Odor detection element

Abstract

To provide an odor detection element capable of achieving both improvement in detection sensitivity of an odor substance and prevention of deterioration in oscillation characteristics.SOLUTION: An odor detection element according to the present invention includes: a piezoelectric oscillator including a piezoelectric layer containing a piezoelectric material as a main component, a first electrode provided on a front surface of the piezoelectric layer, and a second electrode provided on a back surface of the piezoelectric layer; and a sensitive membrane including a first particle layer provided on the first electrode and having a plurality of porous particles and a particle fixing material for fixing the plurality of porous particles, and an adsorption layer provided on the first particle layer and having a second particle layer including a plurality of unfixed porous particles.SELECTED DRAWING: Figure 3

Description

Technical Field

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[0001] The present invention relates to an oscillator-type odor detection element and an oscillation method of the odor detection element.

Background Art

[0002] The oscillator-type odor detection element has a sensitive film that adsorbs a specific odor substance on the oscillator, and the odor substance can be detected by the change in the resonance frequency accompanying the adsorption of the odor substance to the sensitive film. For the oscillator, a QCM (Quartz crystal microbalance) element using a quartz oscillator or the like is used.

[0003] The quartz oscillator has a characteristic called the crystal impedance value (hereinafter referred to as the CI value). The CI value is the equivalent series resistance in the state without additional capacitance, and since the oscillation stability of the quartz oscillator decreases when the CI value is high, reduction is required. For example, Patent Document 1 discloses a quartz oscillator in which the CI value is reduced by defining the ratio of the electrode width to the width of the quartz vibration plate.

Prior Art Documents

[0006] Thus, in odor detection elements using QCM elements, increasing the amount of adsorbent material contained in the sensitive membrane to improve sensitivity leads to a decrease in the CI value and a deterioration in the oscillator's oscillation characteristics. Similarly, in odor detection elements using oscillators other than QCM elements, increasing the amount of adsorbent material contained in the sensitive membrane also leads to a deterioration in the oscillator's oscillation characteristics.

[0007] In view of the above circumstances, the object of the present invention is to provide an odor detection element that can simultaneously improve the detection sensitivity of odor substances and prevent deterioration of oscillation characteristics. [Means for solving the problem]

[0008] The present invention has been made in view of the above-mentioned problems, and firstly solves the problem with an odor detection element comprising: a piezoelectric vibrator comprising a piezoelectric layer mainly composed of a piezoelectric material, a first electrode provided on the surface of the piezoelectric layer, and a second electrode provided on the back surface of the piezoelectric layer; and a sensitive film comprising a first particle layer provided on the first electrode and having a plurality of porous particles and a particle fixing material for fixing the plurality of porous particles, and an adsorption layer provided on the first particle layer and having a second particle layer containing a plurality of unfixed porous particles.

[0009] Secondly, the piezoelectric material is a quartz crystal, and the first electrode provided on the quartz crystal is a gold electrode with irregularities. A gap is provided between the porous particles on the first electrode side of the first particle layer and the first electrode, and the porous particles on the first electrode side are fixed by a particle fixing material provided on the first electrode, thereby resolving the issue.

[0010] Thirdly, the gold electrode is resolved by having an average surface roughness of 0.01 or more and 0.03 or less, as defined by the unfolded area ratio (Sdr).

[0011] Fourthly, the piezoelectric oscillator is solved using an FBAR (Film Bulk Acoustic Resonator) element.

[0012] Fifth, the problem is solved by blocking the pores of at least some of the porous particles constituting the first particle layer with the particle fixing material, while the pores of the porous particles in the second particle layer are exposed to the external atmosphere.

[0013] Sixth, the porous particles can be resolved by using MOF (Metal Organic Frameworks) particles. In particular, when porous particles are fixed with a particle fixing material, this fixing material blocks the adsorption sites of the porous particles. Therefore, the material is divided into a first particle layer and a second particle layer, and the second particle layer is configured so that the adsorption sites can function. [Effects of the Invention]

[0014] As described above, the present invention makes it possible to provide an odor detection element that can simultaneously improve the detection sensitivity of odor substances and prevent deterioration of oscillation characteristics. [Brief explanation of the drawing]

[0015] [Figure 1] This is a plan view of an odor detection element (QCM type) according to an embodiment of the present invention. [Figure 2] The above is a cross-sectional view of the odor detection element, taken along line AA in Figure 1. [Figure 3] It is a cross-sectional view showing the detection element enlarged as described above. [Figure 4] It is a schematic diagram showing the operation of the odor detection element according to the comparative example. [Figure 5] It is a schematic diagram showing the operation of the odor detection element according to an embodiment of the present invention. [Figure 6] It is a schematic diagram showing the manufacturing method of the above odor detection element. [Figure 7] It is a cross-sectional view of the odor detection element (FBAR type) according to an embodiment of the present invention. [Figure 8] It is a graph showing the relationship between the particle ratio of the sensitive film and the CI value (crystal impedance value) according to Example 1 of the present invention. [Figure 9] It is a graph showing the adsorption characteristics of odor substances depending on the presence or absence of the adsorption layer according to Example 2 of the present invention. [Figure 10] It is a graph showing the adsorption characteristics of odor substances depending on the presence or absence of the adsorption layer according to Example 2 of the present invention. [Figure 11] It is a graph showing the relationship between the surface roughness Sdr and the amount of activated carbon in the sensitive film according to Modified Example 1 of Example 2 of the present invention. [[ID=​​​​​​​​​​​​​​​​​​​​​​​​Hereinafter, an odor detection element according to an embodiment of the present invention will be described with reference to the drawings.

[0017] [Configuration of the odor detection element] The odor detection element 100 in this embodiment includes a piezoelectric vibrator 110 and a sensitive film 120, as shown in Figures 1 and 2. The piezoelectric resonator 110 comprises a piezoelectric layer 111, a first electrode 112, and a second electrode 113. The piezoelectric layer 111 is a layer mainly composed of a piezoelectric material. As shown in Figure 2, the surface of the piezoelectric layer 111 is called the surface 111a, and the back surface is called the back surface 111b. The first electrode 112 is provided on the surface 111a, and the second electrode 113 is provided on the back surface 111b. The first electrode 112 and the second electrode 113 sandwich the piezoelectric layer 111 and constitute a pair of electrodes. The first electrode 112 and the second electrode 113 are made of a conductive material, such as gold, silver, chromium, titanium, or other metals. The piezoelectric resonator 110 is, for example, a QCM element in which the piezoelectric layer 111 is made of quartz. Alternatively, it is a resonator formed on a semiconductor substrate such as an FBAR.

[0018] This section discusses the adsorption characteristics of sensitive films. Sensitive films are composed of resins, polymers made of resins, metals, metal oxide films, sintered inorganic materials, or films selected and mixed from these. These sensitive films adsorb multiple gases and multiple odor components, although the amount of adsorption varies. These components include gases, suspended liquids, and water vapor. If sensitive film A is prepared, there are almost no films that adsorb only gas A; other gases B, C, etc., will also be adsorbed, but a film that adsorbs gas A the most will be called a sensitive film that adsorbs a specific odor substance. The sensitive film 120 of the present invention is provided on the piezoelectric vibrator 110 and adsorbs specific odor substances contained in the gas supplied to the odor detection element 100. The type of odor substance that the sensitive film 120 adsorbs varies depending on the material that makes up the sensitive film 120.

[0019] The sensitive membrane 120 comprises a particle immobilization layer 121 and an adsorption layer 122 located on the front side. As shown in Figure 1, the particle immobilization layer 121 is provided on the first electrode 112, and the adsorption layer 122 is provided on the particle immobilization layer 121. As shown in Figure 3, the adsorption layer 122 is a layer containing porous particles 123 and adsorbs odor substances.

[0020] [Explanation of porous particles] First, let's explain the porous particles 123. The porous particles may have complex shapes. As shown in Figure 3 and later, they are layered, much like a filter membrane made of sand. Also, because their shapes differ, they interlock with each other during application, and are mechanically fixed to a certain extent. These porous particles 123 are inorganic particles, organic-inorganic composite particles, or organic particles, and have pores on their surface and inside. Therefore, they adsorb odor substances more effectively. The porous particles 123 do not need to be of only one type; multiple types may be mixed. Here, MOF (Metal Organic Frameworks) were used as the porous particles 123, but other materials such as activated carbon and zeolite are also used. At the very least, if the surface is porous, the specific surface area is large, and the amount of odor substances adsorbed per unit weight increases. Furthermore, it is desirable that the adsorption and desorption response is fast, and more preferably, the porous particles are made of a material with low hygroscopicity. Furthermore, the small gaps that occur between these porous particles are also called pores, and a membrane made up of layered porous particles is called a porous membrane.

[0021] MOFs are not particularly limited as long as they are metal-organic structures; commonly available MOFs such as UiO-66 can be used. It is desirable for MOFs to have a high adsorption capacity for odor substances per unit weight. Furthermore, MOFs may encapsulate some functional molecules within their pores. The metal atoms constituting the MOF can be at least one of the following: zinc, cobalt, niobium, zirconium, cadmium, copper, nickel, chromium, vanadium, titanium, molybdenum, aluminum, etc. However, the metal atoms constituting the MOF are not limited to these. The MOF may consist of one or more metal atoms.

[0022] The porous particles 123 preferably have a volume frequency center particle size (median diameter) of 10 nm to 500 μm as measured by a laser diffraction / scattering particle size distribution analyzer, more preferably 50 nm to 10 μm, and even more preferably 50 nm to 300 μm. When the porous particles 123 are MOFs, having a median diameter within the above range allows for a balance between the function of the MOF and the physical and mechanical properties of the composite. If the particle size of the MOF is too small, the function of the MOF will be limited, and if it is too large, it may cause oscillation failure.

[0023] [Description of the adsorption layer] The adsorption layer 122 may have an upper layer consisting solely of porous particles 123. Depending on the degree of aggregation of these porous particles 123, a dispersant may be used. Furthermore, a binder (particle immobilization layer 121) is used in the lower layer of the adsorption layer 122 to fix it to the electrode 112. This binder binds the porous particles together and also provides adhesion to the first electrode 112. For example, cellulose acetyl butyrate or hydrophobic polymers having silyl groups can be used. In addition, cellulosic polymers, imide polymers, fluorine polymers, acrylic polymers, or ester polymers can be used as binders. In this embodiment, the binder is dissolved in a solvent, such as acetone. As a result, adhesion to the first electrode 112 becomes possible.

[0024] The dispersant has the function of dispersing the porous particles 123 in the solvent. Dispersants include, for example, surfactants and polymeric surfactants, and examples of polymeric surfactants include cationic polymers, nonionic polymers, and anionic polymers. The dispersant is preferably a surfactant having a functional group that is moderately adsorbed to the surface of the porous particles 123, and that does not clog the pores of the porous particles 123.

[0025] The particle immobilization layer 121 is provided on the first electrode 112 and immobilizes the porous particles 123. As described above, the particle immobilization layer 121 mainly consists of a binder and is applied on top of the first electrode 112 in Figure 3, with porous particles 123 embedded within it, or the lower part of the porous particles being embedded while the surface of the remaining porous particles is exposed from the surface of the binder. However, generally, a solvent is mixed with a binder, dispersant, and porous particles before being applied. As a result, the surface of the porous particles 123 is covered with the binder, and the pores opening on the surface of the porous particles are blocked by this binder. In this case, the adsorption and desorption capacity of the porous particles 123 is significantly reduced. Therefore, without using a binder or dispersant, we tried coating a layer of particles with pores covered using a solution in which only porous particles were mixed in the same solvent. (See below.) As a result, the coated porous particles dissolve the binder beneath them. Therefore, in the lower layers, the porous particles 123 are fixed by the binder below, while as you move towards the upper layers, the porous particles 123 are not covered by the binder, and porous particles with unoccluded pores are stacked. These porous particles interlock with each other and are mechanically fixed to some extent, as shown in Figure 3 and subsequent figures. In other words, as shown in Figure 14, the porous particles 123 appear to be the same size macroscopically, but differ in size microscopically. Because the film is formed by spray coating, pressure is applied to the particles during film formation, resulting in amorphous particles stacking randomly. As a result, a film like the one shown in Figure 14 is formed. Furthermore, even when a solution containing a dispersant and porous particles mixed in the same solvent was applied to a particle layer with pores, the porous particles interlocked with each other and were mechanically fixed to a certain extent, similar to the solution containing only porous particles mixed in the solvent described above. In the experiment, the binder coats one to several layers of porous particles 123 from the electrode 112 side, fixing the porous particles 123 in place. This binder is sometimes referred to as the particle fixing layer 121.

[0026] [Operation of the odor detection element] In the odor detection element 100, when a voltage is applied between the first electrode 112 and the second electrode 113, vibration occurs in the piezoelectric layer 111 due to the inverse piezoelectric effect, causing the odor detection element 100 to vibrate at a constant resonant frequency. When odor substances are adsorbed onto the sensitive film 120, the weight of the sensitive film 120 increases and the resonant frequency decreases. On the other hand, when odor substances adsorbed onto the sensitive film 120 are desorbed, the weight of the sensitive film 120 decreases and the resonant frequency increases. Therefore, the odor detection element 100 can detect the amount of odor substances adsorbed onto the sensitive film 120 based on the change in the resonant frequency.

[0027] [Role and effects of porous particles] Before describing the effectiveness of the odor detection element 100, we will explain the problems using a comparative example. A: When porous particles are not adhering to the electrode surface Figure 4 shows a comparative example. Similar to the present invention, the odor detection element 200 comprises a piezoelectric vibrator 210 and a sensitive film 220. The piezoelectric vibrator 210 comprises a piezoelectric layer 211, a first electrode 212 and a second electrode 213, and the sensitive film 220 comprises an adsorption layer 222 made of porous particles 223. Unlike the odor detection element 100 in this embodiment, the odor detection element 200 does not have a particle immobilization layer. Because it is not bonded to the first electrode 212, the first electrode 212 and the porous particles shift during vibration, resulting in energy loss. B: When the lower porous particles are fixed to the electrode surface, and the upper part is formed with individually independent porous particles. Figure 5 shows a mixture of solvent, binder, dispersant, and porous particles, which is then applied to the first electrode 112 and dried. In this process, the binder adheres to the first electrode 112, and the porous particles are embedded, or partially embedded, within the binder, creating multiple layers of porous particles. This layer will be referred to as the first particle layer. Next, a solution containing a solvent, dispersant, and porous particles, with the binder removed, is applied to this first particle layer and dried. In this case, multiple layers of porous particles, which are not fixed and are individually independent, are formed on top of the first particle layer fixed by the binder. This is called the second particle layer. When forming this second particle layer, the same solvent used to dissolve the binder in the first particle layer was employed. As a result, the binder on the surface of the first particle layer dissolved, and the porous particles in the lower layer of the second particle layer were partially fixed by the dissolved binder. Some of the pores of the porous particles were also covered by the binder. This layer of porous particles fixed by the binder is considered the first particle layer. Above this, a layer of individually independent porous particles is provided. This is called the independent particle layer. Therefore, a first particle layer is provided on the first electrode 112, fixed with a binder and with some of its pores blocked, and a second particle layer is formed on top of that, each particle being independent and with most of its pores exposed. When the adsorption layer 222 is made up of individual porous particles 223 that are mechanically stacked, driving the piezoelectric vibrator 210 causes each porous particle 223 to move unstably because they are not fixed by a binder such as resin. This results in frequent collisions C between the porous particles 223, leading to a loss of vibrational energy and thus degrading the oscillation characteristics. First, let's consider this phenomenon using a quartz crystal oscillator. When a quartz oscillator is observed visually or with a metallurgical microscope, it can be seen that the gold electrode has irregularities, as shown in Figure 16. This is because, as shown in Figure 15, the quartz itself has irregularities, and when a thin film of gold is coated onto this quartz by vapor deposition or sputtering, the thinness of the film causes smooth irregularities to develop on the surface of the gold electrode, following the irregularities of the quartz. The irregularities of the gold electrode are between 0.01 and 0.03 in average surface roughness, defined by the unfolded area ratio (Sdr). On this uneven gold electrode, porous particles of varying shapes and sizes are deposited as described in [A: In the case where they are individually independent]. As a result, various gaps are created between the gold electrode and the porous particles, and at the same time, different layered structures are formed. Therefore, the porous particles in the bottom layer that directly receive vibrations may slide, while other porous particles may experience some vibration transmission, resulting in various ways of vibration transmission. Consequently, it is thought that the movement becomes more disorganized as you move towards the upper layers of the adsorption layer 222. This is likely because the porous particles are not fixed in place, making their disorganized movement more pronounced.

[0028] On the other hand, Figure 5 shows the operation of the odor detection element 100 according to this embodiment. In the odor detection element 100, as described above, a particle fixing layer (binder) 121 is provided between the adsorption layer 122 and the piezoelectric vibrator 110. The particle fixing layer 121 fixes the porous particles 123 in the lower layer and matches the vibration of the porous particles 123 in the lower layer to the vibration of the piezoelectric vibrator 110. Therefore, the vibration of the piezoelectric vibrator 110 can be transmitted to the porous particles 123 side. However, as mentioned above, collisions C between porous particles 123 can occur in the upper layer (second particle layer side) of the adsorption layer 122, but the loss of vibration energy mainly occurs in the lower layer of the adsorption layer 122. Therefore, by fixing the porous particles 123 in the lower layer with the particle fixing layer 121, the loss of vibration energy can be suppressed.

[0029] [Manufacturing method for odor detection elements] A method for manufacturing the odor detection element 100 will be described. First, prepare the piezoelectric vibrator 110. The surface of the first electrode 112 may be cleaned with Ar plasma. Next is the process of forming the particle immobilization layer. The particle immobilization layer 121 can be formed by mixing the above-mentioned binder with a solvent and applying the solution to the surface of the first electrode 112 (see Figure 6). This step can be omitted, and you may start from the next step.

[0030] The solution was applied here by spray coating. After application, a particle immobilization layer 121 is formed on the first electrode 112 by evaporating the solvent. The solvent is not particularly limited, but acetone, THF (tetrahydrofuran), or cyclohexanone can be used.

[0031] Next, we proceed to the first particle layer process and the second particle layer process. The porous particles 123 can be formed by mixing them with a solvent containing a binder and then applying the mixture onto the particle-fixed layer 121. Furthermore, the process may be started directly from the first particle layer step without going through the particle immobilization layer formation step.

[0032] Next, in the second particle layer formation process, individual, independent porous particles are formed. The solvent dissolves the upper layer of the particle immobilization layer 121. As a result, the porous particles 123 are embedded within the particle immobilization layer 121 and firmly fixed to it. Alternatively, if a particle immobilization layer is formed directly, the mixed binder is coated onto the electrode 112, and the porous particles are immobilized.

[0033] [Regarding surface roughness of the adsorption layer] The adsorption layer 122 consists of stacked, individually independent porous particles (second particle layer), with amorphous porous particles randomly arranged. As a result, gaps are created between the porous particles, and the pores inside and on the surface of the porous particles are exposed. This increases the surface area of ​​the adsorption sites, improving the amount of odor adsorption, i.e., the sensitivity. Specifically, the adsorption layer 122 preferably has an average surface roughness of 0.01 or higher, defined by the unfolded area ratio (Sdr), more preferably 0.3 or higher, and even more preferably 0.5 or higher. Since the average surface roughness of the first electrode 112 and the particle immobilization layer 121, defined by the unfolded area ratio, is approximately 0.01 to 0.03, it is preferable for the surface roughness of the adsorption layer 122 to be greater than that. The unfolded area ratio (Sdr) can be measured using a laser microscope. The surface roughness of the adsorption layer 122 can also be defined by the arithmetic mean (Sa), in which case, the average value of the arithmetic mean is preferably 0.1 or higher.

[0034] [About piezoelectric vibrators] The piezoelectric resonator 110 of the present invention is an element that generates vibration in the piezoelectric layer 111 due to the inverse piezoelectric effect when a voltage is applied between the first electrode 112 and the second electrode 113. As described above, the piezoelectric resonator 110 is a QCM in which the piezoelectric layer 111 is made of quartz. However, the piezoelectric resonator 110 may also be an FBAR (Film Bulk Acoustic Resonator) element. As shown in Figure 7, the piezoelectric resonator 110, which is an FBAR element, is formed like a bridge on a cavity 132 formed in the substrate 131.

[0035] The substrate 131 is a substrate made of Si or the like, and a cavity 132 is provided on the opposite side of the sensitive film 120. The piezoelectric layer 101 is a thin film made of AlN (aluminum nitride), ZnO (zinc oxide), PZT (lead zirconate titanate), or PbTiO3 (lead titanate), etc. The second electrode 113, the piezoelectric layer 101, and the first electrode 112 are stacked on top of the void. The first electrode 112 and the second electrode 113 can be made of a single layer metal film or a composite film thereof, such as Cr (chromium), Ru (ruthenium), Al (aluminum), Ti (titanium), Cu (copper), Mo (molybdenum), W (tungsten), Ta (tantalum), Pt (platinum), Rh (rhodium), or Ir (iridium). A first electrode pad portion 114 is provided on the first electrode 112, and a second electrode pad portion 115 is provided on the second electrode 113. A single-layer metal film such as Ti (titanium) or Au (gold), or a composite film thereof, can be used for the first electrode pad portion 114 and the second electrode pad portion 115. In this configuration, when a voltage is applied between the first electrode 112 and the second electrode 113, the resonant region S of the piezoelectric layer 111 vibrates in the thickness direction through the cavity 132, and elastic wave resonance occurs in the same direction. Unlike the quartz crystals used to construct the oscillator, aluminum nitride is used, for example, but the layered structure is the same as in the embodiment using a quartz crystal oscillator. A first particle layer, fixed with a binder, is formed on the upper electrode 112, and a second particle layer is formed on top of that. Therefore, the vibrations of the FBAR are transmitted to the first particle layer, and the second particle layer, having unblocked pores, has a structure that is highly sensitive. [Examples]

[0036] [Regarding oscillation characteristics] Odor detection elements having the configuration described above and odor elements according to comparative examples were fabricated and various measurements were performed. The oscillation characteristics of the odor detection elements can be determined from the oscillation margin of the quartz crystal oscillator that constitutes the piezoelectric oscillator in each odor detection element. Oscillation margin represents the margin from the oscillation state to the cessation of oscillation, and can be calculated using the following formula with the negative resistance and the equivalent series resistance standard value of the quartz crystal oscillator. In the formula, |-R| represents the negative resistance and R1spe represents the equivalent series resistance value of the quartz crystal oscillator. Oscillation margin [times] = |-R| / R1spe

[0037] The negative resistance value is measured by adding a pure resistance in series with a crystal oscillator and observing how long the oscillator continues to oscillate using an oscilloscope or similar instrument. The negative resistance value is obtained by adding the effective resistance value of the crystal oscillator used for the measurement to the value of the pure resistance just before the oscillation completely stops. The equivalent series resistance value is also called the crystal impedance value or CI value, and the higher the CI value, the more unstable the oscillation becomes.

[0038] In the odor detection sensor according to the present invention, the oscillation of the piezoelectric vibrator may become unstable depending on the environment in which it is used, so it is preferable to set the oscillation margin to be large. This makes it possible to create an odor detection element with stable sensitivity characteristics under a wide range of environmental conditions. Theoretically, oscillation is possible if the oscillation margin is 1x or more, but malfunctions such as no oscillation or a long oscillation rise time may occur, and it may not operate normally. For this reason, an oscillation margin of at least 5x is generally required. In the present invention, since the QCM element surface is used exposed to the atmosphere, it is determined that it is stable when the CI value is 120Ω or less, somewhat unstable when the CI value is greater than 120Ω, and unstable when the CI value is greater than 150Ω.

[0039] [Example 1] A quartz crystal oscillator with a resonant frequency of 32 MHz, having first and second electrodes made of gold, was used as a piezoelectric oscillator. After plasma treatment of this piezoelectric oscillator, a sensitive film was fabricated on the first electrode by spray coating. Specifically, a 1 wt% cellulose acetyl butyrate acetone solution was spray-coated onto the first electrode for 10,000 Hz, and the acetone was evaporated to form a particle-fixed layer. "10,000 Hz" refers to the amount by which the decrease in resonant frequency due to weight increase reaches 10,000 Hz when the resonant frequency of the piezoelectric oscillator is measured while spray coating is being performed, and the same applies hereafter.

[0040] Next, an activated carbon acetone dispersion was spray-coated onto the particle stationary layer for 90,000 Hz, and the acetone was evaporated to form an adsorption layer. The activated carbon had a median diameter of 800 nm. Using the same method, odor detection elements were prepared by first coating a 1 wt% cellulose acetyl butyrate acetone solution for 30,000 Hz, followed by an activated carbon dispersion for 70,000 Hz, and by first coating a 1 wt% cellulose acetyl butyrate acetone solution for 50,000 Hz, followed by an activated carbon dispersion for 50,000 Hz.

[0041] Furthermore, using a similar method, odor detection elements were prepared by coating a 0.5 wt% THF solution of a hydrophobic polymer containing silyl groups for 20,000 Hz, followed by coating an activated carbon dispersion for 80,000 Hz, and by coating a 0.5 wt% THF solution of a hydrophobic polymer containing silyl groups for 60,000 Hz, followed by coating an activated carbon dispersion for 40,000 Hz. In addition, as a comparative example, an odor detection element was prepared by coating an activated carbon dispersion for 100,000 Hz without forming a particle immobilization layer.

[0042] Figure 8 is a graph plotting the CI value [Ω] against the particle ratio in the sensitive membrane of each odor detection element described above. The CI value was measured using an impedance analyzer 4294A (Keysight Technologies). From this figure, it can be seen that the odor detection element with a particle immobilization layer has a significantly lower CI value compared to the comparative example with a 100% particle ratio, and that the CI value tends to decrease as the proportion of the particle immobilization layer in the sensitive membrane increases. In particular, the increase in the CI value of the two-layer sensitive membrane with cellulose acetyl butyrate as the particle immobilization layer is considerably suppressed. As described above, it has been shown that forming a particle immobilization layer between the adsorption layer containing porous particles and the first electrode has the effect of suppressing the increase in the CI value due to sensitive membrane formation.

[0043] [Example 2] Similar to Example 1, a quartz crystal oscillator with a resonant frequency of 32 MHz, having first and second electrodes made of gold, was used as the piezoelectric oscillator. After plasma treatment of this piezoelectric oscillator, a sensitive film was fabricated on the first electrode by spray coating. Specifically, (1) an odor detection element was fabricated with a particle immobilization layer + adsorption layer: 17162 Hz of hydrophobic polymer solution containing silyl groups + 82893 Hz of activated carbon dispersion, and (2) an odor detection element was fabricated with a particle immobilization layer + adsorption layer: 51177 Hz of cellulose acetyl butyrate solution and 103798 Hz of activated carbon dispersion. As comparative examples of each, (3) an odor detection element was fabricated with a particle immobilization layer: 156488 Hz of hydrophobic polymer solution containing silyl groups, and (4) an odor detection element was fabricated with a particle immobilization layer: 138773 Hz of cellulose acetyl butyrate solution.

[0044] For each of the odor detection elements described above, a gas of a constant concentration was introduced from a gas generator at a flow rate of 300 sccm, and the frequency change associated with the adsorption of odor substances was measured. Figures 9 and 10 show the adsorption characteristics of odor substances with and without the adsorption layer of each odor sensor. From these figures, it can be seen that by using configurations (1) and (2) with activated carbon in the upper layer, an effect of increasing the amount of odor substances can be obtained compared to the comparative examples (3) and (4).

[0045] [Modification 1 of Example 2] Similar to Example 1, a quartz crystal oscillator with a resonant frequency of 32 MHz, having first and second electrodes made of gold, was used as the piezoelectric oscillator. After plasma treatment of this piezoelectric oscillator, a sensitive film was fabricated on the first electrode by spray coating. Specifically, a 1 wt% cellulose acetyl butyrate acetone solution was applied to the first electrode by spray coating to form a particle immobilization layer, and then an activated carbon dispersion was applied to the particle immobilization layer by spray coating for 90,000 Hz to form an adsorption layer, thereby fabricating an odor detection element.

[0046] Furthermore, using the same method, odor detection elements were fabricated by coating an adsorption layer on a particle stationary layer for 70,000 Hz and by coating an adsorption layer on a particle stationary layer for 50,000 Hz. Simultaneously with the fabrication of the odor detection elements, an adsorption layer was also formed on a glass substrate. In addition, as a comparative example, an adsorption layer of activated carbon and a binder mixed in the following ratios was formed on a glass substrate for 100,000 Hz. Specifically, acetone dispersions of activated carbon and the binder cellulose acetyl butyrate mixed in ratios of 9:1, 7:3, and 5:5 were coated onto the glass substrate.

[0047] The surface roughness (hereinafter referred to as surface roughness Sdr), defined by the unfolded area ratio (Sdr), was measured for the adsorption layer formed on the glass substrate as described above. A laser microscope VK-X3000 (manufactured by KEYENCE, lens magnification 150x) was used to measure the surface roughness Sdr. Figure 11 is a graph plotting the surface roughness Sdr against the amount of activated carbon in the sensitive film. In the case of an adsorption layer consisting only of activated carbon, indicated by the legend "Single layer of activated carbon," the surface roughness Sdr was improved compared to an adsorption layer mixed with activated carbon and a binder, indicated by the legend "Mixed layer." By improving the surface roughness of the adsorption layer, the contact area between the adsorption layer and the gas containing odor substances increases, and an improvement in sensitivity can be expected.

[0048] [Example 3] Similar to Example 1, a quartz crystal oscillator with a resonant frequency of 32 MHz, having first and second electrodes made of gold, was used as the piezoelectric oscillator. After plasma treatment of this piezoelectric oscillator, a sensitive film was fabricated on the first electrode by spray coating. Specifically, a particle immobilization layer was formed by spray coating a 1 wt% acetone solution of cellulose acetyl butyrate onto the first electrode for 10,000 Hz, and then an adsorption layer was formed by spray coating an MOF dispersion onto the particle immobilization layer for 113,000 Hz to create an odor detection element. The MOF dispersion was a mixture of UiO-066 particles and cellulose acetyl butyrate (as a binder) as an additive in a 9:1 ratio. In addition, an odor detection element was also created in which an adsorption layer was formed by spraying the MOF dispersion for 177,000 Hz in the same manner. Furthermore, as a comparative example, an odor detection element was also created in which only an adsorption layer was formed by spraying the above MOF dispersion for 130,000 Hz without forming a particle immobilization layer.

[0049] Figure 12 plots the CI value and the film thickness of the adsorption layer for each odor detection element, with the CI value shown as a bar graph and the film thickness as a circle. In Figure 12 and subsequent figures, the legend "Single Layer" indicates the value for the odor detection element having only the adsorption layer according to the comparative example, and the legend "Dual Layer" indicates the value for the odor detection element having both a particle immobilization layer and an adsorption layer according to the above embodiment. The film thickness of the sensitive film is defined by the upper limit of the CI value, but as shown in Figure 12, forming a particle immobilization layer has the effect of raising the upper limit of the sensitive film thickness because the rate of increase in the CI value when the same amount is applied is lower compared to when there is no particle immobilization layer.

[0050] Furthermore, odor detection elements were fabricated by coating an MOF dispersion of UiO-066 and cellulose acetyl butyrate in a 19:1 ratio onto a particle stationary layer as described above, and as a comparative example, an odor detection element was fabricated by coating the MOF dispersion without forming a particle stationary layer. In addition, odor detection elements were fabricated by coating an MOF dispersion of UiO-066 and cellulose acetyl butyrate in a 49:1 ratio onto a particle stationary layer as described above, and as a comparative example, an odor detection element was fabricated by coating the MOF dispersion without forming a particle stationary layer.

[0051] Figure 13 plots the CI value and the film thickness of the adsorption layer for each odor detection element, with the CI value shown as a bar graph and the film thickness as a circle. As shown in the figure, by forming a particle immobilization layer, the film thickness at which the CI value reaches its upper limit in the case of 49:1 is greater than the film thickness at which the CI value reaches its upper limit in the case of 19:1. Therefore, it is shown that it is possible to increase the MOF ratio in MOF:binder. When the absolute amount of MOF in the sensitive membrane increases, the amount of odor substances adsorbed increases, and the decrease in sensitivity due to pore blockage of the MOF is suppressed by the decrease in the amount of binder, so an improvement in the detection sensitivity of odor substances can be expected. Furthermore, a similar reduction in CI value was observed when UiO-66 was mixed with a high molecular weight ionic dispersant in a 100:1 ratio, rather than as a binder.

[0052] [Example 4] A sensitive film of the odor detection element according to the above embodiment was formed on a silicon substrate. Specifically, a 1 wt% cellulose acetyl butyrate acetone solution was spray-coated onto the silicon substrate to form a particle stationary layer, and then an MOF dispersion was spray-coated onto the particle stationary layer to form an adsorption layer. The MOF used was UiO-66. Figure 14 is a Scanning Electron Microscope (SEM) image of this sensitive film. As shown in the figure, a particle stationary layer made of cellulose acetyl butyrate is formed on the silicon substrate, and an adsorption layer made of UiO-66 is formed on the particle stationary layer.

[0053] [Example 5] The surface shape of the piezoelectric resonator according to the above embodiment was observed. The piezoelectric resonator is a QCM element, and a gold electrode is formed on a quartz crystal resonator. Figure 15 is a laser microscope image of the gold electrode surface, and Figure 16 is an image of the height of the gold electrode surface. Figure 17 is an SEM image of the cross-section of this piezoelectric resonator. As shown in Figure 17, the surface of the quartz crystal resonator has an uneven shape. This is due to grinding the quartz crystal resonator in order to adjust the resonant frequency. Because the gold electrode is thin, as shown in Figures 15 and 16, its surface has an uneven shape according to the surface shape of the quartz crystal resonator. If porous particles are deposited directly on this gold electrode surface to form an adsorption layer, the fixation to the gold electrode surface becomes unstable due to the uneven shape, resulting in vibration energy loss. In contrast, if a particle fixing layer is formed on the gold electrode surface as in the above embodiment, and an adsorption layer is formed on top of it, the porous particles of the adsorption layer are stably fixed by the particle fixing layer, making it possible to suppress vibration energy loss. [Explanation of symbols]

[0054] 100... Odor detection element 110... Piezoelectric vibrator 111... Piezoelectric layer 111a…Surface 111b…Back side 112...1st electrode 113…Second electrode 120... Sensitive membrane 121…particle fixation layer 122… Adsorption layer 123…Porous particles 124… Adsorption layer 125…particle fixation material

Claims

1. A piezoelectric vibrator comprising a piezoelectric layer mainly composed of a piezoelectric material, a first electrode provided on the surface of the piezoelectric layer, and a second electrode provided on the back surface of the piezoelectric layer, An odor detection element comprising: a sensitive membrane comprising: a first particle layer provided on the first electrode and having a plurality of porous particles and a particle fixing material for fixing the plurality of porous particles; and an adsorption layer provided on the first particle layer and having a second particle layer containing a plurality of unfixed porous particles.

2. The piezoelectric layer is made of quartz, and the first electrode provided on the quartz is a gold electrode with irregularities. A gap is provided between the porous particles on the first electrode side of the first particle layer and the first electrode. The odor detection element according to claim 1, wherein the porous particles on the first electrode side are fixed by a particle fixing material provided on the first electrode.

3. The odor detection element according to claim 2, wherein the gold electrode has an average surface roughness of 0.01 or more and 0.03 or less, as defined by the unfolded area ratio (Sdr).

4. The odor detection element according to claim 1, wherein the piezoelectric vibrator is an FBAR (Film Bulk Acoustic Resonator) element.

5. The odor detection element according to claim 1 or claim 2, wherein the pores of at least some of the porous particles constituting the first particle layer are blocked by the particle fixing material, and the pores of the porous particles in the second particle layer are exposed to the external atmosphere.

6. The odor detection element according to claim 1 or claim 2, wherein the porous particles are particles of MOF (Metal Organic Frameworks).

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