WO2020021153A1

WO2020021153A1 - Moisture sensor

Info

Publication number: WO2020021153A1
Application number: PCT/ES2019/070531
Authority: WO
Inventors: Álvaro BLANCO MONTES; Miguel Ángel FERNÁNDEZ MORALES; Francisco GALLEGO GÓMEZ; Ceferino López Fernández
Original assignee: Consejo Superior De Investigaciones Científicas
Priority date: 2018-07-26
Filing date: 2019-07-26
Publication date: 2020-01-30
Also published as: ES2738913A1

Abstract

The invention relates to a moisture sensor based on the optical response of an artificial opal to water adsorption. The basic design only needs to read the Bragg diffraction band (its intensity at a predetermined wavelength or its position) of an artificial opal whose manufacture does not require any post-processing (surface functionalisation, infiltration, curing, etc.). The sensor responds directly to the adsorption of water molecules on the opal and therefore does not require intermediate phenomena (such as imbibition, hydration expansion, chemical reactions, etc.). Optionally, controlling the hydrophilicity, porosity, periodicity or fill fraction of the opal makes it possible to adjust the sensitivity of the sensor and/or Bragg peak reading frame. The responsive element (opal) is wireless, can be miniaturised to a few cubic microns and has a response time of typically less than one second.

Description

HUMIDITY SENSOR

DESCRIPTION

OBJECT OF THE INVENTION

The object of the invention is framed in the technical field of physics.

More specifically, the object of the invention is directed to a humidity sensor that makes use of optical means to determine and / or quantify the degree of humidity in the environment in which it is located.

BACKGROUND OF THE INVENTION

First, a photonic crystal is a material in which there is a periodic modulation of the refractive index in one, two or the three directions of space; in such a way that once an electromagnetic radiation is emitted from its interior or affects its surface, the interference that occurs between the different waves reflected in each interface between the means of different refractive index results in certain frequency ranges cannot be transmitted on the glass; said frequency ranges that cannot be transmitted, more specifically, the energy ranges related thereto are called photonic gaps. The value of the dielectric constants of the photonic crystal components and the spatial period of their variation determine the position and width of these gaps; having in a three-dimensional photonic crystal the gaps are associated to each direction of propagation.

On the other hand, relative humidity is, together with temperature, the physical magnitude most frequently measured today. The interest in humidity monitoring and control is so extensive and concerns such a variety of purposes, that it has generated a large number of different transduction methods. The most common transducers are currently MEM (MicroElectroMechanical). Among the MEMS, the capacitive type are replacing the resistives given their better performance. Capacitive sensors typically base their response on the intrusion of air into a polymer of the sensor and on the change in the capacity of the system that it produces, the transduction process being on the scale of ten seconds being also sensitive to electromagnetic radiation and not allowing remote measurements.

There are many subtypes of this type of humidity sensors based on different physical properties, such as condensation in a mirror (chilled mirrors), polymer hydration, optomechanical network variation, etc. whose response times typically range from several seconds to minutes. Other optical sensors of complex architectures (photometric fiber interferometry, side-coupling induction technologies, surface plasmonic resonance, etc.) may have response times around the second or less, but they are expensive to manufacture and implement and require advanced instrumentation. .

The determination of humidity levels is of vital importance in some environments that require careful monitoring of working conditions.

In Mathew, J. et al Humidity sensor based on photonic crystal fiber interferometer Electronics Letters, vol. 46, no. 19, pp. 1341-1343, 2010, a moisture sensing device is detailed that makes it possible to dispense with the use of any hygroscopic material being based on an interferometer based on a photonic glass fiber that works in reflection mode and is made of a material based on Si; The described sensor works in reflection mode and allows relative humidity measurements.

On the other hand, document W02007027792A2 details a method and a humidity detection apparatus using a multilayer photonic material with structures that have a photonic hollow. Said structure has a series of photonic gaps that are formed in the reflection spectrum from the alternation of layers of materials of different index of refraction which can be deposited or arranged on an optically transparent substrate or a reflecting face of a prism. Through this arrangement, the light directed towards the prism is directed to the multilayer structure and reflected from the prism, where it is captured and analyzed. Also, in W02007027792A2, sensor configurations are described in which the fixed wavelength or coupling angle is maintained, while monitoring the change in the other parameter.

Document JP2008232925 details a refractive index sensor and a refractive index measuring device that allows reducing the number of components and reduce the cost in the refractive index sensor since it uses a photonic crystal; Likewise, the refractive index measuring device having said refractive index sensor is detailed. The refractive index sensor is formed by a collection (matrix) of 2D photonic crystals (resonators) in which laser radiation is affected. The spectral variation of each resonator depends on the change in refractive index. The refractive index measuring apparatus described in JP2008232925 comprises said refractive index sensor, an image forming means for capturing an image of the matrix that includes a near-field visual image of the resonator and measuring means for determining the variation of the image generated by the imaging device and from there being able to carry out a measurement of the refractive index of the medium to be measured from this image variation, the described sensor being based on the emission of light from a two-dimensional Si photonic crystal that only works in the infrared spectrum.

DESCRIPTION OF THE INVENTION

The object of the invention is a humidity sensor, which is based on a responsive element based on a photonic crystal whose dielectric material contains interstitial voids that are filled with air, said photonic crystal being preferably based on an artificial opal. Artificial opals are three-dimensional periodic arrangements of dielectric spheres of varying sizes and compositions. Its response to electromagnetic radiation depends on the ordering of these spheres, their size and composition, and this response can be scaled according to these parameters (and, for this reason, be extensible to any electromagnetic range). Inverse opals are known as the structures resulting from filling the gaps between spheres with some other material and eliminating those spheres. The possible structural variations (two-dimensional or 1-dimensional arrangements), although they are no longer commonly called opals, their optical properties and electromagnetic responses are essentially similar to three-dimensional ones.

The sensor described here is based on the effect of optical transduction and allows moisture to be measured by analyzing the optical response to obtain the value of the light intensity at wavelengths of the flanks of the photonic gap (from now, Bragg diffraction peak or simply Bragg peak) where the variation in intensity with humidity is maximized.

Since the humidity sensor described here is based on optical measurements, the detection and possible quantification of humidity is carried out wirelessly, which makes the sensor object of the invention and its possible implementation immune to radioelectric noise, Likewise, the sensor object of the invention provides detections with response times below a second given that the responsive element (an artificial opal) is wireless, miniaturizable to a few cubic microns and its response time is typically less than a second.

As indicated, the humidity sensor object of the invention is based on an optical response to the adsorption of water of a photonic crystal, more specifically of artificial opal to the adsorption of water. In its basic design, it is only necessary to read the Bragg peak (its intensity at a pre-established wavelength or its position) of an artificial opal whose manufacture does not require any post-processing (surface functionalization, infiltration, curing, etc. .) in this way the humidity sensor responds directly to the adsorption of water molecules in the artificial opal and therefore does not require intermediate phenomena (such as imbibition, hydration dilation, chemical reactions, etc.).

Optionally, by controlling the hydrophilicity, porosity, periodicity or filling fraction of the opal, the sensitivity of the sensor and / or reading region of the Bragg peak can be adjusted.

In an embodiment of the object of the invention there is an optical humidity sensor with transduction based on the reading of a distinctive quality of the Bragg peak (hereinafter, Bragg peak) exhibited by an artificial opal.

The humidity sensor described herein object of the invention consists mainly of the following elements: • A responsive element, such as an artificial opal (or colloidal crystal), which is a type of photonic crystal comprising self-assembled monodispersed hydrophilic material spheres between which interstitial voids are defined that are filled with air. The spheres may be of silica or other similar material. The photonic crystal, in a preferred embodiment of the humidity sensor described herein, has a level of reflectance of the Bragg peak equal to greater than 80% and does not require any further processing after its manufacture.

• A light source that illuminates the opal to examine its optical response. Such a source may consist of low power LED emitters.

• An optical response transducer to electrical signal, either analog or digital.

Since the response of the artificial opal to moisture requires only adsorption / desorption of water molecules on its surface, the humidity sensor object of the invention exceeds in response time to existing sensors of usual use, whose response necessarily involves some intermediate phenomenon Thus, for example, the response time in such sensors is determined by the rate of change in capacitance of a polymer (as in capacitive sensors) or in size (as in optical devices based on hydrogels). In such cases, response times range from several seconds to minutes. Ultimately, such differentiation in the physical principle of the humidity sensor object of the invention results in that an increase in its sensitivity does affect the speed of the response. This is a radical advantage over the behavior of existing humidity sensors, which commonly exhibit an inverse relationship between the sensitivity and the speed of response of the sensor.

The object of the invention exhibits a high sensitivity (which we call as the "S" value that is defined as the normalized variation of the reflectance, in percent, divided by the variation in relative humidity, in percent, which describes how much the monitored magnitude varies per unit of HR.

By measuring the reflectance at a wavelength located on one of the flanks of the Bragg peak, the opal's response to moisture changes is more sensitive (it shows a higher S value). This wavelength is also selectable based on the availability of the corresponding light source. To the being the Bragg peak much more defined than in other systems known in the prior art (multilayer reflectors, two-dimensional photonic crystals, three-dimensional photonic crystals infiltrated by hydrogels, etc.), the sensitivity of the object of the invention is significantly higher. Since S is defined as the normalized variation of the reflectance (in%) divided by the variation in relative humidity (in%), an opal of 370 nm silica spheres exhibits an approximate value of S = 30 in the range of low humidity (0-20% RH). Values of S in the range 0.5-15 have been reported in the best humidity sensors based on photonic response.

The nature of the artificial opal, composed of typically resistant and very inert materials, and its optical response give the humidity sensor great robustness and durability, and makes it suitable for use in all types of fields, even in those of high radiation Electromagnetic, nuclear, great corrosion, etc.

As alternatives to the artificial opal described, the photonic crystal that acts as a responsive element used can consist of an inverse opal photonic crystal, resulting from the infiltration of the interstices between the opal spheres and the subsequent selective elimination of the spheres, or other structures similar self-assembled lower dimensionality (monocas, bilayers, etc).

DESCRIPTION OF THE DRAWINGS

To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, according to a preferred example of practical implementation thereof, a set of drawings is attached as an integral part of said description. where, for illustrative and non-limiting purposes, the following has been represented:

Figure 1.- Shows a graph showing a relative humidity (RH) reading made by the sensor object of the invention during a variation of 10 to 70% RH

Figure 2.- Shows a graph showing a relative humidity (RH) reading made by the sensor object of the invention during a variation of 70 to 10% RH. Figure 3.- Shows a graph showing the sensitivity to very fast wet currents on the surface

PREFERRED EMBODIMENT OF THE INVENTION

In a preferred embodiment of the object of the invention there is a humidity sensor based on a responsive element comprising a photonic crystal, which in turn comprises self-assembled monodispersed spheres of hydrophilic material, such as a silica-based material, a source of light that illuminates the photonic crystal to examine its optical response, such as a light source based on low-power LED emitters whose wavelength of the light source preferably overlaps with the Bragg peak of the responsive element and a transducer of the optical response to signal, signal that can be analog electrical or digital.

The humidity sensor object of the present invention is based on the dependence of the optical response of the photonic crystal with the relative humidity of the environment. The sensitivity and speed of variation of the optical response to a change in humidity results in a very sensitive and ultrafast humidity sensor. As a responsive element, the characteristics of the photonic crystal (its manufacture, range of application, sensitivity to moisture, etc.) determine the distinctive qualities of the sensor, for this example of preferred embodiment of the invention and non-limiting use is made artificial opal, which can be both direct and inverse, as photonic crystal of the responsive element

The wavelength at which the Bragg peak appears depends, in addition to the relative humidity of the air, on the size and composition of the constituent spheres. The easy availability of a wide range of types of monodisperse spheres (from few nanometers to many microns, compact or porous, of oxides, polymers, etc.) allows a great versatility in the selection of the detection range of the Bragg peak (depending on, for example, from the light source or, by extension, any electromagnetic wave chosen). The position of the Bragg peak can be estimated very roughly by the simple Bragg equation (hence its name):] = 2-nd (where n is the average index of refraction of the structure, easily calculable, and d the periodicity, which depends only of the chosen sphere size, D, of the shape d = 0.816 D). This simple relationship allows us to tune the sensor response without varying the diameter of the constituent sphere.

Thus, in a preferred embodiment of the humidity sensor object of the invention, the photonic crystal is an artificial opal. The appearance of the high reflectance Bragg peak in an artificial opal only requires a lateral area of a few periodicities (each determined by the diameter of the spheres) and a thickness of about fifteen layers of spheres. Thus, for example, if a Bragg peak in the visible spectrum (400-700 nm) is required, the artificial opal should consist of spheres of ~ 250-350 nm (by the Bragg equation described above, 2- 1.3 0.816-300 = 636, approximately the wavelength of the He-Ne laser, red), so that the minimum extension of the responsive element will be ~ 15 pm2 and a thickness of ~ 4 pm. This quality allows a high miniaturization of the humidity sensor.

The humidity sensor's ability to detect moisture is given by how it affects opal optics. The physical principle is that the humidity of the surrounding air, which also fills the interstices of the opal, determines the number of water molecules adsorbed on the surfaces of the opal (both internal and external). The amount of water in the liquid phase determines the refractive index in the interstices of the water and, consequently, the position and intensity of the Bragg peak. Once the size and refractive index of the spheres are known, the Bragg peak is determined by the adsorbed water and, therefore, is highly correlated with the humidity of the surroundings.

The sensitivity of the optical response of the sensor to humidity is given by the hydrophilicity of the opal: the greater the hydrophilicity, the greater the adsorbed water (for any value of the humidity) and the greater dependence of the Bragg peak with the ambient humidity. Therefore, the materials to be used to make the opal will be essentially oxides such as silica (Si0 ₂ ), alumina (Al ₂ 0 ₃ ), zircona, (ZnO), etc. Even in the case of polymer opals (significantly hydrophobic), their sensitivity could be increased by coating hydrophilic material (e.g., oxides) by techniques known as atomic layer deposition (ALD) or chemical vapor deposition (CVD ). Conversely, if one wanted to reduce the sensitivity of the opal, the hydrophilicity of the material could be reduced. In the case of silica, for example, the concentration of silanoles, chemical groups that determine the affinity of the material to water, on the surface of the spheres is easily adjustable by different thermal processes.

On the other hand, the sensitivity to moisture can increase if, together with the phenomenon of adsorption, the condensation of hair is present. The latter appears when the pore size is small enough for a given humidity range. Thus, a strategy to increase sensitivity, for example, in the range of very low RH (<1%) is to use an opal formed by microporous spheres (with pores of diameters smaller than 2 nm). Thus, in addition to maintaining the sensitivity to moisture in the rest of the range, the appearance of capillary condensation in the micropores in the range 0-1% RH causes an abrupt change in the refractive index of the sphere and induces a response Opal optics much more pronounced. If increased sensitivity is required within a certain range of the humidity range, opals formed by mesoporous spheres (of selectable pore size) could be used, such as Carbon.

The speed of the response of the sensor object of the invention is fundamentally determined by the optical response speed of the opal. Given the physical principle of this sensor, the response of the artificial opal to changes in ambient humidity depends directly on the dynamics of adsorption / desorption of water molecules in the interstices of the artificial opal, which instantly causes the change in refractive index and, consequently, the Bragg peak. Therefore, the response of the artificial opal is delimited by the process of diffusion of the air from the outside of the artificial opal to its interstices. In an artificial opal with a Bragg peak in the visible range, whose thickness is ~ 5 microns, the response time is less than 1 s for a change of 10 to 70% RH, as shown in Figure 1 or vice versa of 70 to 10% RH, as seen in figure 2, from said figures 1 and 2 it follows that the response time (considered as 1 / e of the total signal increase) is less than 1 second having used an artificial opal comprising silica spheres of ~ 280 nm. By comparison, the reading of RH, under identical conditions, of a traditional capacitive silicon-based sensor is also shown in FIG.

In possible alternative embodiments of the humidity sensor object of the invention, even shorter response times can be obtained. For this purpose, in an even preferred embodiment of the humidity sensor object of the invention, interstices are provided. larger to favor the diffusion of air into the artificial opal (by means of, for example, larger spheres or by using inverse opals); while in a still more preferred embodiment of the humidity sensor object of the invention there is a smaller opal volume, reducing both the lateral extent and the thickness (in this respect, it should be noted that optical response times of < 50 ms in an opal of silica spheres of ~ 330 nm against temperature changes - and, therefore, of water adsorption - minimizing the volume affected.

Once the characteristics of the artificial opal have been determined, a light source is directed to the responsive element to examine its optical response once the light strikes the latter (by extension, any electromagnetic wave that covers the spectral range of the Bragg peak ). It is sufficient that the wavelength (or lengths) of the light source overlap with the Bragg peak. The sensitivity of the humidity sensor will increase if a wavelength coinciding with the flanks of the Bragg peak is used, where the intensity of the peak is much more sensitive to any humidity change. For this, a monochromatic light can be used or, alternatively, an optical filter can be used to select the working wavelength (all these elements are economical and available in a wide range); in any case the wavelength of the light source must have a wavelength value comprised in the Bragg peak values of the responsive element.

The light that examines the optic of the opal has to be collected and converted by, for example, a photosensitive cell that transforms the optical signal into an electrical signal operating as an optical response transducer to an electrical signal. Since the Bragg peak is detectable in transmission or reflection, light source and transducer can be arranged, respectively, in opposite or coincident planes of the opal. If the intensity of the light source requires amplification, optical collection elements (lenses) or electronic amplifiers can also be used if necessary.

Finally, optionally, an ADC (Analog to Digital Converter) digital analog converter can be available to digitize the electrical signals and be treated by a calibration and visualization software.

Claims

1. Humidity sensor characterized by comprising:

to. a responsive element comprising a photonic crystal which in turn comprises self-assembled monodisperse spheres hydrophilic material between which interstitial voids are defined,

b. a light source intended to illuminate the responsive element to examine its optical response, and

c. an optical response transducer to an electrical signal, intended to collect and convert the optical response of the responsive element into a signal.

2. Moisture sensor according to claim 1 characterized in that the photonic crystal is an artificial opal.

3. Moisture sensor according to claim 1 characterized in that the photonic crystal is a reverse artificial opal.

4. Moisture sensor according to any one of claims 1 to 3 characterized in that the photonic crystal has a reflectance of the Bragg peak equal to greater than 80% in its entire length.

5. Moisture sensor according to any one of the preceding claims characterized in that the hydrophilic material of the spheres is selected from: silica (Si0 ₂ ), alumina (Al ₂ 0 ₃ ), zircona, (ZnO) or any oxide or compound hydrophilic or any non-hydrophilic hydrophilized by coating hydrophilic material.

6. Humidity sensor according to any one of the preceding claims characterized in that the spheres have a diameter between about 250 nm and about 350 nm, such that the responsive element has an extension of about 15 pm2 with a thickness of about 4 pm .

7. Moisture sensor according to any one of claims 1 to 6 above characterized in that the spheres have a diameter of 330 nm.

8. Moisture sensor according to any one of the preceding claims characterized in that the spheres are microporous.

9. Moisture sensor according to claim 8 characterized in that the spheres are microporous with pores of diameter less than 2 nm.

10. Humidity sensor according to any one of claims 1 to 7 characterized in that the spheres are mesoporous.

11. Humidity sensor according to any one of the preceding claims characterized in that the wavelength of the light source has a wavelength value comprised in the Bragg peak values of the responsive element.