Flexible resistance type pressure sensor based on candle ash and preparation method

Abstract

The invention provides preparation and application of a flexible resistance type pressure sensor taking candle ash as a conductive material. The sensor is composed of a pressure-sensitive layer and a flexible electrode package. The pressure-sensitive layer is porous PDMS foam soaked and attached with candle ash, and the foam is prepared from a PDMS prepolymer and a pore-foaming agent. The pressure-sensitive layer converts pressure change into resistance change, and the resistance is reduced along with the increase of the pressure. The flexible pressure sensor prepared by packaging the flexible electrode has good biological adaptability, and can be used for human body mechanical movement and physiological signal detection, including pressure detection of finger and arm bending, walking and other activities.

Description

Technical Field

[0001] The present invention relates to the field of pressure sensor devices, with classification number G01L1 / 00, and specifically to a flexible resistive pressure sensor and a preparation method thereof. Background Art

[0002] The performance of flexible resistive pressure sensors depends on the performance of pressure-sensitive materials. In order to improve the sensitivity and working range of the resulting sensor, porous conductive materials are often selected as the pressure-sensitive layer. Porous PDMS has attracted widespread attention in the fields of smart wearables, sports monitoring, biometrics, etc. due to its high specific surface area, high porosity, low density, good flexibility and biocompatibility. Based on the porous structure of PDMS foam, when subjected to external force, the contact area between the foam pores will change. After the addition of conductive materials, this change will increase the conductive paths inside the material, thereby causing resistance changes to achieve the perception of external force. In the existing technology, the preparation methods of porous PDMS mainly include particle template method, emulsion foaming method, gas foaming method, solvent evaporation method, printing method, etc. The particle template method adds pore-forming agents such as nickel foam, salt particles, sugar particles, etc. to the PDMS prepolymer, and removes the pore-forming agent after the mixture is solidified, leaving pores to form PDMS foam. The emulsion foaming method is to disperse at least one liquid in micron-sized droplets in an incompatible liquid to form a heterogeneous mixture, which forms pores after solidification. Part of the emulsion is retained inside the material, which will affect the performance of the device during use. The gas foaming method is to add a foaming agent (such as sodium bicarbonate) to the PDMS prepolymer. During heating and curing, the foaming agent decomposes to produce gas, thereby preparing porous foam. The solvent evaporation method is to add a solvent to the PDMS prepolymer. During the curing process, the solvent evaporates and the concentration of the prepolymer increases, becomes saturated, and phase-separates. After the solvent evaporates, a porous PDMS foam is obtained. The porous PDMS prepared by the emulsion foaming method, gas foaming method, and solvent evaporation method has poor controllability and repeatability. The printing method requires high costs and a complicated preparation process.

[0003] In existing technologies, the pressure-sensitive layer of most resistive PDMS pressure sensors typically incorporates conductive materials such as graphene, carbon nanotubes, and silver nanowires into the PDMS material, significantly enhancing its conductivity and creating a variable conductive network under pressure. While pressure sensors fabricated with these conductive materials perform well, they are relatively expensive, and their preparation often requires complex steps, expensive precursors, multiple chemical reagents, and harsh reaction conditions. Summary of the Invention

[0004] To address the above issues, the present invention provides a simple and inexpensive method for preparing a flexible resistive PDMS pressure sensor. The method uses PDMS and NaCl particles to prepare porous PDMS foam, and then uses candle ash as a conductive material to enhance the foam's resistance conversion ability under pressure.

[0005] To achieve the above purpose, the technical solution of the present invention is as follows:

[0006] The PDMS pressure sensor proposed in the present invention comprises two parts: porous PDMS with candle ash attached as a pressure-sensitive material and flexible electrodes attached to the upper and lower surfaces of the pressure-sensitive material.

[0007] The present invention provides a method for preparing a conductive porous PDMS foam. The method involves heating and curing a mixture of a PDMS prepolymer, NaCl particles, a curing agent, and n-hexane solvent. The mixture is then immersed in deionized water to remove the NaCl particles and form the PDMS foam. Finally, the porous PDMS foam is immersed in a mixture of candle ash and ethanol to absorb the conductive material and enhance the foam's electrical conductivity.

[0008] Preferably, the NaCl particles are ground and screened, and the NaCl particle size is 500 μm to 200 μm.

[0009] Preferably, the prepared candle ash is heated to above 400° C. for 30 minutes to remove residual paraffin.

[0010] Preferably, the prepared candle ash is heated, purified and pulverized to obtain powdered candle ash.

[0011] Preferably, the ratio of PDMS to NaCl particles during the preparation of the porous PDMS foam is 1:3.

[0012] Preferably, the ratio of PDMS to curing agent during the preparation of the porous PDMS foam is 10: 1. Preferably, an appropriate amount of n-hexane is added to the mixture during the preparation of the porous PDMS foam to soften the PDMS and facilitate the removal of bubbles remaining in the mixture by vacuuming.

[0013] Preferably, the solvent n-hexane needs to be removed before PDMS solidifies, and the removal method is to heat it at 40° C. for 2 hours.

[0014] Preferably, the porous PDMS foam is cured by heating the mixture to 80° C. for 2 hours.

[0015] Preferably, the NaCl is removed by soaking in 500 ml of deionized water for 12 hours twice.

[0016] Preferably, the concentration of the soaked candle ash ethanol solution is 0.01 g / ml.

[0017] Preferably, the soaking and adsorption process uses ultrasound-assisted treatment for 10 minutes, and the soaking and adsorption operation is repeated three times.

[0018] Beneficial effects: 1. The flexible resistive pressure sensor prepared by the present invention can change the mechanical properties of the pressure-sensitive layer by changing the size of NaCl particles to prepare pressure sensors with different sensitivities.

[0019] 2. The present invention soaks porous PDMS foam in conductive candle ash to enhance the foam's conductivity as a pressure-sensitive layer. When the pressure-sensitive layer is under pressure, more conductive paths are formed, causing the resistance of the pressure-sensitive layer to change under pressure. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The resistance of the PDMS pressure sensors prepared in Example 1, Comparative Example 1, and Comparative Example 2 varies with pressure.

[0021] Figure 2 The linear fitting of the mean value of the resistance change with pressure of the PDMS pressure sensor prepared in Example 1;

[0022] Figure 3 The linear fitting of the mean value of the resistance change with pressure of the PDMS pressure sensor prepared in Comparative Example 1;

[0023] Figure 4 The linear fitting of the mean value of the resistance change with pressure of the PDMS pressure sensor prepared in Comparative Example 2;

[0024] Figure 5 Preparation process of the pressure-sensitive layer of the flexible sensor. DETAILED DESCRIPTION

[0025] All raw materials involved in the present invention have no special requirements except that candle ash is prepared by burning purchased candles.

[0026] The present invention provides a flexible resistive PDMS pressure sensor based on candle ash, which can be used for detecting human mechanical motion and physiological signals, including finger and arm bending and pulse beating.

[0027] The conductive material used in the present invention is candle ash, which is the product of incomplete combustion of a candle. Preparation process: Place an iron sheet clamped by pliers into the flame of a candle and heat it, reducing the contact between the flame and the air to cause incomplete combustion, and depositing more carbon black particles on the iron sheet. In order to ensure that there is no paraffin residue on the prepared candle ash, which in turn affects the conductive properties of the prepared pressure-sensitive layer, it needs to be purified. Purification process: Clean the candle ash on the iron sheet into an iron can, and place the iron can on a heating table and heat it to above 400°C for 30 minutes to ensure that the residual paraffin therein is removed.

[0028] The pressure-sensitive material of the present invention is a porous PDMS foam, and the porogen used is NaCl particles. The salt particles are ground in a mortar and sieved to obtain particles with sizes of 200-300 μm, 300-400 μm, and 400-500 μm. The porogen of the present invention is replaceable, stable, and easy to remove solid particles.

[0029] The pressure-sensitive material of the present invention is a porous PDMS foam, which is specifically prepared as follows: PDMS and a curing agent are first mixed uniformly, and then NaCl and n-hexane are added and stirred to ensure uniform mixing. The n-hexane is used to soften the PDMS and enhance its fluidity, facilitating vacuum treatment to remove internal bubbles after the mixture is filled into a mold. The vacuum-treated mixture is heated to 40°C to remove the n-hexane, and finally heated and cured. Heating to 40°C is intended to volatilize the n-hexane while preventing excessive curing of the PDMS.

[0030] The present invention also provides a flexible resistive pressure sensor, which is composed of a pressure-sensitive layer and a flexible electrode. The flexible electrode is obtained by cutting copper-nickel conductive cloth.

[0031] Example 1:

[0032] Preparation process: 1. Mix PDMS: 400-500 μm NaCl: curing agent: n-hexane in a mass ratio of 10:30:1:5;

[0033] 2. Transfer the mixture into a square mold with a length, width and height of approximately 5 cm and 1.7 cm.

[0034] 3. Place the mold containing the mixture in a vacuum chamber and evacuate for 60 minutes to remove bubbles inside the mixture;

[0035] 4. Place the mold in an oven at 40°C for 2 hours to remove the n-hexane, then heat to 80°C for 2 hours to completely cure the PDMS.

[0036] 5. Remove the mold and peel the PDMS off the mold;

[0037] 6. Cut the PDMS peeled off from the mold into thin pieces with a length, width and thickness of 2cm*1cm*0.1cm;

[0038] 7. Soak the cut PDMS in deionized water for 12 hours, then take it out and dry it. Repeat the above steps to ensure that the NaCl is completely removed.

[0039] 8. Soak the cut porous PDMS foam in a 0.01 g / ml candle ash ethanol solution and ultrasonicate for 10 min;

[0040] 9. Place the treated PMDS foam in an oven and heat to 80°C for 20 minutes to evaporate and remove the ethanol.

[0041] 10. Repeat steps 8 and 9 two more times to ensure that the candle ash is adsorbed on the PDMS foam.

[0042] 11. Place electrodes on the top and bottom of the PDMS foam and seal it with tape.

[0043] Comparative Example 1

[0044] In this comparative example, only the size of the NaCl particles used in preparing the porous PDMS foam was changed to 300 μm to 400 μm.

[0045] The other steps are the same as in Example 1.

[0046] Comparative Example 2

[0047] In this comparative example, only the size of the NaCl particles used in preparing the porous PDMS foam was changed to 200 μm to 300 μm.

[0048] The other steps are the same as in Example 1.

Claims

1. A flexible resistive pressure sensor, characterized in that It consists of two parts: a flexible electrode and a pressure-sensitive layer. The pressure-sensitive layer is a porous PDMS foam that absorbs conductive material and changes its resistance under pressure.

2. The flexible resistive pressure sensor according to claim 1, wherein: The flexible electrode material is conductive cloth.

3. The flexible resistive pressure sensor according to claim 1, wherein: The porous PDMS foam is prepared from PDMS prepolymer and porogen.

4. The porous PDMS foam according to claim 3, characterized in that The pore size of the PDMS porous foam is affected by the particle size of the porogen, and the size of the porogen NaCl is 50 to 500 μm.

5. The porogen according to claim 4, characterized in that The porogens can be NaCl, sodium citrate, or sugar cubes.

6. The flexible resistive pressure sensor according to claim 1, wherein: The conductive material is purified candle ash, which is produced by incomplete combustion of paraffin.

7. The conductive material adsorption according to claim 1, characterized in that: The conductive material and the solvent are first mixed, and then put into the PDMS foam to absorb the conductive material with the assistance of ultrasound.

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