at least one LNT film is chosen between a use range of 5 to 50 micrometers (μm).
Thus, the device according to the disclosure allows harvesting energy in an efficient way while respecting both constraints of flexibility, durability and toxicity.
According to a particular feature, thickness tf, of said at least one LNT film, which is comprised between 5 to 50 μm, is adapted according to a target output power to deliver by said flexible piezoelectric device during use.
Thus, the device according to the disclosure is interfaceable with various microelectronic devices so as to deliver a regular power adapted for consumption of such microelectronic devices. According to a particular feature, said flexible substrate layer is made of a metallic material.
Thus, this allows obtaining a thin, while resistant layer, adapted for operational and industrial use.
According to a particular feature, said flexible substrate comprises at least one of nickel (Ni), copper (Cu), iron (Fe), aluminum (Al), titanium (Ti), as well as alloys and combinations thereof, such as brass, stainless steel, Ta6V . . . .
Thus, this allows manufacturing the device in a cheap and reliable process.
According to a particular feature, geometry of said flexible piezoelectric device is adapted according to a target output power to deliver by said flexible piezoelectric device during use.
According to a particular feature, the total thickness (ts+tf+electrode thicknesses, . . . ) of said flexible piezoelectric device is adapted so as to achieve a predetermined magnitude of deflection of said flexible piezoelectric device according to a target resonance frequency.
According to a particular feature, the film thickness ti of said flexible piezoelectric device is adapted so as to achieve a predetermined capacitance of said flexible piezoelectric device according to a target resonance frequency.
Hence, the device is able to deliver power as a function of a predetermined application.
According to a particular feature, film thickness tf of said LNT film and substrate thickness ts of said flexible substrate layer are selected so as to optimize the effective electromechanical coupling k2 of said flexible piezoelectric device, as a function of thickness ratio
Thus, the properties of the LNT materials are optimized as a function of the film thickness, tf.
According to a particular feature, the film thickness, tf, of said LNT film is selected so as to extend deflexional limit of said LNT film during use.
Thus, this allows increasing durability of the device.
According to a particular feature, orientation of piezoelectric tensor of crystals of LiNbO3 and/or LiTaO3 forming said LNT film is chosen so as to optimized the deflexional coupling factor value k23 of said LNT film.
Thus, this makes it possible to take maximum advantage of the LNT material properties.
According to a particular feature, orientation of piezoelectric tensor defined the material coupling coefficient (equation 1). Among commercial wafers of LiNbO3 and LiTaO3, the standard cuts correspond to crystal orientated by a rotation angle, around X-axis (also wrote (YXI)/θ cuts defined by IEEE standard where, θ is the angle of rotation around X-axis). In these crystals, the better coupling is found for bending in the plane perpendicular to X-axis. Then, according to figure the crystals are chosen into the group of LiNbO3 and LiTaO3 wafers:
According to a particular feature, width of the device is about 10 mm, length is comprised between 40 mm and 100 mm and resonance frequency is comprised between 10 Hz and 200 Hz.
According to another aspect, the disclosure is also directed to a method of manufacturing the piezoelectric device as depicted above. The method of manufacturing comprises:
According to a specific feature, the step of preparing the LNT substrate comprises:
Embodiments of the present disclosure can be better understood with reference to the following description and drawings, given by way of example and not limiting the scope of protection, and in which:
The present disclosure relates to a piezoelectric device comprising at least one bonded LiNbO3 and LiTaO3 flexible single crystalline films (LNT Films) of a predetermined film thickness, tf. The present disclosure also relates to method for manufacturing said piezoelectric device. Three methods are disclosed. As previously exposed, commonly used piezoelectric materials in energy harvesting are usually not compliant with environmental constraints. The inventors investigated the use of a lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) crystals which present similar efficiency in energy harvesting as commonly used PbZr1-xTixO3 (PZT). While promising, the use of these materials raised several problems that the inventors successively solved so has to obtain an industrially makeable device. For example, to respond to a problem of flexibility and brittleness of silicon wafer, the inventors had to develop some strategies to use metallic and polymer substrates. These strategies had conducted to implement several methods of manufacturing and integration of LiNbO3 and LiTaO3 flexible single crystalline films.
More generally, the inventors also determined the importance of optimizing the effects of crystal orientation, film thickness, tf, flexibility and capacitance, which are all considerations necessary for energy harvesting applications. LiNbO3 and LiTaO3 flexible single crystalline films (LNT Films) have high sensitivity and interest in several applications such as sensors, micro-devices (called MEMS), Micro and Macro devices for energy harvesting, actuators, and more generally a component of a device using piezoelectric or pyroelectric robust and flexible films such as loTs, autonomous wireless sensors, flexible devices, wearable devices, high-temperature devices/local power supply, etc. This necessitate however, according to the invention, to configure the parameters of the LNT films which are used in order to obtain piezoelectric devices which are similar, in terms of performances, to the ones manufactured with commonly used PZT.
Thus, it is proposed, in a general approach, to use widely commercially available and high-quality single crystal lead-free piezoelectric materials (LiNbO3 and LiTaO3), with special care on optimization of film thickness, tf, capacitance and orientation. The disclosure comprises five optimizations (combinable) for the production of flexible single piezoelectric materials:
The disclosure hence proposes a suitable microfabrication process and the application to vibrational energy harvesters based on film thickness, tf, (5 to 50 μm) of single-crystal LiNbO3 and LiTaO3 films bonded on flexible substrate. Indeed, according to the disclosure, as a function of the application, the thickness tf of the LNT film (or films) is adapted. More specifically, given several optimizations (disclosed herein after, electromechanical coupling, size, etc.), total thickness t of the piezoelectric device is also optimized so as to provide the desired power output (in calculation thickness of electrode telectrode is considered as negligeable). Once determined an output power needed for the device, and once determined an orientation of the piezoelectric tensor around x-axis by ψ angle (which allows a first optimization of the piezoelectric electromechanical coupling of the material kij2), several optimizations are implemented, mainly based film thickness tf of the LNT film (or films) and on size of the device (length, width).
More specifically, the harvester is deemed to be capable of generating a voltage, V, that can be converted into electrical power whenever connected to a load. If the excitation of the beam is sinusoidal, the voltage response is AC (alternative current), therefore one has to consider the root mean squared (RMS) value of V. In consequence, the power dissipated in the resistive load, R 1, is given as:
If one examines the instantaneous power response for a piezoelectric generator in impedance matching condition, one has:
From equation 1, according to the disclosure the inventors identified the parameters to optimize:
For example, in order to implement the material on a hosting substrate of substrate thickness ts and with a given stiffness, one has to choose correctly the film thickness tf of the piezoelectric material. As exposed above, one parameter to consider is the effective electromechanical coupling k2, which is the ratio between the converted energy and total input energy.
The inventors determined how k2 depends on the thickness ratio
and ruled that is possible to investigate its maximum value as a function of this thickness ratio. For example, using the LiNbO3 orientation (YXI)/128°, the inventors ruled that the optimized thickness ratio condition to be in the rage of 2.3, which correspond to a 30 μm piezoelectric film thickness, tf. For this range of piezoelectric film thickness, tf, the inventors determined that a change the length of the beam, do not imply considerable variation in terms of k2. Moreover, the inventors determined that it is possible to further increase the value of k2 using different substrate thicknesses and stiffness of the substrate. The capacitance for such device is about 4 nF, for an active surface area of 270 mm2.
With regards to the frequency of the beam, the inventors ruled that it is possible to lower its response and increase the displacement considering different thickness of the substrate. The bending resonance is thus tuned in terms of the ratio
Therefore, it one reduces the substrate thickness, it is possible to attain lower frequency response. Other possibilities are to change the length of the beam once the optimized total thickness of the cantilever is found or to add a mass tip to the unclamp side of the beam. For instance, considering a beam width of 10 mm, and varying the beam length from 20 mm to 100 mm, one can tune the frequency from 200 Hz down to 10 Hz. The width of the beam can be as low as 5 mm.
Thus, as example, for a cantilever with these features, one can estimate the power response while varying the length of the piezoelectric device. The inventors ruled that the maximum instantaneous power took at resonance increases with the length of the cantilever, ranging from 1 μW up to 400 μW. In fact, the available energy increases with the volume of piezoelectric material under strain and for longer beams one can attain higher displacement. Eventually, metal substrates like stainless steel have high quality factor and reliability, and they can be exploited to increase the power output of the harvesters.
Usually, the voltage delivered to a sensor, by a piezoelectric device of the type described above, he has to be in DC, whereas the most common electronic configuration to convert AC to DC signals is a standard full-bridge rectification circuit. This electronic interface consists in 4 diodes and a smoothing capacitor, Cr, able to store the rectified voltage, VDC. Considering standard dimensions for the beam (e.g. 60 mm by 10 mm), the inventors ruled the rectified power in terms of the coupling k2. One defines the normalized frequency
where ω0 is the resonance frequency of the beam, thus the optimal power in function of k2 is given as:
Where F is the excitation force, M the mass of the harvester and ζ the mechanical damping of the structure. In order to operate a small sensor, one assumes that the power delivered to the load has to be above 20 μW. Hence, one can estimate the power output of the harvester considering a reasonable structural damping (ζ=0.01) and an excitation force driven by an acceleration of 2 m/s2. In
The optimizations on film thickness, tf, of the piezoelectric material and of the substrate thus have consequence both on the coupling factor and on the power delivered by the device. These optimizations are presented herein after. In consequence, the piezoelectric device produced with one or several of the optimizations, has the following advantages:
Once the piezoelectric element and the metal substrate, are bonded together, the harvester can withstand higher acceleration levels and target application with lower resonance frequencies (from 10 to 500 Hz). In addition, even if the dielectric constant of the piezoelectric materials is low, it is possible to reduce the film thickness locally to achieve reasonable capacitance (nF scale). In this way, one can develop prototypes attaining realistic impedance-matching conditions between the piezoelectric transducer and the electronic interface.
According to the disclosure, in one embodiment, the inventors demonstrated that optimization of flexible single crystal LNT film depends on size effect. Considering, a simple LNT film beam, the decrease of the film thickness, tf, increases the deflection. In the frame of a simple beam with uniform rectangular section, the result can be at first order described by the equation of isotropic beams deflection in the frame of originates Euler-Bernoulli formalism but can be extended to more complex modelling (
In a simplistic way, one can define the deflection d, in function of a force F and length L, with Young's modulus E as:
With an inertia moment defined as
where w is the width and tf the film thickness of the piezoelectric device respectively. From this equation one can see that the deflection is proportional to
so that the film thickness has a strong effect on the magnitude of deflection. This equation is valid with linear materials that are isotropic, but if one considers anisotropic properties of LiNbO3 (or LiTaO3), the inventors state that it can be also modeled with finite element simulations to reach the optimized thickness, tf. By extension to the exposed beam theory, the addition of the substrate material of thickness, ts, the electrodes, the shape of the structure that can be supported by 1, 2 or more distributed points (i.e. not limited to a beam) can be modeled with finite element simulations to reach optimized parameters whose the total thickness t=ts+tf, of the composite device.
Optimization of resonance frequency can also be achieved with optimization dimension of dimension of the piezoelectric device. The vibration of a simple—one-end clamped structure can be solved using continuous mechanic of beams. A simplified expression gives for the first vibration mode a resonant frequency f1:
Where ρ is the density and A=wt the area of the section. The product ρA is the mass per unit length supposed uniform. Others parameters where defined in equation (3).
Considering a composite structure made of a host substrate, or more than one, the deflection depends on the inertial moment of the composite structure. It includes one, two (bimorph) or more multiple piezoelectric elements (stack), or fiber those the ferroelectric polarization can be controlled to more efficiency. For instance, in the case of bimorph, the structure is composed by two active piezoelectric layers, and one central passive layer that represents the host. The piezoelectric elements can be connected either in parallel or series connection. Given two identical piezoelectric elements, in the first case, since the two capacitors are in parallel, the equivalent capacitance is doubled and so is the current. The series connection instead reduces the capacitance but increases the voltage. Both the connections can be implemented taking into account the polarization of the piezoelectric layers. Multi-morph structures instead, are usually implemented with several piezoelectric and passive layers in between, and are mostly used for actuation purposes.
The range of deflection is limited by the fracture's mechanisms of the composite. In particular LNT wafers are brittle under loadings. To ensure high flexibility and viable structure with LNT films, one condition is to reduce the thickness tf of the film.
In order to prevent tensile stresses, pre-constrained piezoelectric film can be undertaken under compressive stresses, inventors have found that method known in the art can be applied.
Another way is to transfer the film on others flexible host substrates composite so that the stresses is always low or in compression in the LNT film. The engineer calculation, will determine the position of the neutral axis in function of the applied loads and in function of parameters such as the shape geometry, the thicknesses, ts and tf, and widths of the beams, the number of the composite beams, the elastic modulus of the LNT film and of the host flexible substrates.
For instance, if one will reduce the stress in the LNT film, and, considering a simple two-composite beam with a single host substrate having a Young modulus, E, higher than LNT (n=Ehost/ELNT>1, for instance stainless steel). Our calculation under bending load, gives a stress in LNT film reduced by 1/n for a host substrate have a thickness, ts, lower than the one of LNT film ti. More complex beam with multiple piezoelectric LNT films and flexible hosts of different material can ensure better viability and lifetime.
The piezoelectric harvesting system can include all possible shapes described in literature such as: MEMS devices, cantilever, spiral, multi-resonant structures, bimorph and stack structure with polarity inversion or periodically poled structures. These specific shapes can be obtained by different techniques: saw cutting, ions beams cutting, etching with acid solution, reactive ion etching (DRIE), laser cutting and electrical poling or a combination of these techniques.
The host crystal is not necessarily conductive, therefore the thin metallic films used in the transfer will act as an electrode (also called burry electrode). The second electrode of the structure can be deposited on the top of the structure, or, in the case of a stack composite, the second electrode can be buried.
Considering two electrodes on top and bottom of a flat piezoelectric film, and from the general theory of linear dielectric, the capacitance is inversely proportional to the thickness, tf of the piezoelectric element and proportional to the areas of the electrode,
Where ε is the dielectric permittivity of the piezoelectric material, S the surface of the electrodes and tf the thickness of the dielectric layer (
For instance, if the goal is to achieve a capacitance superior to 1 nF (nano-farads), to have realistic impedance matching with electronic interface, the thickness tf would be inferior to 15 μm, having a surface of approximately 40 mm2 (
In terms of electronic configuration, the internal resistance of the piezoelectric element follows the equation:
Where f is one of driving frequencies of the ambient vibration. For an optimized piezoelectric layer, the driving frequencies is the resonant frequency of the structure adapted to the ambient vibrations. The optimized internal resistance is in the kΩ range, compatible with low power electronics.
Commercial wafer of LNT are of hundreds μm to 1 mm thick poled single crystal and oriented by IEEE convention: (YXI)/θ LiNbO3 or LiTaO3. The piezoelectric element orientation is important in order to have optimal values for the piezoelectric coefficients. One can choose carefully among the commercially available cuts, to have the desired properties.
Eventually, (YXI)/128° orientation can be chosen as they are commercially available.
This optimization can be coupled with the two previous ones so as to obtain desired characteristics of the piezoelectric device.
The transfer to flat host-substrate or the stack of any composition has to be taken into consideration from the manufacturing origin of the substrate specimen. Here one assumes the transfer to a flat solid materials which can be characterized by a high viscosity (viscosity>104 Pa·s) or elastic Young modulus (E>1 GPa). It includes materials such as:
The first element is flatness (or TTV) and roughness of the host, to allow the gluing process. The inventors state that the preparation of the surface of the substrate has to be made by several step comprising:
According to the disclosure, the inventors state to use gluing by thermocompression, for instance with Cr/Au-Au/Cr bonding on prepared flat host material, and by extension the method described applies also to molecular gluing. For replacing Cr, other adhesive metals on lithium niobate may by used: aluminum, titanium, copper, . . . . In practice, since gold does not directly adhere to niobate, one tends to use chromium or another metal as an interface between the two. On the second side (i.e. the second side of the substrate or the thermo-compressed side of the film), one could use another metal combination if needed. More specifically, the inventors propose three possible processes for the production of flexible single crystal of LiNbO3 or LiTaO3 films of thicknesses, tf, below conventional wafers. The proposed processes are simple and reproducible because LiNbO3/LiTaO3 is a commercial product whose characteristics are stable and inexpensive and the bonding of such LNT films ensure stability and durability of the resulting piezoelectric device.
Unlike PZT, which necessitate a polarization process, during which a strong DC electric field is provided through the electrodes at a temperature a little below the Curie temperature, the proposed technique does not need any polarization. This polarization step is omitted in the disclosed processes because the Curie temperature is >600° C. for LiTaO3 and >1150° C. for LiNbO3. For avoiding this step, the inventors state it is preferable to use the orientation (see above) to improve the coupling, whereas this one is fixed by the direction of polarization in the case of PZT.
Additionally, unlike in the classic solution which is to attach a piezoelectric element on a flexible beam structure, the proposed processes differ because the complete structure of the device is created on a wafer. It is thus not a system comprising several attached elements, but an integrated system. It has thus to be understood that one realizes the composite structure before shaping, whereas in general the piezo element is added after.
A first process consists essentially in direct wafer gluing on any host material or composite, and is described according to
The LiNbO3 or LiTaO3 wafer is initially prepared on an intermediary or sacrificial crystal that can be prepared in laboratory, directly bought from a company or obtained by smart-cut or ions implanted sliding.
However, the smart-cut process does not allow to attain thick films (with thickness, tf>1 micron) necessary for efficient energy harvesting. For instance, one uses silicon intermediate crystal prepared by Au—Au thermocompression. The process is described according to
The process consists essentially in direct growth of the host structure on LiNbO3 (or LiTaO3) wafer and is described in
It is to note that electro-deposition of metallic substrate could be realized onto an initial structure made of LiNbO3 (or LiTaO3)/Cr/Au-Au/Cr/Si prepared by Au—Au thermocompression,
| Number | Date | Country | Kind |
|---|---|---|---|
| 20203952.5 | Oct 2020 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/078543 | 10/14/2021 | WO |
