Capacitive flexible pressure sensor based on barium titanate composite dielectric layer and preparation method thereof

Abstract

The invention relates to the technical field of flexible electronics and sensing, in particular to a capacitive flexible pressure sensor based on a barium titanate composite dielectric layer, which sequentially comprises a lower electrode layer, an upper electrode layer, a lower electrode layer and an upper electrode layer from bottom to top, the composite dielectric layer is arranged on the lower electrode layer and is formed by compounding an elastic polymer matrix and a high-dielectric-constant barium titanate (BaTiO3) filler; the upper electrode layer is made of a flexible conductive material and is arranged on the composite dielectric layer; the lower electrode layer and the upper electrode layer are connected with an external capacitance measuring circuit through wires. According to the sensor disclosed by the invention, the comprehensive performance of high sensitivity, wide detection range, low hysteresis and excellent cycle stability is realized by optimizing the shape of the barium titanate filler, surface modification and composite structure design.

Description

Technical Field

[0001] This invention relates to the fields of flexible electronics and sensing technology, specifically a capacitive flexible pressure sensor based on a barium titanate composite dielectric layer and its fabrication method. Background Technology

[0002] Flexible pressure sensors have attracted much attention due to their broad application prospects in wearable devices, electronic skin, human-computer interaction, and health monitoring. Among them, capacitive flexible pressure sensors have advantages such as simple structure, low power consumption, fast dynamic response, and sensitivity to static pressure. Their core working principle is that external pressure causes a change in the sensor's capacitance, which is typically achieved by altering the thickness or dielectric constant of the dielectric layer.

[0003] Traditional capacitive flexible pressure sensors often use purely elastic polymers (such as polydimethylsiloxane PDMS, Ecoflex, etc.) for their dielectric layer, which have a low dielectric constant (usually 2-3), resulting in low sensor sensitivity. ,in The initial capacitance. This represents the change in capacitance. The sensitivity is limited by the amount of pressure change. To improve sensitivity, researchers often add conductive or dielectric fillers with high dielectric constants, such as carbon nanotubes, graphene, and metal nanoparticles, to the polymer matrix to form a composite dielectric layer. However, these conductive fillers tend to form conductive pathways in the matrix, leading to increased dielectric loss and even seepage at higher concentrations, making the material resemble a conductor. This can damage the capacitive structure and affect the stability and linearity of the sensor.

[0004] Barium titanate (BaTiO3) is a classic perovskite-structured ferroelectric material with extremely high intrinsic dielectric constants (reaching thousands at room temperature), and its dielectric properties can be effectively tuned through particle size, morphology, and doping. Introducing barium titanate as a functional filler into an elastic polymer matrix holds promise for significantly increasing the effective dielectric constant of the composite dielectric layer without excessively increasing dielectric loss, thereby significantly improving sensor sensitivity. However, optimizing the dispersibility, surface modification, and interfacial bonding of barium titanate filler with the polymer matrix to fabricate flexible pressure sensors that combine high sensitivity, wide linear range, low hysteresis, and good stability remains a key technical challenge. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and propose a capacitive flexible pressure sensor based on a barium titanate composite dielectric layer and its preparation method. This sensor achieves comprehensive performance with high sensitivity, wide detection range, low hysteresis and excellent cycle stability by optimizing the morphology, surface modification and composite structure design of the barium titanate filler.

[0006] To achieve the above objectives, the technical solution specifically adopted by the present invention is as follows: A capacitive flexible pressure sensor based on a barium titanate composite dielectric layer, comprising, from bottom to top: The lower electrode layer is made of a flexible conductive material; A composite dielectric layer, disposed on the lower electrode layer, is composed of an elastic polymer matrix and a high dielectric constant barium titanate (BaTiO3) filler. The upper electrode layer, made of a flexible conductive material, is disposed on the composite dielectric layer; The lower electrode layer and the upper electrode layer are connected to an external capacitance measurement circuit via wires.

[0007] Furthermore, the barium titanate filler is nanoparticles, nanowires, or barium titanate powder that has been surface modified (e.g., treated with silane coupling agents or titanate coupling agents), and its volume fraction in the elastic polymer matrix is ​​5% to 50%.

[0008] Furthermore, the elastic polymer matrix is ​​one or more of polydimethylsiloxane (PDMS), Ecoflex, thermoplastic polyurethane (TPU), or styrene-ethylene-butene-styrene block copolymer (SEBS).

[0009] Furthermore, the composite dielectric layer has a porous or microstructured surface; the porous structure is formed by adding a pore-forming agent (such as sugar particles or salt particles) and subsequently dissolving it, or by using a foaming process; the microstructured surface is formed into an array structure such as a pyramid, cylinder or hemisphere by a mold transfer method.

[0010] Furthermore, the lower electrode layer and the upper electrode layer are metal thin films (such as gold, silver or aluminum), conductive nanomaterial thin films (such as carbon nanotubes, graphene or silver nanowires) or conductive polymers (such as PEDOT:PSS); and their preparation methods are coating, spraying, vapor deposition or transfer printing.

[0011] This invention also provides a method for fabricating a capacitive flexible pressure sensor based on a barium titanate composite dielectric layer, comprising the following steps: S1. Barium titanate filler pretreatment: Barium titanate powder was surface modified to improve its dispersibility and interfacial compatibility in polymer solutions. S2. Preparation of composite dielectric layer paste: The surface-modified barium titanate filler was dispersed in an organic solvent, and an elastic polymer prepolymer and a curing agent were added. After stirring and ultrasonic treatment, a composite slurry was obtained. S3. Composite dielectric layer molding: The composite slurry is poured onto a flat substrate, cast into a film, cured, and then demolded to obtain a flat composite dielectric film; or The composite slurry is poured onto a mold with a microstructure array, cured, and then demolded to obtain a composite dielectric layer with a microstructured surface; or Soluble pore-forming agent particles are added to the composite slurry. After molding and curing, the pore-forming agent is dissolved and removed to obtain a porous composite dielectric layer. S4. Electrode fabrication and integration: A lower electrode layer is prepared on a flexible substrate, wherein the flexible substrate is polyethylene terephthalate (PET), polyimide (PI), or a cured polymer; The composite dielectric layer prepared in step S3 is bonded or transferred to the lower electrode layer; A top electrode layer is fabricated on a composite dielectric layer; The lower electrode layer and the upper electrode layer are prepared by coating, spraying, vapor deposition or transfer printing processes; S5. Lead wires: Connect the upper and lower electrodes to the measuring leads using conductive silver paste or metal foil.

[0012] Furthermore, the flexible substrate is polyethylene terephthalate (PET), polyimide (PI), or a cured polymer.

[0013] Furthermore, in step S4, after preparing the upper electrode layer, the following steps are also included: A flexible encapsulation layer is covered on the upper electrode layer, wherein the flexible encapsulation layer is a polydimethylsiloxane (PDMS) or thermoplastic polyurethane (TPU) film.

[0014] 1) High Sensitivity: The high dielectric constant of barium titanate filler significantly enhances the effective dielectric constant of the composite dielectric layer. Under external pressure, the microstructure or pores are compressed, which not only reduces the dielectric layer thickness but also may cause further nonlinear growth of the dielectric constant due to changes in filler spacing (such as enhanced interfacial polarization effect), resulting in an extremely high capacitance change rate. In the low-pressure range (<10 kPa), the sensitivity can reach 0.1-1 kPa. -1 above.

[0015] 2) Wide detection range and good linearity: By optimizing the filling ratio of barium titanate and the microstructure design of the dielectric layer, the sensor can maintain high sensitivity at low pressure, while its capacitance change can maintain a good linear or piecewise linear relationship with the pressure in a wide pressure range (such as 0-100kPa or even higher), making it suitable for detecting a variety of pressure signals from slight touch to human movement.

[0016] 3) Low hysteresis and high stability: Barium titanate, as an inorganic dielectric filler, possesses stable mechanical properties and, after surface modification, bonds firmly to the polymer matrix, reducing energy loss caused by polymer chain slippage and interfacial slippage, thereby effectively reducing the sensor's mechanical hysteresis. Simultaneously, the stable material system ensures the sensor's performance reliability under long-term cyclic loading.

[0017] 4) Flexible and highly compatible preparation process: The preparation method described in this invention can be combined with existing mature micro-nano processing, coating, transfer printing and other processes, which is suitable for large-area and large-scale production, and the morphology and performance of the sensitive unit can be easily changed by adjusting the mold. Attached Figure Description

[0018] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings: Figure 1 This is a schematic diagram of the layered structure of the flexible pressure sensor.

[0019] Figure 2 The sensitivity curve of the sensor prepared for the implementation of this invention.

[0020] Figure 3 The image shows the stability test results of the capacitance response of the sensor prepared for this invention under 7000 cycles of cyclic loading. Detailed Implementation

[0021] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0022] In a first aspect, the present invention provides a capacitive flexible pressure sensor based on a barium titanate composite dielectric layer, comprising, from bottom to top: The lower electrode layer is made of a flexible conductive material; The composite dielectric layer, disposed on the lower electrode layer, is composed of an elastic polymer matrix and a high dielectric constant barium titanate (BaTiO3) filler. The addition of BaTiO3 filler can significantly increase the overall dielectric constant of the composite material, directly amplifying the capacitance signal output under pressure. The polymer matrix (such as PDMS or silicone) can provide excellent elastic deformation capability, allowing the dielectric layer to be significantly compressed under pressure (reducing the electrode spacing d), further amplifying the capacitance change.

[0023] The upper electrode layer, made of a flexible conductive material, is disposed on the composite dielectric layer; The lower electrode layer and the upper electrode layer are connected to an external capacitance measurement circuit via wires.

[0024] In one embodiment, the barium titanate filler is nanoparticles, nanowires, or surface-modified (e.g., treated with silane coupling agents or titanate coupling agents) barium titanate powder, with a volume fraction of 5% to 50% in the elastic polymer matrix. By optimizing the filler morphology and interfacial compatibility, the dielectric constant is improved and agglomeration is reduced. At the same time, the filling ratio is controlled to balance flexibility (low volume fraction) and dielectric properties (high volume fraction), ensuring the sensor's capacitive response sensitivity and mechanical stability under pressure.

[0025] In one embodiment, the elastic polymer matrix is ​​one or more of polydimethylsiloxane (PDMS), Ecoflex, thermoplastic polyurethane (TPU), or styrene-ethylene-butene-styrene block copolymer (SEBS). Utilizing the high elastic modulus, biocompatibility, and easy processing properties of these materials, the composite dielectric layer is provided with reversible deformation capability, adapting to the bonding requirements of irregular surfaces and reducing the risk of permanent plastic deformation under cyclic loading.

[0026] In one embodiment, the composite dielectric layer has a porous or microstructured surface; the porous structure is formed by adding a pore-forming agent (such as sugar or salt particles) and subsequently dissolving it, or by using a foaming process; the microstructured surface is formed into an array structure such as a pyramid, cylinder, or hemisphere using a mold transfer method. By introducing a porous structure or microstructured surface (such as a pyramid array) into the composite dielectric layer, the compressible deformation space of the material under pressure is significantly increased. Thus, when an external force is applied, the deformation of the dielectric layer is greater, resulting in a more pronounced change in capacitance (in capacitive sensors), thereby improving the sensitivity to a level beyond conventional expectations. The specific mechanism is as follows: First, pores are formed by adding a pore-forming agent (such as sugar or salt particles) and dissolving it, or by using a foaming process. These pores increase the internal porosity of the dielectric layer, making it easier to compress under pressure, expanding the deformation space, and enhancing the magnitude of capacitance change. Second, an array structure such as a pyramid, cylinder, or hemisphere is formed using a mold transfer method. These micro-geometry features provide more deformation points, which can generate local high strain under pressure, further amplifying the overall deformation of the dielectric layer and improving signal response sensitivity.

[0027] In one embodiment, the lower electrode layer and the upper electrode layer are metal thin films (such as gold, silver or aluminum), conductive nanomaterial thin films (such as carbon nanotubes, graphene or silver nanowires) or conductive polymers (such as PEDOT:PSS); the preparation method is coating, spraying, vapor deposition or transfer printing.

[0028] In a second aspect, the present invention provides a method for fabricating a capacitive flexible pressure sensor based on a barium titanate composite dielectric layer, comprising the following steps: S1. Barium titanate filler pretreatment: Barium titanate powder was surface modified to improve its dispersibility and interfacial compatibility in polymer solutions. S2. Preparation of composite dielectric layer paste: The surface-modified barium titanate filler was dispersed in an organic solvent, and an elastic polymer prepolymer and a curing agent were added. After stirring and ultrasonic treatment, a composite slurry was obtained. S3. Composite dielectric layer molding: The composite slurry is poured onto a flat substrate, cast into a film, cured, and then demolded to obtain a flat composite dielectric film; or The composite slurry is poured onto a mold with a microstructure array, cured, and then demolded to obtain a composite dielectric layer with a microstructured surface; or Soluble pore-forming agent particles are added to the composite slurry. After molding and curing, the pore-forming agent is dissolved and removed to obtain a porous composite dielectric layer. S4. Electrode fabrication and integration: A lower electrode layer is prepared on a flexible substrate, wherein the flexible substrate is polyethylene terephthalate (PET), polyimide (PI), or a cured polymer; The composite dielectric layer prepared in step S3 is bonded or transferred to the lower electrode layer; A top electrode layer is fabricated on a composite dielectric layer; The lower electrode layer and the upper electrode layer are prepared by coating, spraying, vapor deposition or transfer printing processes; S5. Lead wires: Connect the upper and lower electrodes to the measuring leads using conductive silver paste or metal foil.

[0029] Furthermore, the flexible substrate is polyethylene terephthalate (PET), polyimide (PI), or a cured polymer.

[0030] Preferably, in step S4, after preparing the upper electrode layer, the method further includes: covering the upper electrode layer with a flexible encapsulation layer, wherein the flexible encapsulation layer is a polydimethylsiloxane (PDMS) or thermoplastic polyurethane (TPU) film.

[0031] A third aspect of this invention is to provide a specific embodiment to demonstrate that the sensor obtained by this invention possesses the characteristics of high sensitivity, wide detection range and good linearity, low hysteresis and high stability. This embodiment includes the following steps: S1. 10g of barium titanate nanoparticles with an average particle size of 200nm were surface modified with an ethanol solution of 3-aminopropyltriethoxysilane (APTES) and dried for later use.

[0032] S2. Add the modified barium titanate nanoparticles (approximately 20% by volume) to ethyl acetate and ultrasonically disperse for 30 minutes. Add PDMS prepolymer (Sylgard 184A) and curing agent (Sylgard 184B, with a weight ratio of modified barium titanate:A:B ≈ 16.07:10:1), and mechanically stir for 2 hours to obtain a homogeneous slurry.

[0033] S3. The obtained uniform slurry is poured onto a silicon template with a pyramid array (10 μm wide at the base and 10 μm high) on the surface, placed in a vacuum drying oven to remove air bubbles, and then cured at 80°C for 2 hours. After cooling, the PDMS / BaTiO3 composite dielectric film with a pyramid microstructure surface is peeled off from the template.

[0034] S4. Fingered gold electrodes are prepared on a flexible PET film by magnetron sputtering using a mask as the lower electrode.

[0035] S5. Attach the flat surface of the composite dielectric film obtained in step S3 to the sensitive area of ​​the lower electrode.

[0036] S6. On the microstructure surface of the composite dielectric film, the upper electrode is formed by spraying silver nanowire ink through a soft contact mask.

[0037] S7. Encapsulate with another PDMS film and lead out wires with conductive silver paste.

[0038] Tests show that the sensor has a sensitivity of 0.85 kPa within a pressure range of 0-5 kPa. -1 The sensitivity is 0.15 kPa in the range of 5-30 kPa. -1 The response time is <100ms, and the capacitance response decays by less than 5% after 5000 cycles of loading.

[0039] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A capacitive flexible pressure sensor based on a barium titanate composite dielectric layer, characterized in that, From bottom to top, they include: The lower electrode layer is made of a flexible conductive material; A composite dielectric layer, disposed on the lower electrode layer, is composed of an elastic polymer matrix and a high dielectric constant barium titanate (BaTiO3) filler. The upper electrode layer, made of a flexible conductive material, is disposed on the composite dielectric layer; The lower electrode layer and the upper electrode layer are connected to an external capacitance measurement circuit via wires.

2. The capacitive flexible pressure sensor based on a barium titanate composite dielectric layer as described in claim 1, characterized in that: The barium titanate filler is nanoparticles, nanowires, or surface-modified barium titanate powder, and its volume fraction in the elastic polymer matrix is ​​5% to 50%.

3. The capacitive flexible pressure sensor based on a barium titanate composite dielectric layer as described in claim 1, characterized in that: The elastic polymer matrix is ​​one or more of polydimethylsiloxane (PDMS), Ecoflex, thermoplastic polyurethane (TPU), or styrene-ethylene-butene-styrene block copolymer (SEBS).

4. The capacitive flexible pressure sensor based on a barium titanate composite dielectric layer as described in claim 1, characterized in that: The composite dielectric layer has a porous or microstructured surface; the porous structure is formed by adding a pore-forming agent and then dissolving it, or by using a foaming process; the microstructured surface is formed into an array structure such as a pyramid, cylinder or hemisphere by a mold transfer method.

5. A capacitive flexible pressure sensor based on a barium titanate composite dielectric layer as described in claim 1, characterized in that: The lower electrode layer and the upper electrode layer are metal thin films, conductive nanomaterial thin films, or conductive polymers; their preparation methods include coating, spraying, vapor deposition, or transfer printing.

6. A method for fabricating a capacitive flexible pressure sensor based on a barium titanate composite dielectric layer as described in any one of claims 1-5, characterized in that, Includes the following steps: S1. Barium titanate filler pretreatment: Surface modification treatment of barium titanate powder; S2. Preparation of composite dielectric layer paste: The surface-modified barium titanate filler was dispersed in an organic solvent, and an elastic polymer prepolymer and a curing agent were added. After stirring and ultrasonic treatment, a composite slurry was obtained. S3. Composite dielectric layer molding: The composite slurry is poured onto a flat substrate, cast into a film, cured, and then demolded to obtain a flat composite dielectric film; or The composite slurry is poured onto a mold with a microstructure array, cured, and then demolded to obtain a composite dielectric layer with a microstructured surface; or Soluble pore-forming agent particles are added to the composite slurry. After molding and curing, the pore-forming agent is dissolved and removed to obtain a porous composite dielectric layer. S4. Electrode fabrication and integration: A lower electrode layer is fabricated on a flexible substrate; The composite dielectric layer prepared in step S3 is bonded or transferred to the lower electrode layer; A top electrode layer is fabricated on a composite dielectric layer; S5. Lead wires: Connect the upper and lower electrodes to the measuring leads using conductive silver paste or metal foil.

7. The method for fabricating a capacitive flexible pressure sensor based on a barium titanate composite dielectric layer as described in claim 6, characterized in that, The flexible substrate is polyethylene terephthalate (PET), polyimide (PI), or a cured polymer.

8. The method for fabricating a capacitive flexible pressure sensor based on a barium titanate composite dielectric layer as described in claim 6, characterized in that, In step S4, after preparing the upper electrode layer, the following steps are also included: A flexible encapsulation layer is covered on the upper electrode layer, wherein the flexible encapsulation layer is a polydimethylsiloxane (PDMS) or thermoplastic polyurethane (TPU) film.

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