Field of the Invention

The invention relates to a planar resistive humidity sensor enabling measurement with direct measuring signal (hereinafter also referred to as “DC measuring signal”), which means that the humidity sensor according to the invention can be supplied directly from a DC voltage source, and further relates to a method of production of such a humidity sensor.

Background of the Invention

Relative humidity is one of the most monitored environmental parameters. At present, humidity sensors essentially consist of three main parts, which include a carrier substrate, an electrode system and a sensitive layer.

The carrier substrate is a carrier pad made of non-conductive material, on which the electrode system and the sensitive layer are implemented. Many different materials can be used as the carrier substrate, e.g. rigid materials to mechanically carry the entire humidity sensor, such as ceramics, glass, silicon, etc. At present, however, flexible materials such as PET, polyimide, and even cellulose-based materials are preferably used in the manufacture of the substrate. These materials are used in substrates especially when we want to incorporate a humidity sensor into the area of flexible electronics.

The most commonly used electrode system is the interdigital electrode (IDE) system, which is used for electrical contact between the sensitive layer and the evaluation device, and which can be created by conventional printing methods, such as screen printing and stencil printing. The gap between the electrodes is usually from 50 to 200 pm depending on the electrical conductivity of the sensitive layer. In addition to common metal-based materials, it is also possible to use printing pastes based on conductive polymers or carbon. The combination of substrates and different materials of IDE electrodes opens up a wide range of applications, e.g. in the field of flexible printed electronics.

The sensitive layer is a layer that covers the electrode system and interacts with the target analyte, while changing its physical or chemical properties. In the case of patent application, humidity is the analyte. Depending on the change in the above properties caused by the analyte, the electrical parameters of this layer, such as impedance, change. The materials used as sensitive layers can range from conventional ionic salts through hygroscopic polymers to relatively new conductive polymers and various composite materials.

The above principle of construction of resistive humidity sensors is generally known. The material used for the construction of the electrode system can be, for example, based on graphene oxide, as disclosed in the invention RU 2682259 (Cl).

Materials based on Cu, Zn, W and ferrite can be used for application in the sensitive layer. This construction of a humidity sensor is described in the patent application RO 132 780 (A2). However, the construction of this sensor is not planar, but it is a sandwich structure. This sandwich structure is also used by the invention known from US 2018/136266 (Al), where a dielectric medium is placed between two electrodes, which changes its resistance depending on the humidity.

An example of a planar humidity sensor with a graphene -based sensitive layer is given in the invention KR 2018/0052807 (A), while using a metal electrode system for its operation. Another known example is a heat-resistant humidity sensor using semiconductor materials, which is described in JP 2018/077129 (A). This sensor uses inorganic semiconductor materials for its operation, including carrier substrate. Similarly, the invention is described in CN104 655 688 (A), which discloses a humidity sensor implemented on a glass carrier substrate and using semiconductor technology. Another known invention from CN 106 248 735 (A) describes a humidity sensor based on ultra-thin sulphide films and a method of its preparation. The sensitivity of the sensor to humidity is created by thin layers of tungsten disulphide by metal plating. A gas detection sensor is also known, which is used for humidity detection in patent application CN 107 525 825 (A). It is a sensor where a sensitive layer based on copper (II) oxide (CuO), carbon nanotubes with copper oxide (CuO-MWCNTs) and a composite material of copper (II) oxide and graphene (CuO-Graphene) is formed by means of screen printing. Polyethylene film (PET) is used as the carrier substrate. The published humidity sensor has the advantages of a low detection limit, high sensitivity and good stability.

CN 102 495 106 (A) discloses an invention which describes a humidity sensitive material for a flexible humidity sensor, including a method of its preparation. The material for the formation of sensitive layer uses multi-walled carbon nanotubes and liquid organosilicon.

Each of the above inventions shares at least some of the disadvantages, including the impossibility of using a DC measuring signal, since polarization takes place in materials of the sensitive layer during the passage of direct electric current, a complex and economically expensive process of production of a humidity sensor, environmental non-friendliness, whether due to the toxic properties of the materials used, or due to the poor waste degradability of the entire humidity sensor, and last but not least, the unsuitability for use, for example, in textiles.

The purpose of the invention is to provide a humidity sensor for measurement with DC measuring signal, which could be supplied directly from a DC voltage source (e.g. battery), which would be made of materials naturally degradable in the environment, which could be produced by means of standard printing processes, which would have high operational reliability, and which would be flexible enough for use, for example, in smart textiles.

Summary of the Invention

This set task is solved by providing a humidity sensor for measurement with DC measuring signal according to the invention below. The humidity sensor for measurement with DC measuring signal consists of a carrier substrate, which serves to create mechanical support for other components of the humidity sensor. Furthermore, the humidity sensor consists of a sensitive layer for its interaction with the measured analyte, which changes its behaviour in conducting electric current. Last but not least, the humidity sensor consists of at least one electrode system for ensuring electrical contact with the sensitive layer.

The core of the invention is based on the fact that the sensitive layer is formed on the basis of carbon nanotubes functionalized with at least one carboxyl (COOH) or SO ₃ H side chain group, and modified by substitution of an atom at the end of their chain with an atom from the group of Li, Na, K, Cl or the amino group (NFb).

This is convenient because the humidity dependence is determined primarily by the chemical functionalization of carbon nanotubes, while depending on the type and degree of functionalization. The carbon nanotubes are functionalized with the above-mentioned side chain groups (COOH, SO ₃ H) and then the chemically reactive part of the side chain group (e.g. hydrogen at the end of the chain) is chemically replaced by another chemical element. For proper function, it is necessary to use an element that increases the dissociation constant of the whole group, e.g. sodium. As a result, the side chain group will become more dissociated, making it easier for water to bind in these sites. The resulting effect increases the sensitivity of the sensor to air humidity. The carbon nanotubes are used for transferring the charge between the electrodes of the electrode system, and also between the individual nanotubes; modification of the side chain groups allows to detect the analyte also by means of a DC measuring signal. Their high conductivity allows the formation of a very thin sensitive layer, and this significantly improves the dynamic properties of the entire humidity sensor, such as response time and recovery time. It is also advantageous that the active layer can be applied to the electrode system of the sensor by conventional printing techniques.

The use of DC measuring signal for evaluating the humidity level is undoubtedly a great advantage of the invented humidity sensor. In commonly available commercial resistive humidity sensors, this method of measurement cannot be used due to the polarization of the material. While the DC signal measurement provided by the invention allows the use of the invented humidity sensors directly in wireless devices powered only by batteries, such as RFID tags, smart clothing, without the need for additional electronic components for signal transmission and evaluation.

In another preferred embodiment of the humidity sensor according to the invention, the carbon nanotubes have a single-walled or multi-walled carbon structure.

In another preferred embodiment of the humidity sensor according to the invention, the electrode system is made of a material based on carbon materials or electrically conductive polymers. And the carrier substrate may be preferably made of a biodegradable material. This means that another preferred aspect of the invention is that the humidity sensor uses completely new combination of materials which are easily environmentally degradable, and thus the humidity sensor does not significantly burden the environment at the end of its service life and can be easily recycled. The carrier substrate may be formed of various non- conductive materials, but it is preferable to use readily biodegradable materials, such as cellulose. The connection of carbon electrodes with a biodegradable substrate, e.g. based on cellulose, and a sensitive layer based on carbon nanotubes is very advantageous, because the result is a humidity sensor containing no metals or plastics. Such a humidity sensor can be recycled very easily and efficiently at the end of its service life, with almost no environmental impact. This solution is very advantageous in the case of using a large number of sensors, e.g. in applications for smart cities and for the so-called Internet of Things.

In another preferred embodiment of the humidity sensor according to the invention, the carrier substrate is planar and the electrode system is formed by interdigital electrodes. The planar substrate can be made from large sheets that are printed and then cut (roll to roll technology). The planar solution of the substrate is very advantageous for mass production of humidity sensors with minimal costs.

In another preferred embodiment of the humidity sensor according to the invention, the carrier substrate is formed by a fibre and the electrode system is formed by at least two parallel electrically conductive paths running along the fibre of the carrier substrate. This form of humidity sensor in the form of fibre allows it to be easily integrated into smart textiles.

The invention also relates to a method of production of the above-mentioned humidity sensor. The production method includes the following process steps: a) at least one carrier substrate is produced from an electrically non-conductive material, b) at least one electrode system is applied to the carrier substrate from an electrically conductive material by printing, c) a sensitive layer is applied over the electrode system.

The core of the invention is based on the fact that in process step c) the sensitive layer is applied by printing, using a printing dispersion composed of carbon nanotubes functionalized with at least one carboxyl (COOH) or SO3H side chain group and modified by substitution of an atom at the end of their chain with an atom from the group of Li, Na, K, Cl or the amino group (NH2), and further composed of a volatile liquid carrier medium.

This is particularly advantageous because digital printing methods are very accurate even with a low resolution of the printed pattern. The printing dispersion composed of carbon nanotubes can be used in a printer, and thanks to the volatile liquid carrier medium, after application to the carrier substrate, it evaporates, leaving a clean sensitive layer without foreign matters that would affect its function. The electrode system and the sensitive layer can be implemented on the carrier substrate by means of common printing technologies, which has an advantage in terms of energy and financial savings compared to most of the described solutions. With great advantage, the electrode system formed on the carrier substrate can be implemented by means of carbon pastes also applied by conventional printing technologies.

In the preferred embodiment of the production method according to the invention, the AerosolJet printing method or the airbrush method is used in process step c). The AerosolJet method allows the application of a printing dispersion in the form of aerosol drops; in addition it allows printing from a distance of up to 10 mm, which makes it possible to print on carrier substrates formed, for example, by relief. In another preferred embodiment of the production method according to the invention, in process step a) the carrier substrate is produced as a planar body or as a longitudinal body in the form of a fibre. It is also preferred if in process step b) an electrode system is produced in the form of an interdigital electrode or in the form of parallel electrically conductive paths running along the fibre of the carrier substrate.

It is preferred to produce the carrier substrate as a planar substrate or as a fibre to expand the range of possible applications of the humidity sensor, in particular in applications for the Internet of Things, for smart textiles, for smart cities, etc.

The advantages of the invention include the operation of the sensor with DC measuring signal, as well as high operational reliability, accuracy and speed. Furthermore, the advantages of the invention include fast and cheap production, environmental friendliness, and a wide range of applications. Use of printing technology and environmental compatibility allow mass production of the humidity sensor.

Explanation of drawings

The present invention will be explained in detail by means of the following figures where:

Fig. 1 shows a planar embodiment of a humidity sensor in a perspective view,

Fig. 2 shows a humidity sensor in the form of a fibre in a top view,

Fig. 3 shows a graph with the change of electrical resistance of the humidity sensor to a change in relative humidity.

Example of the invention embodiments

It shall be understood that the specific cases of the invention embodiments described and depicted below are provided for illustration only and do not limit the invention to the examples provided here. Those skilled in the art will find or, based on routine experiment, will be able to provide a greater or lesser number of equivalents to the specific embodiments of the invention which are described here.

Fig. 1 shows a planar embodiment of a humidity sensor, which consists of a carrier substrate 1, an electrode system 2, and a sensitive layer 3 based on a side chain group of functionalized carbon nanotubes with modification of side chain group. The electrode system 2 here consists of interdigital electrodes (IDE), which ensure contact with the sensitive layer 3 and at the same time enable the connection of measuring devices or evaluation electronic circuits. This embodiment of the sensor can be implemented by commonly available printing technologies, such as screen printing, stencil printing, airbrush and other known printing techniques.

Fig. 2 shows the humidity sensor implemented on a fibre. Thus, it is a sensor where the carrier substrate 1 has an elongated design (fibre), the electrode system 2 is printed on the carrier substrate 1, as well as the sensitive layer 3 based on the side chain group of functionalized carbon nanotubes with modification of side chain group. Here, the electrode system 2 is formed by two parallel conductive paths, through which a sensitive layer 3 is applied. This geometric arrangement of the sensor on the fibre can be preferably further integrated, for example, in smart textiles. To prepare this geometry of the sensor on the fibre, special printing methods and technologies are required, such as the Aerosol Jet system.

Fig. 3 shows a graph with the change of electrical resistance of the humidity sensor according to relative humidity.

Example 1

The carrier substrate 1 is made of cellulose. The electrode system 2 is made of carbon paste. The sensitive layer 3 is made on the basis of carbon nanotubes functionalized with the carboxyl (COOH) side chain group modified by substitution of the hydrogen atom from the end of the chain with the Na atom. Example 2

The carrier substrate 1 is made of polylactic acid (PLA). The electrode system 2 is made of electrically conductive polymer. The sensitive layer 3 is made on the basis of carbon nanotubes functionalized with the SO ₃ H side chain group modified by substitution of the hydrogen atom from the end of the chain with the Na atom.

Example 3

The carrier substrate 1 is made of polyhydroxyalkanoate (PHA). The electrode system 2 is made of carbon paste. The sensitive layer 3 is made on the basis of carbon nanotubes functionalized with the carboxyl (COOH) side chain group modified by substitution of the hydrogen atom from the end of the chain with the K atom.

Example 4

The carrier substrate 1 is made of cellulose. The electrode system 2 is made of electrically conductive polymer. The sensitive layer 3 is made on the basis of carbon nanotubes functionalized with the SO3H side chain group modified by substitution of the hydrogen atom from the end of the chain with the K atom.

Example 5

The carrier substrate 1 is made of polylactic acid (PLA). The electrode system 2 is made of carbon paste. The sensitive layer 3 is made on the basis of carbon nanotubes functionalized with the carboxyl (COOH) side chain group modified by substitution of the hydrogen atom from the end of the chain with the Li atom.

Example 6

The carrier substrate 1 is made of polyhydroxyalkanoate (PHA). The electrode system 2 is made of electrically conductive polymer. The sensitive layer 3 is made on the basis of carbon nanotubes functionalized with the SO3H side chain group modified by substitution of the hydrogen atom from the end of the chain with the Li atom. Example 7

The carrier substrate 1 is made of cellulose. The electrode system 2 is made of carbon paste. The sensitive layer 3 is made on the basis of carbon nanotubes functionalized with the carboxyl (COOH) side chain group modified by substitution of the hydrogen atom from the end of the chain with the Cl atom.

Example 8

The carrier substrate 1 is made of cellulose. The electrode system 2 is made of carbon paste. The sensitive layer 3 is made on the basis of carbon nanotubes functionalized with the carboxyl (COOH) side chain group modified by substitution of the hydrogen atom from the end of the chain with the amino group (NH2).

Industrial applicability

The humidity sensor for measurement with DC measuring signal according to the invention can be used in areas where it is necessary to monitor humidity and at the same time it is necessary to implement this sensor in systems where its flexibility is required, such as smart textiles, for ecological safety or ability to measure by means of supply from a DC power supply.