KR20170108476A

KR20170108476A - Gas detecting element and gas sensor using same

Info

Publication number: KR20170108476A
Application number: KR1020160032437A
Authority: KR
Inventors: 이정우; 김유석; 김정규; 유진형; 장은수
Original assignee: 주식회사 엘지화학
Priority date: 2016-03-18
Filing date: 2016-03-18
Publication date: 2017-09-27
Also published as: KR102009938B1

Abstract

The present invention relates to a sensing element having nanostructures in the pores of a porous substrate. When a DC voltage is applied to the sensing element, the nanostructure acts as an oxidizing tip, causing corona discharge in the pores. The breakdown voltage and breakdown voltage The present invention provides a gas detecting element capable of detecting the type and concentration of a charged gas by using the measured current magnitude.

Description

TECHNICAL FIELD [0001] The present invention relates to a gas sensing element and a gas sensor using the gas sensing element.

The present invention relates to a gas detection element capable of detecting the type and concentration of a measurement gas and a gas sensor using the same.

Gas sensors generally refer to sensor devices that detect toxic gases such as carbon monoxide, sulfur dioxide, and nitrogen oxides. Currently, gas sensors can be classified into contact type small-sized gas sensors and semiconductor type gas sensors depending on the mechanism of operation. Among them, the semiconductor type gas sensor is composed of metal oxide tin oxide, zinc oxide, indium oxide, . The metal oxide used as the gas sensing material may change its electrical resistance depending on the presence or concentration of a specific gas in the air. By observing the change in the electrical resistance, the presence or absence of the specific gas can be determined.

However, since the conventional semiconductor type gas sensor detects a characteristic change occurring when the gas molecules are adsorbed on the surface of the detecting element, a process of desorbing the gas molecules by heating after the detection is necessary, and the disadvantage that the regeneration time for re- And the heater must be installed together, and the gas sensor portion may be contaminated. In addition, since the selectivity to a specific gas is high, it is difficult to detect various kinds of gases and it is difficult to cope with various environments.

A problem to be solved by the present invention is to provide a gas detection element capable of analyzing the concentrations of various kinds of gases and gases.

Further, the present invention includes a gas sensor including the gas detecting element.

Further, the present invention provides a gas detection method using the gas sensor.

In order to solve the above-described technical problem,

There is provided a gas detection element comprising a porous substrate having a plurality of voids and sensing means formed of a conductive nanostructure formed in the void.

Also, the conductive nanostructure is characterized in that a corona discharge occurs when a voltage is applied.

According to one embodiment,

Wherein the sensing means is provided on the metal electrode layer, And a counter electrode positioned on the sensing means at a predetermined interval by a spacer,

A corona discharge may be caused by the conductive nanostructure formed on the pores of the porous substrate by applying a voltage to the counter electrode.

Further, the present invention provides a gas sensor including the detecting element.

According to one embodiment, the gas sensor

A housing having an inlet through which the sample gas flows;

A detector body including a detection element for detecting gas introduced through said inlet; And

And a discharge port for discharging the sample gas into the detector body.

The present invention also provides a gas detection method using the gas sensor.

According to one embodiment, the gas detection method comprises

Supplying a sample gas to the gas sensor;

Applying a voltage to the gas sensor to generate a corona discharge inside the porous electrode having the nanostructure formed therein;

Ionizing and destroying the sample gas by the corona discharge; And

And measuring an insulation breakdown voltage of the sample gas.

The present invention provides a gas sensing element having nanostructures in the pores of a porous substrate, so that when the DC voltage is applied, the nanostructure acts as an oxidizing tip, causing a corona discharge inside the pores. It is possible to provide a gas detecting element capable of detecting the type and concentration of the introduced sample gas by measuring the magnitude of the current generated at the insulation breakdown voltage and the breakdown voltage generated at this time.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a process diagram showing a method of manufacturing a porous substrate according to an embodiment.
2 is a SEM image of a porous substrate manufactured according to one embodiment.
3 is a conceptual diagram showing the principle of gas detection by corona discharge.
4 shows a module of the gas detecting element.
5 is a structural view and a SEM image of a porous substrate having SWNTs fabricated in accordance with an embodiment of the present invention.
6 shows the current density-current curve of the gas measured by the gas sensor.
Figure 7 is a Fowler-Nordheim plot (FN plot) measured with a gas sensor.
FIG. 8 shows the change of the breakdown voltage according to the concentration of the sample gas.
Fig. 9 shows the change in the current density with the concentration of the sample gas at the breakdown voltage.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention is capable of various modifications and various embodiments, and specific embodiments are illustrated in the drawings and described in detail in the detailed description. It is to be understood, however, that the invention is not to be limited to the specific embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

The gas detecting element according to the present invention

And a structure in which a conductive nanostructure is formed on the pores of the porous substrate,

When a DC voltage is applied, a corona discharge is generated inside the gap by the nanostructure.

More specifically, the gas detecting element may include:

A metal electrode layer;

The porous substrate provided on the metal electrode layer;

A spacer disposed on the porous substrate; And

And a counter electrode positioned at a predetermined distance from the porous substrate in the spacer, wherein a DC voltage is applied to cause a corona discharge by a nanostructure provided in a gap of the porous substrate, and a sample which is insulated by the corona discharge It is possible to provide a gas detecting element capable of analyzing the type of the sample gas by measuring the dielectric breakdown voltage of the gas.

Fig. 1 shows the structure of the detecting element as described above, and shows the principle of detection of the sample gas by the corona discharge using the detecting element.

In addition, the nano structure provided inside the pores of the porous substrate can increase the contact area with the sample gas compared with the flat plate structure, and by acting as a needle-like structure, the generated electric field can be formed densely in the pores Therefore, it is possible to minimize the loss of the electric field, so that the ionization and dielectric breakdown of the sample gas due to the corona discharge can be more efficiently performed, and the damage caused by the external vibration and impact can be minimized, thereby improving the durability of the device .

The sensing element may be a single element, or may be modularized as in Fig. 2 to improve the performance of the element.

The modularization of the sensing element is achieved by forming a plurality of gas sensing elements, thereby enhancing the sensitivity of the sensing signal, thereby reducing false positives and improving the accuracy and reliability of the data, thereby enhancing the effectiveness of the gas sensing system.

A corona discharge is an electrical discharge phenomenon caused by ionization of a fluid around a conductor. Specifically, there are an anode corona and a cathode corona. The anode corona means a corona when a tip electrode is used as an anode, and insulation breakdown occurs as the voltage across the electrodes rises.

The cathode corona is formed at a lower voltage than the anode corona, and according to the present invention, the voltage to the converter breakdown can be measured lower than that of the anode corona, and the discharge current generated by the DC voltage in the anode corona environment is more stable .

According to the present invention, in such corona discharge, the nanostructure formed on the porous electrode is made to be a needle electrode for inducing the corona discharge, and the gas can be detected by using the dielectric breakdown of the sample gas from the corona discharge by applying the DC voltage have.

This is because a certain gas has a specific breakdown voltage and the breakdown voltage has a substantially constant value even when the concentration is changed so that it can serve as a fingerprint and thus the breakdown voltage of the gas It becomes possible to distinguish the types.

In addition, a discharge current of a certain magnitude is induced at the breakdown voltage of the sample gas, and the discharge current shows a tendency to increase in proportion to the concentration of the sample gas, so that the concentration of the sample gas can also be measured.

According to one embodiment, the porous substrate may be made of metal foam or may be electrochemically oxidized to produce a porous substrate.

In the present invention, metal foam of a conductive metal may be used, or a porous metal may be prepared by using an electrochemical etching method of a conductive metal. For example, a method for producing porous silicon produced by using an electrochemical etching method includes:

Preparing a silicon wafer on the metal layer;

Immersing the silicon wafer in an HF solution to remove a native oxide film;

Depositing a metal such as Cr (Cr) or gold (Au) on the back surface of the wafer from which the oxide film has been removed;

Anodizing the surface-roughened wafer using the apparatus of FIG. 3; And

The anodic oxidation process may be followed by washing with distilled water and drying with nitrogen or argon gas. The electrode used as a counter electrode in the oxidation process may be platinum (Pt) or the like.

Figure 2 shows a SEM image of a porous Si wafer etched with electrochemical oxidation according to one embodiment.

The size of porous silicon pores can be controlled from a few nanometers to a few microns, which is proportional to the amount of current flowed during electrochemical corrosion, the amount of additive added as an impurity to silicon, the amount of HF, and the type of carrier (n , p). The process of fabricating such porous silicon is advantageous in that it is highly reproducible, can be rapidly fabricated, and has a low cost to manufacture.

Or forming a perforated mask on the silicon substrate;

Contacting a surface of the silicon substrate exposed by the mask with a hydrogen fluoride (HF) solution to which hydrogen peroxide and ethanol are added, applying a positive voltage to the metal layer under the silicon substrate, and applying a negative voltage to the HF solution;

Forming a microporous silicon substrate on an exposed silicon surface of the HF solution by anodic oxidation electrochemical etching;

And removing the mask and then converting the porous silicon structure to a microporous silicon oxide structure by thermal oxidation.

The porous substrate may be made of a material selected from the group consisting of Al ₂ O ₃ , AlN, ZrO ₂ , MgO, SiC, Si, Ceramics such as boron (B ₄ C) and boron nitride (BN), and equivalents thereof.

The nanostructure may be formed by a chemical vapor deposition (CVD) method or a physical vapor deposition (PVD) method. Preferably, the nanostructure may be formed by chemical vapor deposition CVD).

For example, a method of manufacturing a nanostructure includes

Porous nano substrates;

Introducing a metal catalyst into the pores of the porous substrate;

Introducing the nanostructure material gas into the pores of the porous substrate; And

The metal catalyst and the raw material gas react with each other to form a nanowire. The metal nanoparticle may be coated with a material that does not react with the raw material gas so that portions other than the voids of the porous nano- The nanowires can be formed.

According to one embodiment, the conductive nanostructure may be composed of carbon, ZnO, In ₂ O _3, and Bi ₂ S ₃ , and may be preferably a carbon nanostructure. For example, SWNTs (single wall carbon nanotubes), DWNTs (double wall carbon nanotubes), MWNTs (multi wall carbon nanotubes), carbon nanowires, carbon nanofibers, or carbon nanorods.

In addition, the conductive nanostructure should have a high charge density per unit area for corona discharge. Therefore, the conductive nanostructure should have a sharp or long shape. For example, the conductive nanostructure may have a needle shape, a wire shape, a rod shape, Nano-cone, or nanotube type. By using the hollow nanotube type, the surface area with the sample gas can be widened, and the corona discharge can be more efficiently performed.

The metal catalyst may be at least one selected from the group consisting of Ni, Co, Fe, Pt, Au, Al, Cr, Cu, (Mn), molybdenum (Mo), rhodium (Rh), silicon (Si), tantalum (Ta), titanium (Ti), tungsten (W), uranium (U), vanadium (V) and zirconium One or more metals or alloys selected from the group can be used.

The spacer may be an insulating material capable of electrically isolating the porous substrate from the counter electrode. Examples of the spacer include silicon dioxide (SiO ₂ ), aluminum oxide (Al ₂ O ₃ ), tantalum oxide Ta ₂ O _5), zirconium oxide (ZrO _2), dioxide, hafnium (HfO _2), titanium dioxide (TiO ₂₎ a nitride film containing an oxide film and a silicon oxynitride (SiON), silicon nitride (Si ₃ N _4), etc., including such Or a hafnium (Hf) -based insulating film including hafnium silicon oxynitride (HfSiON), hafnium silicate (HfSiXOY, where 0.1 <X <9 and 2 <Y <4).

The space between the porous substrate and the counter electrode formed by the spacer can be kept vacuum, and the sample gas can flow through the space.

Further, according to an embodiment, the gas sensor including the detecting element may be configured such that, as shown in FIG. 3,

A housing having an inlet through which the sample gas flows;

And a discharge port for discharging the sample gas of the detector body.

The discharge port may discharge the remaining sample gas from the detector body without ionization or dielectric breakdown by the corona discharge in the sample gas supplied to the detector body.

According to one embodiment, the detector body portion including the detection element may be in a vacuum state.

According to one embodiment, a flow rate regulator for regulating the flow rate of the sample gas may be provided at an inlet of the sample gas, for example, a needle valve may be provided.

The present invention also provides a method for measuring the type and concentration of a sample gas using the gas sensor.

A gas detection method according to the present invention comprises:

Supplying a sample gas to the gas sensor;

Applying a DC voltage to the gas sensor to generate a corona discharge inside the porous electrode having the nanostructure formed therein;

Ionizing and destroying the sample gas by the corona discharge; And

And measuring the dielectric breakdown voltage of the sample gas.

A method of detecting a sample gas by a corona discharge is a method in which a sample gas is applied to a detecting element and then a DC voltage is gradually increased to induce dielectric breakdown by ionization of the sample gas. The voltage and current of the sample gas can be measured to analyze the composition and concentration of the sample gas.

Best Mode for Carrying Out the Invention Hereinafter, preferred embodiments are described to facilitate understanding of the present invention. The following examples are provided to aid understanding of the present invention, but the present invention is not limited by the following examples.

&Lt; Preparation Example > Electrochemical production method of porous silicon wafers

A pure p-type silicon monocrystalline wafer (boron doped, <100>, 0.0008 ~ 0.0012 Ω / cm) was used and a porous silicon was fabricated by electrochemical etching by flowing a constant current using a Kithley 2420 model. HF solution (48% by weight: Fisher Scientific) and anhydrous ethanol (Fisher Scientific) were used as etching solvents. As a target substrate, a 2 inch wafer was used. Before electrochemical etching, the substrate is washed with ethanol 2-3 times so that there is no foreign substance on the surface of the wafer, and the surface is treated with Ar gas. For electrochemical etching, a silicon wafer is immobilized on the apparatus of FIG. 1 and then a constant current is flowed using a current source controlled by a computer. Solvents for electrochemical corrosion were mixed with 48% HF and ethanol in a volume ratio of 3: 1. Electrochemical etching was performed in Teflon cell using two electrodes. Platinum wire was used as anode and aluminum foil was used as cathode. The current used for the etching was proceeded at a current density of 50 mA / cm < ^{2 &} gt ;.

< Example 1> Porosity Si On By CVD The deposited SWCNT Fabrication of Substrate

A metal such as Fe, Ni, Co, or the like is deposited on the porous silicon substrate through anodic oxidation using a magnetron sputtering method. The sample is charged into a thermochemical vapor deposition apparatus, and NH ₃ , H ₂ The catalytic metal of the sample is etched with the gas to form nano-sized small grains. At this time, the diameter of the final catalyst is determined to be several tens of nanometers or less by controlling the treatment time. The synthesis of carbon nanotubes proceeds by supplying various hydrocarbon gases at a flow rate of 20 sccm onto nano-sized catalyst metal grains. At this time, the diameter of the carbon nanotubes is determined by the size of the catalyst metal determined by controlling the flow rate of the etching gas. An SEM image of the porous substrate prepared as described above is shown in FIG.

< Comparative Example 1> Porosity Si On By spraying Deposited SWCNT Fabrication of Substrate

A SWCNT - based substrate was fabricated on a porous silicon wafer prepared by electrochemical oxidation using a spraying method. The carbon nanotube dispersion solution used in the spraying method was prepared by dispersing single wall carbon nanotubes synthesized by 5 wt% vapor deposition in DMF solvent using ultrasonic waves.

< Experimental Example 1> Comparison of detection performance between porous structure and plate structure

A spacer and a counter electrode were bonded to the substrate prepared in Example 1 and Comparative Example 1 to prepare a gas sensing element. After putting the gas detection element into the chamber, connect the electrodes, inject gas such as N ₂ , O ₂ , Ar into the device, and then use a keithely 2400 power supply to measure the voltage at which an electric field of 3 V / " Electrical breakdown voltage (breakdown voltage and current) generated during corona discharge of the gas was measured and shown in FIG. 6 and FIG.

6 shows a higher current density in the detection device of Example 1 using a structure in which a SWCNT was formed on a porous substrate. This shows that the substrate structure having a larger surface area from the porous substrate structure reacted around the SWCNT structure It may mean that the concentration of the sample gas is higher and the detector element of the porous structure exhibits a smaller value when the magnitude of the electric field at which the dielectric breakdown occurs occurs. From this, it can be seen that the reaction in the detection element of the porous structure is more efficient.

In FIG. 7, since the FN plot shows the shape of a straight line, it can be seen that the current component of the device is the electron emission current, and since the porous structure exhibits a higher current amount than that of the flat plate structure, The electron emission current is induced.

< Experimental Example 2> Porosity Si - SWCNT Detection of sample gas using substrate

The dielectric breakdown voltage and discharge current of N ₂ , O ₂ , and Ar gases were measured using the porous substrate prepared in Example 1 and Comparative Example 1, and the results are shown in FIGS. 8 and 9.

According to the measurement results of Fig. 8, the breakdown voltage of each gas is almost constant even with the change of the concentration of the sample gas, which can act like a fingerprint in gas detection. That is, it is possible to deduce the component of the gas contained in the unknown sample by using the dielectric breakdown voltage value for each gas as an index.

8 shows that a sensing element comprising a nanostructure formed on a porous substrate by chemical vapor deposition exhibits a lower dielectric breakdown voltage than a porous substrate deposited by spraying, May be related to characteristics.

According to the measurement results of FIG. 9, it can be seen that when a discharge current induced when the dielectric breakdown voltage of each gas is applied to the sample gas is measured, a discharge current of a specific magnitude is induced. It can be seen that as the concentration increases, the discharge current increases in proportion to the concentration. From this, the concentration of the unknown sample can be measured by measuring the discharge current at the dielectric breakdown voltage of the unknown sample.

The present invention includes a substrate on which a nanostructure is formed on a porous structure as a gas sensing element as described above. By using a detection method through a corona discharge with the above element, various kinds of sample gases can be detected, A possible gas sensor can be provided.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, This is possible.

Claims

And a sensing unit comprising a porous substrate having a plurality of voids and a conductive nanostructure formed in the void.

The method according to claim 1,
Wherein the conductive nanostructure is capable of causing corona discharge upon application of a voltage.

The method according to claim 1,
Wherein the sensing means comprises a counter electrode disposed on the metal electrode layer and spaced apart by a spacer on the sensing means,
And a corona discharge is caused by a conductive nanostructure formed in a gap of the porous substrate by applying a voltage to the counter electrode.

The method according to claim 1,
Wherein the material of the conductive nanostructure includes at least one selected from the group consisting of carbon, ZnO, In ₂ O ₃ and Bi ₂ S ₃ .

The method according to claim 1,
Wherein the conductive nanostructure has at least one shape selected from an acicular shape, a wire shape, a rod shape, a nano hair, a nanofiber, and a nanotube shape.

The method according to claim 1,
Wherein the porous substrate is obtained by electrically oxidizing a metal substrate.

The method according to claim 1,
Wherein the nanostructure is a carbon nanotube.

8. The method of claim 7,
Wherein the carbon nanotubes are formed by chemical vapor deposition (CVD).

The method of claim 3,
Wherein a space between the porous substrate formed by the spacer and the counter electrode is held in vacuum.

3. The method of claim 2,
And the corona discharge is an anode corona discharge.

A gas sensor comprising a gas detection element according to any one of claims 1 to 10.

12. The method of claim 11,
A housing having an inlet through which the sample gas flows;
A detector body including a detection element for detecting gas introduced through the inlet; And
And a discharge port for discharging the sample gas into the detector body.

Supplying a sample gas to the gas sensor according to claim 11;
Applying a voltage to the gas sensor to generate a corona discharge in the pores of the porous substrate on which the nanostructure is formed;
Ionizing and destroying the sample gas by the corona discharge; And
And measuring an insulation breakdown voltage of the sample gas.

14. The method of claim 13,
Wherein the applied voltage is gradually increased to measure an insulation breakdown voltage at which insulation breakdown of the measurement gas occurs and to distinguish the kind of gas from the breakdown voltage.

15. The method of claim 14,
Wherein the magnitude of the breakdown voltage is not affected by the concentration change of the sample gas.

16. The method of claim 15,
And the magnitude of the discharge current increases as the concentration of the sample gas increases.