Description:
TECHNICAL FIELD OF THE INVENTION
[001] The present invention is related to the field of sensors. In particular, the invention is related to a pressure sensing element comprising of a diaphragm made up of a layer-by-layer spray coating of zinc oxide functionalized multiwalled carbon nanotube.
BACKGROUND OF THE INVENTION
[002] Sensor technology is a rapidly growing field which has the potential to improve operation, utility, and serviceability of various engineering systems. Sensors are basically devices which detect and respond, to any input from the physical environment, in the form of signals. The inputs could be light, heat, pressure, or any other form of stimulus. Sensors have diverse usages and applications in monitoring functionality of various systems used in different spheres of present-day life like health care, human machine interfacing, prosthetics safety controls etc.
[003] Different systems are available for sensing pressure and strain and a typical electronic sensor is based on the principle of strain induced changes in capacitance, resistance, triboelectricity or piezoelectricity. Recent advancement in design and development of pressure sensor led to various pressure sensors with different sensing principles such as piezoresistive, capacitive, electromagnetic, piezoelectric, optical, potentiometric, resonant, thermal, ionization etc. Piezoresistive pressure sensor due to their simple operating principle, are widely used in various applications [1-4].
[004] Piezoresistive pressure sensors use the piezoresistive effect of a material to detect pressure induced strain. Since the discovery of piezo resistance in silicon by C.S Smith in 1954 [5] and subsequent identification of piezoresistive effect in polysilicon in 1970s [6], silicon(mono-crystalline), and polysilicon owing to their much higher sensitivity to strain changes than metals [4] are considered as suitable piezoresistive materials for fabrication of pressure sensor. However, end application of such sensors is seriously affected due to their lower sensitivity due to operational pressure being required is in the range of kilopascals. Moreover, these sensors are extremely sensitive to the variations in the ambient temperature conditions. On the other side, carbon nanotubes (CNTs) have drawn much attention since their discovery in 1991[7]. The carbon nanotubes have remarkable mechanical, electrochemical, piezoresistive, temperature independence and other physical properties. CNTs are considered as popular choice for making pressure sensors for elevated temperature applications. Carbon nanotubes serves as an effective for fabrication of pressure sensing element due to its high gauge factor (200 to 1000), high sensitivity, and improved temperature independent response.
[005] Nanocomposite thin films are fabricated by adding CNTs to polymers like Poly(methyl methacrylate) (PMMA), Polydimethylsiloxane (PDMS), and polyimide. Although, it is easy to fabricate nanocomposite thin films, such films suffer from serious drawbacks of being highly temperature dependent for pressure sensing, and of reduced pressure sensitivity due to use of polymer matrix [8]. A ring shaped piezoresistive pressure sensor using carbon nanotube (CNT)-polyimide (PI) nanocomposite uses an easy fabrication process and showed good flexibility. However, the pressure sensor lacked the temperature independency due to the presence of polymer [9]. There are several similar reports in various publications [10-12]. However, there are no reports on fabrication of pressure sensing elements/ pressure sensor provided with a film obtained through layer-by-layer spray coating of suspended CNTs to improve sensitivity and thermal stability over a large temperature range.
[006] Thus, there exists a long-standing need for pressure sensing element/ pressure sensor with improved pressure sensitivity and wider working temperature range so as to demonstrate greater tolerance to temperature variations and enhanced functional attributes. Inventors of the present invention by using layer-by-layer spray coating technique have prepared a multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film), which is substantially free of polymers to overcome the drawbacks of the pressure sensing elements/ pressure sensors known in the art, particularly with respect pressure sensitivity and range of working temperature. Integration of such multi-layered MWCNT-ZnO film in free-standing form to a pressure sensing element is particularly useful in improving the pressure sensitivity by 20 - 30%. Use of said film also makes the pressure sensor element/ pressure sensor compatible to wider working temperature range and tolerant to temperature variations, as compared to pressure sensors element/ pressure sensor based on nanocomposite thin films.

SUMMARY OF THE INVENTION
[007] This summary is provided to introduce a selection of concepts in a simplified format that are further described in the detailed description of the invention.
[008] The present invention provides a pressure sensing element (1) with an improved thermal stability and pressure sensitivity, said pressure sensing element (1) comprising of: a diaphragm (2); and a trench electrode (3), wherein the diaphragm (2) is a free-standing film directly suspended above the trench (4) present in the trench electrode (3), wherein the electrodes (5 and 6) in trench electrode (3) are made of low work function and non-oxidizing material selected from a group consisting of Au, Ag, Pt, Pd, and wherein the free-standing film is a multi-layered film of a piezoelectric nanomaterials, wherein the piezoelectric nanomaterials is a piezoresistive fibrous material selected from the group consisting of zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film), which is substantially free of polymers.
[009] In one of the aspects, the present invention provides a pressure sensing element (1) with an improved thermal stability and pressure sensitivity, wherein the electrodes (5 and 6) in trench electrode (3) are made of Au.
[0010] In one of the aspects, the present invention provides a pressure sensing element (1) with an improved thermal stability and pressure sensitivity, wherein the trench (4) in the trench electrode (3) has a width of 5 to 30 µm and a depth of 5 to 30 µm.
[0011] In one of the aspects, the present invention provides a pressure sensing element (1) with an improved thermal stability and pressure sensitivity, wherein the trench (4) in the trench electrode (3) has a width of 10 µm and a depth of 10 µm.
[0012] In one of the aspects, the present invention provides a pressure sensing element (1) with an improved thermal stability and pressure sensitivity, wherein the trench electrode (3) is made of a material selected from the group consisting of silicon (Si), and silicon dioxide (SiO2).
[0013] In one of the aspects, the present invention provides a pressure sensing element (1) with an improved thermal stability and pressure sensitivity, wherein the trench electrode (3) is made of silicon dioxide (SiO2).
[0014] In one of the aspects, the present invention provides a pressure sensing element (1) with an improved thermal stability and pressure sensitivity, wherein the piezoelectric nanomaterials selected from the group consisting of as zinc oxide (ZnO) in MWCNT-ZnO film is present on the surface of the piezoresistive fibrous material selected from the group consisting of multiwalled carbon nanotubes (MWCNT) and provides a strain gradient induced polarization for self-powering of the pressure sensing element.
[0015] In one of the aspects, the present invention provides a pressure sensing element (1) with an improved thermal stability and pressure sensitivity, wherein the multi-layered MWCNT-ZnO film is made of 5-10 layers having a total thickness of 0.5 to 2.0 µm.
[0016] In one of the aspects, the present invention provides a pressure sensing element (1) with an improved thermal stability and pressure sensitivity, wherein the pressure sensing element (1) has a temperature stability (working temperature) in the range of -40 to 30℃.
[0017] In one of the aspects, the present invention provides a pressure sensing element (1) with an improved thermal stability and pressure sensitivity, wherein the pressure sensing element (1) has a 25 to 30% increased sensitivity as compared to pressure sensing element provided with diaphragm made of nanocomposite-based films.
[0018] In another aspect, the present invention provides a process for preparing a pressure sensing element comprising a diaphragm and a trench electrode, said process comprising steps of: (i) preparing a multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film); and (ii) positioning the multi-layered MWCNT-ZnO film as diaphragm in a free-standing, suspended form directly above the trench present in the prefabricated trench electrode.
[0019] In one of the aspects, the present invention provides a process for preparing a pressure sensing element comprising a diaphragm and a trench electrode, wherein the process for preparing a multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film) comprises of following steps: (a) preparing a zinc oxide functionalized multiwalled carbon nanotubes (MWCNT-ZnO) dispersion; (b) spray coating the MWCNT-ZnO dispersion obtained in step a) on a copper substrate to form a layer-by-layer multi-layered MWCNT-ZnO film resting on copper substrate; (c) dipping the multi-layered MWCNT-ZnO film resting on copper substrate as obtained in step b) into polycarbonate solution to coat the multi-layered MWCNT-ZnO film resting on copper substrate with a sacrificial polycarbonate layer, followed by curing by heating; (d) exuviating the polycarbonate layer from the surface of copper substrate of cured polycarbonate coated MWCNT-ZnO film resting on copper substrate as obtained in step c); (e) etching the copper substrate by dipping the polycarbonate coated MWCNT-ZnO film resting on copper substrate as obtained in step d) in a solution of ferric chloride and obtaining a multi-layered MWCNT-ZnO film with sacrificial polycarbonate layer on one side; and (f) removing the sacrificial polycarbonate layer by repetitive washing with chloroform and deionized water and obtaining a multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes, substantially free of polymers.
[0020] In one of the aspects, the present invention provides a process for preparing a pressure sensing element comprising a diaphragm and a trench electrode, wherein the copper substrate is at a temperature of 150 to 170℃, while being spray coated the MWCNT-ZnO dispersion to form a layer-by-layer multi-layered MWCNT-ZnO film resting on copper substrate.
[0021] In one of the aspects, the present invention provides a process for preparing a pressure sensing element comprising a diaphragm and a trench electrode, wherein the spray coating of the MWCNT-ZnO dispersion on copper substrate is carried out by injecting the MWCNT-ZnO dispersion at the rate of 200 µl/minute.
[0022] In one of the aspects, the present invention provides a process for preparing a pressure sensing element comprising a diaphragm and a trench electrode, wherein the dipping in a solution of ferric chloride for etching of copper substrate is carried out for a duration of 2 to 4 hours.
[0023] Still in another aspect, the present invention provides a process for preparing a pressure sensing element comprising a diaphragm and a trench electrode, wherein the zinc oxide functionalized multiwalled carbon nanotubes (MWCNT-ZnO) dispersion is prepared by a process comprising steps of: (a) dispersing multiwalled carbon nanotubes in N, N-dimethylmethanamide (DMF) by ultra-sonication at room temperature; (b) adding zinc acetate dehydrate to the solution obtained in step a), followed by ultra-sonication; (c) stirring the mixture obtained in step b) at room temperature, followed by heating; and (d) allowing the solution obtained in step c) to slowly cool down to room temperature.
[0024] In one of the aspects, the present invention provides a process for preparing a pressure sensing element comprising a diaphragm and a trench electrode, wherein the zinc oxide functionalized multiwalled carbon nanotubes (MWCNT-ZnO) dispersion is prepared by dispersing 35 to 45 mg, preferably 40 mg of the multiwalled carbon nanotubes in 100 ml of DMF and the ultrasonicating for 4 to 6 hours.
[0025] In one of the aspects, the present invention provides a process for preparing a pressure sensing element comprising a diaphragm and a trench electrode, wherein the zinc oxide functionalized multiwalled carbon nanotubes (MWCNT-ZnO) dispersion is prepared by adding 3 to 7 g, preferably 5 g of zinc acetate dihydrate to the dispersion of multiwalled carbon nanotubes in DMF, and ultrasonicating for 15 to 45 minutes.
[0026] In one of the aspects, the present invention provides a process for preparing a pressure sensing element comprising a diaphragm and a trench electrode, wherein the zinc oxide functionalized multiwalled carbon nanotubes (MWCNT-ZnO) dispersion is prepared by stirring the solution obtained in step c) for 30 to 100 minutes and heating it for 4 to 6 hours at 100 to 110℃.
[0027] In still another aspect, the present invention provides a pressure sensor (10) with improved thermal stability and pressure sensitivity, said sensor (10) comprising of: (a) a pressure sensing element (1) comprising a diaphragm and a trench electrode as described herein in present application; (b) a printed circuit board (20) provided with units selected from the group consisting of controlling unit, processing unit, amplifying unit, storage unit, and transmitting units and having at least one electrical contact with the sensor element (1); and (c) a sensor casing (30) with provisions for conveying pressure, ports and housing the pressure sensing element (1), printed circuit board (20), wherein said pressure sensor (10) is characterized by comprising a pressure sensing element (1) comprising a diaphragm and a trench electrode as described herein in present application.
[0028] In another aspect, the present invention provides a pressure sensor (10), wherein the pressure sensor detects the pressure by measuring a change in resistance of the multi-layered MWCNT-ZnO film provided in the pressure sensing element (1).
DESCRIPTION OF THE ACCOMPANYING DRAWINGS
[0029] Distinctive features, aspects and advantages of the present invention will become better understood, when the detailed description of the invention is read with reference to the appended drawings.
[0030] Figure 1: (a) Shows scanning electron images showing MWCNT-ZnO film; (b) Shows a sensor element with free-standing MWCNT-ZnO film suspended on the micro-fabricated trench between the Au trench electrodes; and (c) Schematic representation depicting broad scheme for fabrication of MWCNT-ZnO pressure sensing element.
[0031] Figure 2: Shows a different components of pressure sensor, (a) Shows wire bonding on printed circuit board; (b) Shows casing made up of aluminium/nylon packaging of the pressure sensor; (c) Shows electrical connectors for measurement; and (d) Shows detailed layout of pressure sensing element (1) with diaphragm (2), which is a free-standing multi-layered MWCNT-ZnO film directly suspended above the trench (4) in a trench electrode (3) made up of Si/SiO2 and electrodes (5) and (6).
[0032] Figure 3: COMSOL simulation was conducted to estimate the behavior of the thin MWCNT-ZnO film for the applied pressure. A scaled-down model of electrode with trench as the gap between two electrodes and a diaphragm of MWCNT-ZnO film between the trench (ends on the electrodes) was constructed. Gold was selected as the material for trench electrode and a user-defined MWCNT-ZnO material was selected e.g., density of 2090 kg/m3, relative permittivity 4.5, electrical conductivity of 6.30x107 S/m and thermal capacity of 2800W/(m-K). Solid-mechanics (solid) physics module was used for applying pressure on the film as boundary load. Stationary study with a parametric sweep of pressure from 1 to 10 bar was conducted. The resulting displacement of the MWCNT-ZnO film was measured using a boundary probe. Surface and probe plots were used to depict the results. (a) Shows the action site for pressure on free-standing multi-layered MWCNT-ZnO film suspended above the trench in the trench electrode (left) and the surface plot shows the displacement response against the pressure on free-standing multi-layered MWCNT-ZnO film suspended above the trench in the trench electrode using a solid mechanics study (right); and (b) The probe plot shows pressure v/s displacement plot for a varied pressure of 1-10 bar and the measured displacement linearly ranges in the scale of microns.
[0033] Figure 4: Shows a current-voltage response of the sensor for -1 to +1V sweep at atmospheric pressure.
[0034] Figure 5: Shows a representative sensor assembly for static and dynamic pressure measurements.
[0035] Figure 6: Shows a static response of the sensor at pressure of 2 and 4 bar.
[0036] Figure 7: Shows dynamic characterization in resistance variation for 5 cycles under the pressure varying from 1 bar to 6 bar.
[0037] Figure 8: Shows sensor sensitivity, (a) Shows sensor response to pressure in the range from 1 bar to 6 bar in terms of resistance; and (b) Shows a graph plotting a change in resistance with change in the pressure and identifying the sensitivity.
[0038] Figure 9: Shows thermal stability tests of the sensor for reversible heating conditions.
[0039] Figure 10: Shows self-powered characteristics of the pressure sensing element in accordance with an embodiment of the present invention (a) Shows measured voltage at different pressure over several cycles; and (b) Shows the magnitude of measured voltage at different pressure.
DETAILED DESCRIPTION OF THE INVENTION
[0040] Those skilled in the art will be aware that the present disclosure is subject to variations and modifications other than those specifically described herein. It is to be understood that the present disclosure includes all such variations and modifications. The disclosure also includes all such features of the invention, and steps of the process that are referred to or indicated in this specification, individually or collectively, and any and all combinations of any or more of such features or steps.
[0041] Definitions: For convenience, before further description of the present disclosure, certain terms employed in the specification and examples are collected here. These definitions should be read in the light of the remainder of the disclosure and understood as by a person skilled in the art. The terms used herein have the meanings recognized and known to those of skill in the art, however, for convenience and completeness, particular terms and their meanings are set forth below.
[0042] “Pressure sensing element” is functional component a pressure sensor, which participates in receiving the pressure as stimulus and comprising of a diaphragm and a trench electrode. “Diaphragm” is a free-standing multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film) positioned above the trench in the trench electrode and prepared by layer-by-layer spray coating. “Trench electrode” is an assembly of two electrodes made up low work function and non-oxidizing material and separated by a trench having a width and depth of 1-30 µm. “MWCNT-ZnO” is a multiwalled carbon nanotubes functionalized with zinc oxide.
[0043] The articles “a”, “an” and “the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article. The terms “comprise” and “comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as “consists of only”. Throughout this specification, unless the context requires otherwise, the word “comprise”, and variations such as “comprises” and “comprising”, will be understood to imply the inclusion of a stated element or step or group of elements or steps but not the exclusion of any other element or step or group of elements or steps.
[0044] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods, and materials are now described. All publications mentioned herein are incorporated herein by reference. The present disclosure is not to be limited in scope by the specific examples described herein, which are intended for the purposes of exemplification only. Functionally equivalent products and methods are clearly within the scope of the disclosure, as described herein.
[0045] The present invention provides a novel pressure sensing element (1) relying on the resistance change of a diaphragm (2) made up of a multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film). The inventors of present application in order to obtain a pressure sensing element with improved pressure sensitivity and enhanced working temperature range so as to tolerate greater temperature variations have developed the multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes, which is substantially free of polymers and then suspended directly above the trench (4) of a trench electrode (3). The pressure sensing element (1) described herein can be considered to be independent of temperature to an extent and thus is suitable for diverse applications. Inventors of present application have also developed a process for the fabrication of multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film), which is substantially free of polymers, wherein a piezoelectric nanomaterials zinc oxide (ZnO) nanoparticles functionalize a piezoresistive fibrous material selected from the group consisting of multiwalled carbon nanotubes (MWCNT), which is spray coated layer-by-layer on a copper substrate. The copper substrate act as base substrate for fabrication of multi-layered film of zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film) and is eventually etched off. The ZnO nanoparticles on the MWCNT surface provide strain gradient induced polarization due to the piezoelectric properties of the ZnO for self-powering of the pressure sensing element. This also helps in achieving excellent sensitivity towards pressure change with a significant enhancement in static and dynamic pressure response.
[0046] The functionalization of MWCNT with ZnO improved both the static and dynamic responses of the pressure sensing element (1) or pressure sensor (10) based upon said pressure sensing element. The pressure sensor (10) based upon the pressure sensing element (1) of present invention is found to demonstrate sensitivity of 2.3kPa-1 upon being evaluated by measuring a change in resistance for changes in pressure, while maintaining a constant bias voltage. Thus, pressure sensing element (1) or pressure sensors based upon said pressure sensing element (1) having enhanced working temperature range finds a wider utility across diverse applications.
[0047] The multi-layered MWCNT-ZnO film described herein with combined piezoresistive, and piezoelectric properties is particularly suitable for use in pressure sensing element or the pressure sensor based upon such pressure sensing element with self-powering capability. Further, a method for fabrication a free-standing multi-layered film of ZnO functionalized MWCNT (multi-layered MWCNT-ZnO film) with acceptable mechanical stability without using any polymer matrix is described herein below.
A. Functionalization of MWCNT using ZnO:
[0048] To have uniform growth of ZnO nanoparticles on MWCNT for proper functionalization first dispersion of MWCNT in N, N-dimethylmethanamide (DMF) is prepared by dispersing 35-45 mg, preferably 40 mg of MWCNT in 100 ml of DMF, followed by ultra-sonication for 4-6 hours at a constant power preferably for 5 hours at room temperature. To said first dispersion of MWCNT is added 3-7 g, preferably 5 g of zinc acetate dihydrate, followed by ultra-sonication for 15-45 minutes at a constant power, preferably for 30 minutes for proper dispersal. The mixture so obtained is then vigorously stirred for 30-100 minutes, preferably for 60 minutes at room temperature and then heated to a temperature of 100-110℃, preferably to temperature of 105℃ for 4-6 hours, preferably 5 hours. Thereafter, the suspension of MWCNT-ZnO so obtained is allowed to slowly cool down to the room temperature.
B. Fabrication of multi-layered MWCNT-ZnO film:
[0049] Suspended diaphragm or free-standing multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes is fabricated using spray coating the ZnO functionalized MWCNT suspension as obtained in preceding section on a clean copper (Cu) substrate heated to a temperature of 150-170℃, preferably to a temperature of 160℃. The thickness of the MWCNT-ZnO film is controlled by regulating by injection of MWCNT-ZnO dispersion at the rate of 180-220µl/minute, preferably at the rate of 200µl/minute layer-by-layer coating. The multi-layered MWCNT-ZnO film is made up of 5-10 layers having a total thickness of 0.5 – 2.0 µm. After obtaining multi-layered MWCNT-ZnO film on copper substrate, it is dipped into polycarbonate (PC) solution prepared by dissolving bisphenol A (BPA) in suitable solvent selected from the group consisting of cresol, dichloromethane, dimethyl formamide and chloroform. The multi-layered MWCNT-ZnO film on copper substrate with a sacrificial coating of polycarbonate layer on both sides is then heated to a temperature of 80-100℃ for polycarbonate (PC) curing. Thereafter, the polycarbonate layer present on the surface of copper substrate of cured polycarbonate coated multi-layered MWCNT-ZnO film is exuviated to expose the copper substrate. Thereafter, polycarbonate coated multi-layered MWCNT-ZnO film with exposed copper substrate is dipped in freshly prepared ferric chloride etch solution for 2-4 hours, preferably for ~3 hours to etch away the copper substrate and thereby giving us free-standing multi-layered film of MWCNT-ZnO with polycarbonate coating on one side as sacrificial layer. The sacrificial polycarbonate layer is then removed by repetitive washing for 5-10 times with chloroform and deionized water to obtain a multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film).
[0050] The multi-layered MWCNT-ZnO film as fabricated in preceding section is eventually positioned as free-standing film directly suspended above the trench of a suitable trench electrode as described herein below to form pressure sensing element.
Fabrication of pressure sensing element comprising of multi-layered MWCNT-ZnO film:
[0051] To fabricate pressure sensing element (1), the multi-layered MWCNT-ZnO film as described in preceding section is then suspended directly above a trench (4) of a suitable trench electrode (3). Trench (4) in trench electrode (3) is having a width and depth of 5-30 µm, preferably of ~10 µm, which separate two electrodes (5 and 6) in said trench electrode (3). Electrodes (5 and 6) of trench electrode (4) are made up of low work function and non-oxidizing material selected from the group consisting of Au, Ag, Pt, Pd, and preferably of Au and are positioned on base of trench electrode made up of material selected from the group consisting of silicon (Si), and silicon dioxide (SiO2), and preferably of silicon dioxide (SiO2). The pressure sensing element (1) comprising the multi-layered MWCNT-ZnO film as described herein is having a temperature stability in the range of -40 – 30℃. Figure 1(c) provides a broad schematic for fabrication of pressure sensing element (1) comprising of multi-layered MWCNT-ZnO film.
Fabrication/Assembly of pressure sensor comprising the pressure sensing element comprising of multi-layered MWCNT-ZnO film:
[0052] Different sensors to meet different requirements of diverse application can be fabricated using the pressure sensing element (1) comprising of multi-layered MWCNT-ZnO film as fabricated in preceding section. Integral packaging plays a significant role in the performance of sensors in particular environment. Therefore, it is necessary to have an integration of required modules to meet specific requirements of end application and pack them together in a good packaging to eliminate errors while pressure sensing. Firstly, the pressure sensing element (1) is positioned on a printed circuit board (PCB) (20) provided with units selected from the group consisting of controlling unit, processing unit, amplifying unit, storage unit, and transmitting units. PCB (20) is having at least one electrical contact with the positioned sensor element (1). Figure 2 shows a general arrangement of pressure sensing element (1), PCB (20), ports, and casing (30) for packing/housing of a typical sensor (10). Electrical contacts are made using wire bonding and soldering. A sensor casing (30) with provisions for conveying pressure, attaching of ports, and housing the pressure sensing element (1), printed circuit board (20) is used to pack pressure sensing element (1) along with other parts in a manner so as to match specific end application. Sensor casing (30) is made up of any material to meet the requirement of senor application and can be of materials selected from the group consisting of aluminum and nylon or any other suitable material as per environmental requirement of sensor. In one of the embodiment sensor casing (30) is provided with a 2 mm inlet aperture for communication of pressure stimulus onto the pressure sensing element (1) and two apertures for communication of electrical connectors in the bottom side of casing as shown in Figure 2.
EXAMPLES:
[0053] The following examples are set forth below to illustrate the pressure sensing element of the present invention and results according to the disclosed subject matter. These examples are not intended to be inclusive of all aspects of the subject matter disclosed herein, but rather to illustrate representative product and results. These examples are not intended to exclude equivalents and variations of the present invention, which are apparent to one skilled in the art.
Example 1
[0054] Finite element analysis: A finite element analysis is performed using a Multiphysics simulation software (COMSOL Multiphysics) to depict the principle of operation of the pressure sensor (10) based upon pressure sensing element (1) with free-standing multi-layered MWCNT-ZnO film. Solid mechanics and electric current study are conducted to investigate the response of the free-standing multi-layered MWCNT-ZnO film diaphragm (2) on application of pressure. Figure 3(a) (left frame) depicts the displacement of diaphragm (2) with respect to applied pressure and stimulus receiving site for pressure on free-standing multi-layered MWCNT-ZnO film suspended above the trench (4) in the trench electrode (3). Figure 3 (a) (right frame) shows the displacement response shown against the applied pressure on free-standing multi-layered MWCNT-ZnO film suspended above the trench (4) in the trench electrode (3) in a solid mechanics study. The pressure is set at 10 bar and displacement is shown with color coding. For example, red color shows 0.14 X103 µm displacement, orange shows 0.12 X103 µm displacement, yellow color shows 0.1 X103 µm displacement, green color shows 0.08 X103 µm displacement, turquoise color shows 0.06 X103 µm displacement, dark blue color shows 0.02 X103 µm displacement. Figure 3(b) shows a linear response for pressure v/s displacement plot, wherein Y-axis shows total displacement in multiples of X 103 µm as obtained upon by applying corresponding pressure (in bar) as given on X-axis.
Example 2
[0055] Resistance analysis: Measurement of resistance of the pressure sensor based upon pressure sensing element (1) with free-standing multi-layered MWCNT-ZnO film is performed by using Keithley source-meter at atmospheric pressure and applying -1V to +1V voltage sweep. The results as shown in Figure 4 demonstrate a linear response, which shows that said pressure sensor is purely resistive with a resistance of ~19.3Ω.
Example 3
[0056] Static and dynamic pressure characterizations: The pressure sensor based upon pressure sensing element (1) with free-standing multi-layered MWCNT-ZnO film is evaluated for its static and dynamic characterizations by using a 6*14 mm 2 pneumatic connector for the ease of connecting it to the pressure line. The assembly was made airtight using superglue, as shown in Figure 5.
[0057] Static pressure characterization: Static characterization of the pressure sensor based upon pressure sensing element (1) with free-standing multi-layered MWCNT-ZnO film is conducted under 0.5V bias at room temperature for 2 bar and 4 bar static pressures. The test data as shown in Figure 6 demonstrate a stable response for period of around 120 seconds with an initial decay, which could be related to the leakage of gas.
[0058] Dynamic pressure characterization: Dynamic characterization of the pressure sensor based upon pressure sensing element (1) with free-standing multi-layered MWCNT-ZnO film is conducted at pressure of 1-6 bar with 0.5 V bias. For each set of experiment with predefined pressure, predefined pressure was held for 10 seconds and repeated for 4 to 5 cycles, the response of the sensor was measured using Keithley source-meter. Figure 7 shows variations in resistance in 5 cycles upon applying pressure of 1, 2, 3, 4, 5, and 6 bar in the dynamic characterization. In graphs Y-axis depicts resistance (in ohms) and X-axis depicts time in seconds and interestingly, a high stability for the change in resistance was obtained in response to the dynamic pressure.
Example 4
[0059] Sensitivity analysis: Response of the pressure sensor based upon pressure sensing element (1) with free-standing multi-layered MWCNT-ZnO film in terms of resistance for different pressure applied in the range of 1-6 bar is evaluated and shown in Figure 8(a). Y-axis represents resistance (in ohms), and X-axis represents time in seconds. Peaks in graph are represented by six colors and each color resistance in response to a specific pressure. Results shows that there is an increase in the resistance with an increase in the pressure from 1-6 bar. Figure 8(b) shows the sensitivity plot for the pressure sensor based upon pressure sensing element (1) with free-standing multi-layered MWCNT-ZnO film by plotting a change in resistance along with a change in the pressure. Change in pressure is depicted on X-axis, while change in resistance denoted by ∆R/R0 (%) is presented on Y-axis in said graph. The resistance change for an increase of pressure by 1 bar is measured and the sensitivity is calculated, which is found to be 2.3 kpa-1(0.023Bar-1), which is better than the sensitivity of known pressure sensors.
[0060] The table below shows comparison of the sensitivity of sensor described in present application with the other reported work. Comparison of sensitivity shows that pressure sensor based upon pressure sensing element with free-standing multi-layered MWCNT-ZnO film showed much greater sensitivity of 2.3 kpa-1, which is substantially improved.
Diaphragm material Sensitivity Limit of detection Reference
Si (single crystal) 8.28x10-8 kPa-1 400 kPa [13]
Si (poly crystalline) 9.03x10-8 kPa-1 [13]
GaAs 1.4x10-8 Pa-1 100 kPa [14]
PDMS/Nanotube 0.23 kPa-1 50 kPa [15]
PDMS/SWNTs 1.8 kPa-1 0.6 Pa [16]
MWCNT-ZnO 2.3 kPa-1 >600 kPa Present work
Table 1. Sensitivity comparison
Example 5
[0061] Thermal stability analysis: Thermal stability of the pressure sensor based upon pressure sensing element (1) with free-standing multi-layered MWCNT-ZnO film is evaluated by measuring change in resistance in response to reversible heating conditions of different temperature. Results obtained at a temperature range 230 – 300 K (-43.15℃ - 26.85℃) as shown in figure 9 shows that the resistance does not change with time and that there is insignificant change in resistance measured at different temperature. Since polymers are not used to make the diaphragm of free-standing multi-layered MWCNT-ZnO film, the sensing element (1) have an improved temperature independent response, which is almost temperature independent and thus have an enhanced working temperature range.
Example 6
[0062] Analysis of self-powered characteristics: The use of ZnO for functionalizing the MWCNTs surface provided a strain gradient induced polarization due to piezoelectric properties of ZnO that induces the self-powering property, and the sensor can be used without external power requirement. Figure 10(a) shows measured voltage at different pressure over several cycles for time duration from 0 to 200 seconds at different pressure conditions. Figure 10(b) shows the response of the sensor in terms of voltage at different pressure Figure 10(b) shows that a voltage of about 16.2 µV is obtained at the pressure of 2 bar, while voltage of about 14.3 µV is obtained at the pressure of 6 bar. It can be inferred from figure 10(b) that the induced voltage decreases with increase in pressure, which can be explained by the increase in resistance with increase in the pressure.
[0063] The pressure sensor based upon pressure sensing element with free-standing multi-layered MWCNT-ZnO film described herein have demonstrated improved static and dynamic responses. Such sensors also demonstrated a high sensitivity of around 2.3 kPa-1, which is approximately 25 – 30% higher than those observed for the sensors known in the art. The pressure sensors based upon pressure sensing element with free-standing multi-layered MWCNT-ZnO film are promising for developing temperature independent pressure sensors capable of in broad range of working temperature.
[0064] Advantages of the pressure sensing element/pressure sensor based upon pressure sensing element as described in present application:
• the pressure sensing element can be microfabricated to develop small scale system, which can be used for the micro arrays applications.
• the pressure sensing element is self-supported without using polymer.
• the pressure sensing element is capable for handling a wider pressure range and have an improved sensitivity by 25-30% as compared to nanocomposite film-based pressure sensing element.
• the pressure sensing element is thermally stable over a wide range of thermal variations.
• the pressure sensing element has a self-powered operational capability.
• the pressure sensing element is chemically inert and thus can be exploited in diverse type of environments.
[0065] References:
[1] K. S. Karimov, M. Saleem, Z. M. Karieva, A. Khan, T. A. Qasuria, and A. Mateen, “A carbon nanotube-based pressure sensor,” Phys. Scr., vol. 83, no. 6, p. 065703, Jun. 2011, doi: 10.1088/0031-8949/83/06/065703.
[2] R. Tripathi, S. N. Majji, R. Ghosh, S. Nandi, B. D. Boruah, and A. Misra, “Capacitive behaviour of carbon nanotube thin film induced by deformed ZnO microspheres,” Nanotechnology, vol. 28, no. 39, p. 395101, Sep. 2017, doi: 10.1088/1361-6528/aa7df7.
[3] S. Nandi and A. Misra, “Spray Coating of Two-Dimensional Suspended Film of Vanadium Oxide-Coated Carbon Nanotubes for Fabrication of a Large Volume Infrared Bolometer,” ACS Appl. Mater. Interfaces, vol. 12, no. 1, pp. 1315–1321, Jan. 2020, doi: 10.1021/acsami.9b16608.
[4] Ajayakumar C Katageri, and B G Sheeparamatti, “Carbon Nanotube based Piezoresistive Pressure Sensor for Wide Range Pressure Sensing Applications - A Review,” IJERT, vol. V4, no. 08, Aug. 2015, doi: 10.17577/IJERTV4IS080660.
[5] C. S. Smith, “Piezoresistance Effect in Germanium and Silicon,” Phys. Rev., vol. 94, no. 1, pp. 42–49, Apr. 1954, doi: 10.1103/PhysRev.94.42.
[6] Y. Onuma and K. Sekiya, “Piezoresistive Properties of Polycrystalline Silicon Thin Film,” Japanese Journal of Applied Physics, vol. 11, p. 20, Jan. 1972, doi: 10.1143/JJAP.11.20.
[7] S. Iijima and T. Ichihashi, “Single-shell carbon nanotubes of 1-nm diameter,” Nature, vol. 363, no. 6430, 1993, doi: 10.1038/363603a0.
[8] Q. Zhou and A. Zettl, “Electrostatic graphene loudspeaker,” Appl. Phys. Lett., vol. 102, no. 22, p. 223109, Jun. 2013, doi: 10.1063/1.4806974.
[9] Q. Li, S. Luo, and Q.-M. Wang, “Piezoresistive thin film pressure sensor based on carbon nanotube-polyimide nanocomposites,” Sensors and Actuators A: Physical, vol. 295, pp. 336–342, Aug. 2019, doi: 10.1016/j.sna.2019.06.017.
[10] D. Lee, H. P. Hong, C. J. Lee, C. W. Park, and N. K. Min, “Microfabrication and characterization of spray-coated single-wall carbon nanotube film strain gauges,” Nanotechnology, vol. 22, no. 45, p. 455301, Nov. 2011, doi: 10.1088/0957-4484/22/45/455301.
[11] N. Yogeswaran, S. Tinku, S. Khan, L. Lorenzelli, V. Vinciguerra, and R. Dahiya, “Stretchable resistive pressure sensor based on CNT-PDMS nanocomposites,” in 2015 11th Conference on Ph.D. Research in Microelectronics and Electronics (PRIME), pp. 326–329, Jun. 2015, doi: 10.1109/PRIME.2015.7251401.
[12] “Paper/Carbon Nanotube-Based Wearable Pressure Sensor for Physiological Signal Acquisition and Soft Robotic Skin,” ACS Applied Materials & Interfaces, https://pubs.acs.org/doi/abs/10.1021/acsami.7b10820.
[13] K. Singh, R. Joyce, S. Varghese, and J. Akhtar, “Fabrication of electron beam physical vapor deposited polysilicon piezoresistive MEMS pressure sensor,” Sensors and Actuators A: Physical, vol. 223, pp. 151–158, Mar. 2015, doi: 10.1016/j.sna.2014.12.033.
[14] A. Dehe, K. Fricke, K. Mutamba, and H. L. Hartnagel, “A piezoresistive GaAs pressure sensor with GaAs/AlGaAs membrane technology,” J. Micromech. Microeng., vol. 5, no. 2, pp. 139–142, Jun. 1995, doi: 10.1088/0960-1317/5/2/021.
[15] D. J. Lipomi et al., “Skin-like pressure and strain sensors based on transparent elastic films of carbon nanotubes,” Nature Nanotech, vol. 6, no. 12, pp. 788–792, Dec. 2011, doi: 10.1038/nnano.2011.184.
[16] Xuewen Wang, Yang Gu, Zuoping Xiong, Zheng Cui, and Ting Zhang, “Silk‐Molded Flexible Ultrasensitive and Highly Stable Electronic Skin for Monitoring,” Adv. Mater., vol. 26, no. 9, pp. 1336–1342, Mar. 2014. , Claims:1. A pressure sensing element (1) with an improved thermal stability and pressure sensitivity, said pressure sensing element (1) comprising of: a diaphragm (2); and a trench electrode (3),
wherein the diaphragm (2) is a free-standing film directly suspended above the trench (4) present in the trench electrode (3),
wherein the electrodes (5 and 6) in trench electrode (3) are made of low work function and non-oxidizing material selected from a group consisting of Au, Ag, Pt, Pd, and
wherein the free-standing film is a multi-layered film of a piezoelectric nanomaterials, wherein the piezoelectric nanomaterials is a piezoresistive fibrous material selected from the group consisting of zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film), which is substantially free of polymers.
2. The pressure sensing element (1) as claimed in claim 1, wherein the electrodes (5 and 6) in trench electrode (3) are made of Au.
3. The pressure sensing element (1) as claimed in claim 1, wherein the trench (4) in the trench electrode (3) has a width of 5 to 30 µm and a depth of 5 to 30 µm.
4. The pressure sensing element (1) as claimed in claim 1, wherein the trench (4) in the trench electrode (3) has a width of 10 µm and a depth of 10 µm.
5. The pressure sensing element (1) as claimed in claim 1, wherein the trench electrode (3) is made of a material selected from the group consisting of silicon (Si), and silicon dioxide (SiO2).
6. The pressure sensing element (1) as claimed in claim 1, wherein the trench electrode (3) is made of silicon dioxide (SiO2).
7. The pressure sensing element (1) as claimed in claim 1, wherein the piezoelectric nanomaterials is zinc oxide (ZnO) in MWCNT-ZnO film is present on the surface of the piezoresistive fibrous material selected from multiwalled carbon nanotubes (MWCNT) and provides a strain gradient induced polarization for self-powering of the pressure sensing element.
8. The pressure sensing element (1) as claimed in claim 1, wherein the multi-layered MWCNT-ZnO film is made of 5-10 layers having a total thickness of 0.5 to 2.0 µm.
9. The pressure sensing element (1) as claimed in claim 1, wherein the pressure sensing element (1) has a temperature stability (working temperature) in the range of -40 to 30℃.
10. The pressure sensing element (1) as claimed in claim 1, wherein the pressure sensing element (1) has a 25 to 30% increased sensitivity as compared to pressure sensing element provided with diaphragm made of nanocomposite-based films.
11. A process for preparing a pressure sensing element comprising a diaphragm and a trench electrode, said process comprising steps of:
i. preparing a multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film); and
ii. positioning the multi-layered MWCNT-ZnO film as diaphragm in a free-standing, suspended form directly above the trench present in the prefabricated trench electrode.
12. The process as claimed in claim 11, wherein the process for preparing a multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes (multi-layered MWCNT-ZnO film) comprises of following steps:
a) preparing a zinc oxide functionalized multiwalled carbon nanotubes (MWCNT-ZnO) dispersion;
b) spray coating the MWCNT-ZnO dispersion obtained in step a) on a copper substrate to form a layer-by-layer multi-layered MWCNT-ZnO film resting on copper substrate;
c) dipping the multi-layered MWCNT-ZnO film resting on copper substrate as obtained in step b) into polycarbonate solution to coat the multi-layered MWCNT-ZnO film resting on copper substrate with a sacrificial polycarbonate layer, followed by curing by heating;
d) exuviating the polycarbonate layer from the surface of copper substrate of cured polycarbonate coated MWCNT-ZnO film resting on copper substrate as obtained in step c);
e) etching the copper substrate by dipping the polycarbonate coated MWCNT-ZnO film resting on copper substrate as obtained in step d) in a solution of ferric chloride and obtaining a multi-layered MWCNT-ZnO film with sacrificial polycarbonate layer on one side; and
f) removing the sacrificial polycarbonate layer by repetitive washing with chloroform and deionized water and obtaining a multi-layered film of a zinc oxide functionalized multiwalled carbon nanotubes, substantially free of polymers.
13. The process as claimed in claim 12, wherein the copper substrate in step b) is at a temperature of 150 to 170℃.
14. The process as claimed in claim 12, wherein the spray coating of the MWCNT-ZnO dispersion in step b) is carried out by injecting the MWCNT-ZnO dispersion at the rate of 200 µl/minute.
15. The process as claimed in claim 12, wherein the dipping in a solution of ferric chloride in step e) is carried out for a duration of 2 to 4 hours.
16. The process as claimed in claim 12, wherein the zinc oxide functionalized multiwalled carbon nanotubes (MWCNT-ZnO) dispersion is prepared by a process comprising steps of:
a) dispersing multiwalled carbon nanotubes in N, N-dimethylmethanamide (DMF) by ultra-sonication at room temperature;
b) adding zinc acetate dehydrate to the solution obtained in step a), followed by ultra-sonication;
c) stirring the mixture obtained in step b) at room temperature, followed by heating; and
d) allowing the solution obtained in step c) to slowly cool down to room temperature.
17. The process as claimed in claim 16, wherein in step a) 35 to 45 mg, preferably 40 mg of the multiwalled carbon nanotubes are dispersed in 100 ml of DMF and the ultrasonication is carried out for 4 to 6 hours.
18. The process as claimed in claim 16, wherein the zinc acetate dihydrate added in step b) is 3 to 7 g, preferably 5 g, and wherein the ultrasonication in step b) is carried out for 15 to 45 minutes.
19. The process as claimed in claim 16, wherein the stirring in step c) is carried out for 30 to 100 minutes, and wherein the heating in step c) is carried out for 4 to 6 hours at 100 to 110℃.
20. A pressure sensor (10) with improved thermal stability and pressure sensitivity, said sensor (10) comprising of:
a) a pressure sensing element (1) as defined in claims 1-10;
b) a printed circuit board (20) provided with units selected from the group consisting of controlling unit, processing unit, amplifying unit, storage unit, and transmitting units and having at least one electrical contact with the sensor element (1); and
c) a sensor casing (30) with provisions for conveying pressure, ports and housing the pressure sensing element (1), printed circuit board (20),
wherein said pressure sensor (10) is characterized by comprising a pressure sensing element (1) as defined in claims 1-10.
21. The pressure sensor (10) as claimed in claim 20, wherein the pressure sensor detects the pressure by measuring a change in resistance of the multi-layered MWCNT-ZnO film provided in the pressure sensing element (1).