PRINTED HYGROSCOPIC ELECTRODES FOR LOW-COST CAPACITIVE RELATIVE HUMIDITY SENSORS
1. A method of forming a capacitive relative humidity sensor comprising:
- optionally pre-treating a water absorptive dielectric layer;
depositing a first electrode on at least a portion of a surface of the water absorptive dielectric layer; and
depositing a second electrode on at least a portion of another surface of the water absorptive dielectric layer, wherein at least one of the first electrode and the second electrode is comprised of a water permeable material.
A capacitive relative humidity sensor includes a first electrode and a second electrode, where at least the one of the electrodes is a water-permeable electrode. The electrodes act in conjunction with a water-sensitive hygroscopic dielectric material. The design enables low-cost and simple fabrication of fast-responding and stable relative humidity sensors using digital fabrication techniques.
- 1. A method of forming a capacitive relative humidity sensor comprising:
optionally pre-treating a water absorptive dielectric layer; depositing a first electrode on at least a portion of a surface of the water absorptive dielectric layer; and depositing a second electrode on at least a portion of another surface of the water absorptive dielectric layer, wherein at least one of the first electrode and the second electrode is comprised of a water permeable material.
- View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
- 10. A method of forming a capacitive relative humidity sensor comprising:
depositing a first electrode on at least a portion of a substrate; depositing a water absorptive dielectric layer on at least a portion of the first electrode; optionally pre-treating the water absorptive dielectric layer; and depositing a second electrode on at least a portion of the water absorptive dielectric layer, wherein the second electrode layer is made of a water permeable material.
- View Dependent Claims (11, 12, 13, 14)
- 15. A capacitive relative humidity sensor, comprising:
a water absorptive dielectric layer having a first surface and a second surface; a first electrode positioned in contact with at least a portion of the first surface of the water absorptive dielectric layer; and a second water-permeable electrode positioned in contact with at least a portion of the second surface of the of the water absorptive dielectric layer.
- View Dependent Claims (16, 17, 18, 19, 20, 21, 22, 23)
This invention was made with government support under contract NL0032095 awarded by the Department of Energy, and through contract 4000152637 awarded by Oak Ridge National Laboratory. The government has certain rights in this invention.
Existing commercial relative humidity (RH) sensors are commonly configured as packaged electronic modules having an accuracy of ±3%. Individually, the packaged modules cost several dollars or more, incorporate a printed circuit board for mounting, and are not planar or flexible. Low-cost alternatives of these modules include interdigitated electrode capacitor type designs and porous parallel-plate electrode arrangements. Such existing devices are manufactured with lithographically patterned or screen printed top electrodes, among other complex manufacturing processes. The present disclosure teaches improved manufacturing processes and device designs that result in cost effective and highly accurate, responsive, planar, and flexible RH sensors.
The capacitive relative humidity (RH) sensor includes a first electrode and a second electrode, where at least one of the electrodes is a water-permeable or water vapor-permeable electrode. The electrodes act in conjunction with a water-sensitive hygroscopic dielectric material. The design enables low-cost and simple fabrication of fast-responding and stable relative humidity sensors using printing fabrication techniques.
Relative humidity (RH) sensors designed in a capacitive (e.g., parallel plate) type arrangement, such as discussed in association with the present disclosure, include a water absorptive dielectric material capable of absorbing fluid such as water from the air (e.g., in the form of moisture, vapor, etc.). One example of such a water absorptive dielectric is polyimide, although other water adsorptive dielectrics are available. The dielectric constant and/or geometry of such water absorptive dielectric material is a function of humidity (i.e., the amount of water the dielectric takes-up is a function of humidity, and so changes with humidity). Therefore, when designing RH sensors in a capacitor type design, this form of water absorptive dielectric will cause the capacitance of the RH sensor to be a function of humidity. To further explain, it is noted that one mechanism for the capacitance to be a function of humidity is that a geometric parameter (e.g., thickness) of the water absorptive dielectric layer changes with humidity, causing changes in capacitance between a first electrode and a second electrode. Another mechanism is that the relative permittivity of the dielectric changes with absorption of water.
A useable relative humidity (RH) sensor in a capacitor (e.g., parallel plate) design needs to be designed to allow the water (e.g., moisture or vapor) to reach the water absorptive dielectric in an appropriate amount of time. For example, if the electrodes in the capacitive structure of a relative humidity (RH) sensor were constructed as solid metal electrodes on both sides of the dielectric, water would need to diffuse to the water absorptive dielectric from the sides of the electrodes. Such a configuration would result in too long of a period for the absorption by the dielectric, resulting in an undesirable response time for the RH sensor.
A proposed solution in the art to this response time issue is to use an inter-digitated electrode geometry instead of a plate geometry on at least one side of a water absorptive dielectric of a relative humidity (RH) sensor structure. This allows the water (moisture, vapor) to reach the water absorptive dielectric through open portions of the inter-digitated design. It is also understood that employing this design with low-cost manufacturing techniques (e.g., inkjet printing, aerosol printing, screen printing, etc.) yields sensors with lower capacitance and sensitivity for a given footprint. Having a lower capacitance is disadvantageous as it reduces the ability to discriminate the sensor signal from parasitic capacitance in the system. Higher capacitances and sensitivities can be obtained by manufacturing interdigitated electrodes with smaller feature sizes using techniques like photolithography, although this dramatically increases the manufacturing cost.
Another potential structure that has been discussed is the use of an electrode which employs a carbon black material. It is intended that the specific carbon black material have sufficient porosity to allow the water to move through the electrode (i.e., made of the carbon black) to the water absorptive dielectric. An issue with using carbon black is that the processes to manufacture such carbon black electrodes require operations such as screen printing or lithographic patterning that are not compatible with digital design and fabrication, as the particulate nature of carbon black makes effective inkjet and/or aerosol printing challenging. Furthermore, the conductivity of carbon black is several orders of magnitude lower than metallic electrodes. This dramatically increases the RC time-constant of the sensor and limits the sensor response time and range of measurement frequencies that can be employed.
Another existing alternative is manufacturing a top electrode as a grid of silver lines, where the water diffuses to the water absorptive dielectric through the open areas in the grid. However, an issue with this design is there will be less capacitance due to the open areas within the grid. Additionally, as noted above, having a lower capacitance makes distinguishing the sensor signal from parasitic capacitance in the system more challenging, which increases the complexity and cost of the sensor read-out electronics.
The present disclosure offers an alternative to the costly, difficult to manufacture, and bulky state-of-the-art RH sensors described above. Removing the need to perform lithographic patterning, screen printing, and other non-digital processes makes the present design and processes an attractive alternative to sensors found in the existing art.
Additionally, a parallel plate capacitive RH sensor device designed according to the following disclosure makes the output capacitance significantly higher than devices with interdigitated electrodes, grid electrodes, etc. by providing more surface area for capacitance and acts to simplify the fabrication process compared thereto.
In addition to the use of a water absorptive dielectric, the capacitive RH sensor according to one embodiment of the present disclosure uses a conductive material having water or water-vapor permeable properties to form at least a first top electrode conductor plate. Examples of such materials include, but are not limited to, poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), polyaniline, etc. In some embodiments a second bottom electrode is formed from a solid conductive material (e.g., metal, aluminum, foil, conductive polymer, etc.), while in other embodiments the second bottom electrode is formed from a water or water-vapor permeable conductive material, as described above. As a result of water transmission through the permeable top-electrode material (e.g. PEDOT:PSS), the top electrode does not need to be engineered to be a porous or open structure (e.g., grids, interdigitated), thus increasing the sensor'"'"'s capacitance and sensitivity while decreasing manufacturing complexity. Furthermore, the compatibility of the water/water-vapor permeable materials with digital printing processes (e.g. inkjet printing, aerosol printing, etc.) enables the digital design and fabrication of RH sensors, which allows for rapid prototyping and design flexibility.
The water absorptive dielectric layer 102 can optionally be printed or coated on a supporting substrate (as shown in
An electrode layer 108 is then deposited on the first surface 104 of the water absorptive dielectric layer 102. Thereafter in a next processing step another electrode layer 110 is deposited on the second surface 106 of the water absorptive dielectric layer 102. The deposition steps can be a printing method (e.g., inkjet, aerosol, screen, gravure, flexographic, etc. printing) or a coating method (e.g., sputtering, slot-die coating, blade coating, spray coating, dip coating, etc. . . . ) or a combination of these methods. In this construction at least one of the electrodes 108, 110 is to be considered a top electrode and is comprised of a water or water-vapor permeable material which may be, but is not limited to, PEDOT:PSS.
When the water permeable material (e.g., PEDOT:PSS) is used for only a single electrode (e.g., the top electrode conductor), the other electrode material may be a metal conductor, aluminum, foil, conductive polymer, etc.
It is understood that the process shown in
The thickness of each of the elements (electrodes, dielectric) are sufficiently thin to allow the overall RH sensor 200 of
RH sensors that employ water permeable electrode(s) 108 and/or 110 made in accordance with the teachings of
Thus, the forgoing discloses the use of a water-permeable conductor (e.g., PEDOT:PSS) which is printed or otherwise coated to act as a capacitive humidity sensor in conjunction with a water absorptive dielectric material (e.g., polyimide) to enable low-cost and digital fabrication of fast-responding and stable humidity (RH) sensors using printing technology.
Thereafter, in step 330, the absorptive dielectric substrate 306 is optionally pre-treated and a conductive water-permeable top electrode, (e.g., PEDOT:PSS) 332, is printed or coated over the non-water absorptive dielectric 322 and at least portions of the water absorptive dielectric 306 (e.g., polyimide). The pre-treatment operations may be, but are not limited to, UV-ozone treatment, plasma treatment, chemical etching, and combinations of these processes.
With regard to the optionally provided non-water absorptive dielectric 322, in some instances this structure may be useful to separate portions of the water permeable electrode layer and the water absorptive dielectric material. For example, this arrangement can be used to limit parasitic capacitance from the sensor leads. It is to be appreciated that this optional arrangement may also be used in the structure of
The dielectric constant of the polyimide changes with humidity since the dielectric is hygroscopic, thus changing the capacitance measured between the first and second electrodes. In alternate embodiments, the water absorptive dielectric can change thickness instead of or in addition to changing dielectric constant as water is adsorbed. Such a dielectric would also exhibit a capacitance that is a function of RH.
In the present disclosure therefore, the electrode plates are easily/commercially printable or coatable (e.g., inkjet printing, aerosol printing, screen printing, gravure printing, flexographic printing, sputtering, slot-die coating, blade coating, spray coating, dip coating, etc.). For example, the top electrode for such a structure may be the PEDOT:PSS formed by inkjet printing or aerosol printing methods. In certain embodiments, the bottom electrode may be a printed solid metal (or other appropriate conductive material) or alternatively, could also be a printed water permeable material such as, but not limited to, PEDOT:PSS. Printed hygroscopic moisture-sensitive dielectrics (e.g., water absorptive material) are thus used herein in one embodiment in conjunction with a PEDOT:PSS conductive electrode. Alternative electrode structures include 1) printing PEDOT:PSS on both sides of the substrate instead of using other conductive material (e.g., metals, aluminum, conductive polymers, etc.) and 2) metal grid lines deposited on the PEDOT:PSS in order to minimize resistivity.
A particular aspect of the foregoing concepts disclosed herein is that a water permeable top electrode can completely cover the top of the RH sensor device, allowing the RH sensor device to have a significant and larger capacitive area resulting in higher capacitance and sensitivity of the structure, but still allowing the structure to be manufactured by digital manufacturing techniques such as inkjet and/or aerosol printing, along with other printing and coating processes. Furthermore, the high conductivity of PEDOT:PSS and similar materials allows the top electrode layer to be thinner than screen-printed carbon layers for a target resistance and RC time constant, thus enabling sensors with faster response times.
Further, in embodiments herein, the water absorptive dielectric (e.g., in certain examples a polyimide) is treated prior to printing or coating of the water or water-vapor permeable top electrode (e.g., PEDOT:PSS). The treatment of the water absorptive dielectric is, (e.g., by UV ozone treatment) to improve wetting of the water absorptive material (e.g., polyimide) for connection to the water permeable material (e.g., PEDOT:PSS). This operation will, in certain embodiments, also improve the response time of the fully manufactured device, such as by controlling the extent and rate of water uptake by the water absorptive dielectric. Additionally, plasma and chemical treatments of the water absorptive dielectric (e.g., polyimide) can also be used to treat the water absorptive dielectric (e.g., polyimide) prior to printing of the water permeable electrode structure.
It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.