Patent Application
20250081853
The present disclosure relates to a composite material with piezoelectric properties, to a process for producing a thin film comprising the composite material and to the electronic component comprising the composite material. The electronic component may be for example a microelectromechanical (MEMS) device such as an acoustic sensor or actuator, in particular a microphone.
Significant progress has recently been made in the development of innovative materials for piezoelectric and acoustoelectric conversion. Acoustoelectric conversion has numerous applications, for example, for sound detection and recording, speed measurement, human activity detection, entertainment, information recognition, elderly care and security.
Acoustoelectric conversion may also be used for small-scale energy harvesting and powering. In fact, acoustoelectric conversion converts acoustic waves into electrical signals and may be obtained by exploiting several phenomena such as, for example, the piezoelectric effect.
Piezoelectric acoustoelectric conversion, based on the deformation of a material caused by the vibrations of acoustic waves, is particularly suitable for developing flexible devices.
Conventionally, piezoelectric films are used to manufacture acoustoelectric transducers. In this context, recently, piezoelectric polymers have aroused particular interest, such as, for example, polyvinylidene fluoride (PVDF) and its copolymers, or composites based on a piezoelectric polymeric matrix that have shown acoustoelectric transduction capabilities.
These materials are particularly advantageous as, compared to piezoelectric ceramics, they have excellent flexibility.
It is generally recognized that PVDF, as a semicrystalline polymer, has five polymorphs: α, β, γ, δ and ε. The piezoelectric property of PVDF is attributed to the existence of polar crystalline phases, including the β-and γ-phase, and the β-phase shows the greatest electric dipole moment among all crystalline phases of the polymer. Therefore, one of the strategies to improve the piezoelectric performances of PVDF-based sensors, actuators and energy harvesting transducers has been that of increasing the β-phase content.
Although the piezoelectric coefficient of PVDF and its copolymers is not as high as that of inorganic piezoelectric materials, their good flexibility, strength, chemical resistance, and biocompatibility make them promising piezoelectric candidates for self-powered sensors, microphones, and energy harvesting transducers.
PVDF-based composites containing piezoelectric ceramic fibers have also been prepared over the years in order to optimize PVDF-based devices and to obtain higher sensitivity and higher output power, thus laying the foundation for various applications, for example, in the wearable devices and implants sectors.
On the other hand, the materials currently available (for example aluminum nitride, AlN) have a long and complex production process, which may envisage over thirty deposition steps to form layers of useful thickness in commercial applications.
New piezoelectric materials with improved performances and easy production are therefore being sought.
The present disclosure provides a new material with improved piezoelectric and ferroelectric properties which overcomes the drawbacks of the prior art and in particular which allows easier manufacturing of devices incorporating it. A composite material, a method for producing a thin film and the electronic component thus obtained are provided.
A composite material comprising a fluoropolymer matrix; and a filler, consisting of nanoparticles of a ceramic of a BZT-αBXT type doped with at least one doping element selected from the group consisting of Nb, La, Mn, Nd and W, wherein X is selected from Ca, Sn, and Mn, and a is a molar fraction in the range between 0.10 and 0.90, provided that when X is Mn, the doping element is not Mn. A process forming a thin film including the composite material is also provided.
For a better understanding of the present disclosure, some embodiments thereof are now described, purely by way of non-limiting example, with reference to the drawings, wherein:
The present description relates to a composite material comprising a fluoropolymer matrix and a filler formed by nanoparticles of a ceramic of the BZT-αBXT type wherein X is selected from Calcium (Ca), Tin (Sn) and Manganese (Mn) and a is a molar fraction selected in the range between 0.10-0.90, including 0.10 and 0.90, doped with at least one doping element selected from the group consisting of Niobium (Nb), Lanthanum (La), Mn, Neodymium (Nd) and Tungsten (W), provided that when X is Mn, the doping element is not Mn. The nanoparticles have an average diameter comprised between 100 and 200 nm, including 100 and 200 nm.
Advantageously, the present composite material has shown improved ferroelectric and piezoelectric properties and inductive behavior.
In particular, the presence of nanoparticles favors an exchange at the matrix-nanoparticle interface. Unlike the use of fibers, which favor the stiffening of the thin film, nanoparticles allow the conductive paths essential for the inductive properties and a flexible thin film to be obtained.
In particular, the size range 100-200 nm maximizes the interface between the polymer and the filler. Average nanoparticle diameters greater than 200 nm do not guarantee an effective interface between the polymer and the ceramic for charge transfer. Instead, nanoparticles with an average diameter of less than 100 nm tend to coalesce, impeding a homogeneous distribution in the material.
The doping elements La, Mn, and Nd can replace the element in a position of the barium and calcium site while Nb and W can replace the element in a position of the titanium and zirconium site.
In one embodiment, the fluoropolymer is selected from the group consisting of polyvinylidene fluoride (PVDF) and its copolymers, in particular, copolymers of PVDF with hexafluoropropylene (HFP) or with trifluoroethylene (TrFE).
In particular, PVDF and its copolymers may have a beta-phase percentage mass fraction, with respect to the sum of the masses of the crystalline phases, of at least 70%, more particularly between 86% and 99%.
The ceramic of the BZT-αBXT type may be a ceramic of the Barium Zirconate Titanate-Barium Calcium Titanate (BZT-BCT) type, in particular a ceramic of the (1-α)Ba(Zr(0.2-0.5y)Ti(0.8-0.5y))NbyO3-αBa0.7Ca0.3(Ti(1-y)Nby)O3 type, where a indicates the ratio between BZT and BCT and y=0.01-5% by weight on the total weight of the ceramic indicates the degree of substitution of Nb with respect to the sum of Ti and Zr, in particular y=2% by weight on the total weight of the ceramic.
The composite material has a ceramic (filler) content comprised between 10% and 25% by weight on the total weight of the composite, in particular 20%. Outside this range, the properties begin to decay to a complete loss of inductive behavior. Filler percentages greater than 25% lead to an onset of coalescence phenomena, the frequency of which increases as the filler percentage increases; such coalescence phenomenon leads to an increased possibility of electrical short circuits. In contrast, a filler percentage of less than 10% results in an uneven distribution of filler within the matrix.
Hereinbelow there is described an embodiment of a process for producing a thin film formed with the composite material described above. In detail, the process comprises:
Advantageously, this method allows a homogeneous and uniform thin film to be formed.
In particular, the first spin-coating step, conducted for example at 1000 rpm, leads to the application of the composite material on the entire surface of the substrate; the second spin-coating step, conducted for example at 4000 rpm, favors a first evaporation of the solvent and the third spin-coating step, conducted for example at 9000 rpm, favors the formation of the beta phase as it allows the complete stretching of the PVDF chains and the rotation of the groups H and F. In some embodiments, the substrate has a first surface opposite a second surface, the first surface of the substrate being entirely covered by the composite material.
Furthermore, the first annealing step favors a slow evaporation of the solvent which makes the surface of the thin film uniform and homogeneous; the second annealing step allows the polymer chains to consolidate after the total evaporation of the solvent.
Thanks to its ferroelectric and piezoelectric properties, the composite material may be used for manufacturing electronic components such as acoustic sensors, in particular microphones, energy harvesting transducers and micro-actuators.
Further characteristics will emerge from the following description of some merely illustrative and non-limiting examples.
17.78 g of PVDF powder are dissolved in a solution of 70 ml of dimethyl sulfoxide (DMSO) and 30 ml of acetone. The solution is placed on a magnetic heated plate and stirred by magnetic stirring at a temperature of 100° C.
Preparation of Nanoparticles of BCT-BZT Doped with 2% of Nb (2% BCNZT)
The BCNZT ceramic is prepared by Sol-Gel technique as described in EP 4074672 (US 2022/0332650). The precursor solution is then dried at 150° C. and the powders are recovered.
Once recovered, the powders are homogenized and dry ground using a planetary ball mill at a speed of 400 rpm for 4 hours. They are subsequently calcined at 1400° C. and wet regrinded for 8 hours with ethanol (1 ml) to obtain nanoparticles of the order of 100-200nm.
The 2% BCNZT nanoparticles are added in different amounts (15%, 20% 25% by weight on the composite weight) to the PVDF solution and are dispersed by ultrasonication and stirring. The solution is left on a plate for approximately 10 days to reach stability.
Once stabilized, the solution is deposited by spin-coating on a substrate to form a thin film. Three spin-coating steps are performed.
The method used envisages three steps:
Subsequently the sample is placed for 10 min on a plate at 70° C. for a complete evaporation and subsequently on a plate at 90° C. for another 10 min for the annealing step, finally it is cooled to room temperature T.
The films have been submitted to X-ray diffraction studies to verify the formation of the PVDF phases.
For the electrical characterization, a sample 10 has been provided as shown in
The sample 10 comprises a semiconductor layer 11, for example of monocrystalline silicon; an insulating layer 12, for example of silicon dioxide; a lower electrode layer 13, formed by a TiO2/Pt multilayer; a composite layer 14 of the present PVDF-BCNZT composite material; and a plurality of upper electrodes 15 of Platinum (Pt).
In the used sample, the stack of layers 11-13 has a total thickness of 700 μm; the composite layer 14 has a thickness of 2 μm and the upper electrodes 15 have a thickness of 120 nm.
The sample is sprayed (Turbo-Pumped Thermal Evaporator K975X) with platinum to form the upper electrodes and etched at the end to contact the lower electrode. Alternatively, a sputtering process may also be used to form the upper electrodes.
For all measurements, the electrodes were contacted with metal tips through manipulators on a hand-held spindle probe. Electrical property tests and impedance spectroscopy were performed with the Keysight B1500A semiconductor device analyzer. The ferroelectric properties were studied with the Aixact TFA TF2000E analyzer, applying a 30V, 5 kHz triangular wave signal to the sample.
In particular, the samples tested are films formed with:
The results of the ferroelectric tests are shown in
PVDF and PVDF_BZT-BCT thin films show broader ferroelectric properties associated with inductive behavior, likely due to conductive paths emerging in the PVDF matrix.
The shape of the ferroelectric curves of
Interesting properties of Nb-doped composites emerged thanks to impedance measurements. The samples were tested with the B1530 module of the B1500A, applying a 50 mV AC signal and no DC polarization to the samples. With this module frequencies from 1 kHz to 5 MHz may be tested. These measurements confirmed the dielectric properties of the PVDF as shown in
The described composites also showed an inductive nature measuring impedance and phase, as shown in
Finally, the piezoelectric properties were measured by a laser vibrometer MSA-500 Micro System Analyzer (Polytec) on a cantilever-shaped device.
Films with different doping percentages were submitted to vibrometric testing using the MSA-500 Micro System Analyzer (Polytec) to estimate the piezoelectric parameters. In particular, the device was submitted to Fast Fourier Transform (FFT) analysis sending a periodic chirp of 3 V and the displacement was measured using a laser vibrometer. This allowed the resonance frequency to be extracted.
Once the resonance frequency was extracted, by setting voltage and frequency in proximity to the resonance, the deflection of the cantilever was estimated. The displacement and the corresponding value of the effective in-plane transverse piezoelectric coefficient e31f were shown in Table 1 according to the formula proposed by Mazzalai:
where Y is the Young's modulus, v is the Poisson's ratio, tSi is the thickness of the substrate (in the sample of
Furthermore, since a resonance frequency close to 4 kHz was detected, the behavior of various devices exposed to sound pressure was studied. The acoustic wave is sent from a speaker to the cantilever and with the Micro System Analyzer MSA-500 the response of a device comprising the described composite material of a certified commercial microphone was studied. The measurement was carried out by driving the speaker with an external sinusoidal signal at various frequencies of the audible range, from 20 Hz to 20 kHz. It has been found that the response of the device comprising the composite material comprising PVDF and 20% of 2% BCNZT follows the signal detected by the microphone, with an amplitude that differs by one tenth from the certified microphone, as shown in
The device 20 is for example a MEMS sensor or actuator; in particular a microphone.
The device 20 has a suspended structure 21, here a cantilever, suspended over a cavity 30.
The cavity 30 is formed in a substrate 31 and delimits the suspended structure 21 at the bottom. The suspended structure 21 may be monolithic with the substrate 31. The suspended structure 21 and the fixed structure 31 are for example of monocrystalline silicon.
An insulating layer 22, for example of silicon dioxide, extends above the suspended structure 21 and an actuation or detection structure 32 extends on the insulating layer 22.
The actuation or detection structure 32 comprises a lower electrode layer 23, for example formed by a TiO2/Pt multilayer; a composite layer 24, of the present PVDF_2% BCNZT composite material; and a plurality of upper electrodes 25, for example of Pt.
Finally, it is clear that modifications and variations may be made to the composite material and the production process described and illustrated herein, without departing from the scope of the present disclosure, as defined in the claims.
A composite material comprising a fluoropolymer matrix and a filler consisting of nanoparticles of a ceramic of the BZT-αBXT type wherein X is selected from Ca, Sn, and Mn, and a is a molar fraction selected in the range between 0.10 and 0.90, including 0.10 and 0.90, doped with at least one doping element selected from the group consisting of Nb, La, Mn, Nd and W, wherein when X is Mn, the doping element is not Mn. Said differently, in some embodiments, when X is Mn, the doping element is selected from the group consisting of Nb, La, Nd, and W.
Said nanoparticles have an average diameter comprised between 100 and 200 nm, including 100 and 200 nm.
The fluoropolymer is selected from the group consisting of polyvinylidene fluoride (PVDF) and its copolymers.
The copolymers are selected from the group consisting of copolymers of PVDF with hexafluoropropylene (HFP) or copolymers of PVDF with trifluoroethylene (TrFE).
The polyvinylidene fluoride (PVDF) and its copolymers have a beta-phase percentage mass fraction, with respect to the sum of the masses of the crystalline phases, of at least 70%.
The polyvinylidene fluoride (PVDF) and its copolymers have a beta-phase percentage mass fraction, with respect to the sum of the masses of the crystalline phases, comprised between 86% and 99%, including 86% and 99%.
The doping element is present in an amount comprised between 0.01% and 5%, including 0.01% and 5%, by weight on the total weight of the ceramic.
The composite material a ceramic content comprised between 1% and 70%, including 1% and 70%, by weight on the total weight of the composite.
A process for producing a thin film comprising the composite material, including the steps of: a) submitting a dispersion of nanoparticles in an organic-phase fluoropolymer solution on the substrate to a first spin-coating step at a speed comprised between 500 and 1500 rpm, including 500 and 1500 rpm; to a second spin-coating step at a speed comprised between 3500 and 5500 rpm, including 3500 and 5500 rpm; and to a third spin-coating step at a speed comprised between 8500 and 12000 rpm, including 8500 and 1200 rpm; and b) submitting the product of step a) to a first annealing step at a temperature comprised between 60 and 70° C., including 60 and 70° C., and to a second annealing step at a temperature comprised between 80 and 90° C., including 80 and 90° C.; wherein the nanoparticles are nanoparticles of a ceramic of the BZT-αBXT type wherein X is selected from Ca, Sn, and Mn, and a is a molar fraction selected in the range between 0.10-0.90 doped with at least one doping element selected from the group consisting of Nb, La, Mn, Nd and W. In some embodiments, when X is Mn, the doping element is selected from the group consisting of Nb, La, Nd, and W.
An electronic component comprising a piezoelectric layer including said composite material.
The electronic component forming an acoustic sensor.
The acoustic sensor is a microphone.
The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.
These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.
| Number | Date | Country | Kind |
|---|---|---|---|
| 102023000018027 | Sep 2023 | IT | national |
