GB2464516A

GB2464516A - Self heated carbon monoxide sensor

Info

Publication number: GB2464516A
Application number: GB0819116A
Authority: GB
Inventors: Alireza Salehi
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-10-17
Filing date: 2008-10-17
Publication date: 2010-04-21
Also published as: GB0819116D0

Abstract

A highly sensitive self heated tin oxide (SnO2) carbon monoxide sensor comprises a SnO2thin film deposited on a glass substrate by chemical vapour deposition (CVD) using diluted SnCl4solution as precursor. The application of the semiconductor sensing film itself as a heater, by applying an ac voltage to it, enhances the sensitivity of the gas sensor over the use of an external heater. The self heated sensor also decreases the power consumption of the sensor and may be smaller and cheaper to produce.

Description

INTELLECTUAL

. .... PROPERTY OFFICE Application No. GBO8 19116.5 RTM Date:19 March 2009 The following terms are registered trademarks and should be read as such wherever they occur in this document: Pyrex Intellectual Property Office is an operating name of the Patent Office www.ipo.gov.uk

DESCRIPTION

Title: Highly sensitive self heated Sn02 carbon monoxide sensor

Background:

Tin Oxide is an n-type transparent semiconductor with a wide energy band gap of about 3.5 eV. The development of a highly sensitive carbon monoxide sensor is described to detect incomplete combustion, the early stages of fire and to prevent accidental asphyxiation. The drawback of usual thin film sensors for detection of carbon monoxide gas is the low sensitivity and usage of heaters beneath the thin film sensors which increases the power consumption. In this experiment I made a novel technique to promote the sensitivity of Sn02 gas sensor without any extra heater. Here the sensing film itself was used as heater to gas sensor. The Sn02 sensing film was used simultaneously as heater and no further external heater was required to operate the gas sensor. Thus, the sensor showed a very high sensitivity, low power consumption and easier operation to gas sensors operated with external heater formed underneath the thin film substrate.

Statement of invention:

The self heated gas sensor invented exhibited an excellent characteristic change of more than 37% in response to 1000 ppm carbon monoxide, which was one order of magnitude higher than the Sn02 sensor operated by an external heater formed beneath the thin film substrate. Scanning Electron Microscopy showed highly porous structure of the Sn02 gas sensor operated without any external heater by ac voltage. It was observed that the structure of the sensing Sn02 thin film changed dramatically when used ac voltage directly to the thin film to heating the sensor.

Advantages: Usually commercial sensors are developed using Sn02 containing small amounts of catalyst like Pt or Pd or Cu as sensing films deposited on ceramic heater substrates.

Beyond this several deposition techniques have been used around the world to grow doped and undoped Sn02 films like sol gel, screen printing, thermal evaporation, spray pyrolysis, sputtering and chemical vapour deposition.

Each of the mentioned techniques has its advantage and disadvantage. However, I used the chemical vapour deposition to grow the thin film Sn02 without any further dopant.

This technique is extremely easy with excellent thin film results. The substrate used in this invention was Pyrex glass of normal size of 1cm x 1cm x 2 mm thick. So with this type of thin film deposition there was no problem to use the Sn02 this film as simultaneous sensor and heater for detection of monoxide carbon gas. With this technique a very high sensitive gas sensor has been achieved showing sensitivity of 37% by applying ac voltage to the sensing film without any external heater. This is one order of magnitude higher than that of the Sn02 sensor operated conventionally with external heater underneath the sensor substrate. The sensor shows a power consumption of 1.9 W at 50 V. The invention clearly shows that using the sensing film of the Sn02 gas sensor simultaneously as heater enhances its sensitivity to carbon monoxide significantly. The advantages of this technique, which avoids external heater, are excellent sensitivity, lower cost, ease of implementation, small size and suitable for microelectronic systems.

Introduction to drawings:

Fig. 1 shows the variation of the sensitivity of the self heated Sn02 gas sensor against ac voltage and temperature towards 1000 ppm carbon monoxide. The sensitivity profile obtained here shows the highest value of 37% at a particular ac voltage of 50 Volt.

Converting the voltage into temperature using the temperature controller it can be seen (upper axis of Fig.!) that as the temperature increases the sensitivity rises significantly up to 37% at 175 degree Celsius (50 V). This is an excellent result when comparing with the result of a similar Sn02 gas sensor operated with an external heater as shown in Fig.2. It is observed in Fig.2 that when the sensor is operated with external heater a considerable lower sensitivity peak of around 3.6% at higher operating temperature of 200 degree Celsius is being obtained. It should be noted that both sensors were tested in multiple cycles and showed responses to carbon monoxide in similar manner mentioned above.

Detailed description:

The project was performed using undoped Sn02 films with thickness of about 200 nm grown by Chemical Vapour Deposition technique onto Pyrex glass substrates of size I cm x 1 cm with 2 mm thickness. The sensing films were prepared using SnC14 diluted in water with a molar concentration of 0.2 M. This was kept in a glass bubbler and heated to degree Celsius. A filament heater kept the substrate temperature at 370 degree Celsius during the deposition. Further, a heating cord kept the temperature of the tube between the bubbler and the reaction chamber at around 150 degree Celsius. Diluted SnC14 was driven into the reaction chamber through capillaries by its own vapour pressure. The Sn02 film was grown onto the heated glass substrate through the following chemical reaction: SnC14+2H20-+ Sn02+4HCI. For evaluating the sensitivity of the sensors two successive operating techniques were used. Thus, for sensitivity measurements two contacts were performed on surface of the films using silver paint.

The first technique was carried out using the sensing film itself as heater. Here ac voltage (50 Hz) was applied to the sensing film of the self heated sensor via the two contacts driven a current through the film, which generates heat on the surface. Using a Keithly I-V measure unit model 238 the current through the sample was measured. After a full measurement cycle the resistance response of the sample was calculated through RV/I with different ac voltages ranging from 10 to 50 Volts.

The second technique was performed using a heater formed underneath the sensor substrate. Here the resistance of the Sn02 gas sensor was measured using the two contacts on the surface by changing the heater temperature ranging from 50 to 250 degree Celsius.

Thus, the entire procedure for evaluation of sensitivities of similar Sn02 gas sensors consists of two heating techniques, self heated and external heated. A gas chamber of 2.5 Litre volume and with inlet and outlet was used to measure the gas sensing response of the sensors. CO gas mixed with dry air was injected into the testing chamber of the gas sensor characterization system through a mass flow controller at a total flow rate of about 3 L/min. In each experiment 1000 ppm of CO was injected into the test chamber. The resistance of the gas sensors was measured after the gas injection and compared to the initial resistance under clean air condition. After a full measurement cycle the resistance response of each sensor was transformed into a sensitivity value using commonly used equation: S(%)=((Ra-Rb)/Ra)x 100 where Ra and Rb denote the resistance of the sensor in reference atmosphere (clean air) and the test gas, respectively. It is worth noting that the sensors were kept in the laboratory for 6 months to check the changes. After this period, no changes in resistance values were observed.

The variation of current of the Sn02 gas sensor versus ac voltage in clean air is shown in Fig. 3a. It is shown that the current through the sample increases almost linear with respect to the voltage resulting in an increase in power consumption. The observed voltage dependence of the current is typical for semiconductor materials. The resistance of the sample was calculated using the voltage-current measurements. The results are sununarized in Fig. 3b. It is shown that the resistance decreases with respect to ac voltage from the initial value of 1.49 k Ohm at 10 V to 1.26 k Ohm at 50 V in clean air and no remarkable changes in resistance was observed. The decrease in resistance with increasing the voltage means that the power consumption increases with respect to voltage shown in Fig. 3c. It is shown in Fig. 3c that power consumption of the self heated Sn02 gas sensor increases from a value of 0.1 up to 1.9 W by applying ac voltages between 10 and 50 V. The changes in surface temperature of the self heated Sn02 gas sensor with respect to voltage were observed using a Ni-Cr alloy thermocouple attached close to the substrate with accuracy of +1-2 degree Celsius. Fig. 3d shows the results of temperature changes against ac voltage. It is observed that the surface temperature of the self heated Sn02 gas sensor increases rapidly with respect to the voltage in a similar manner to current and power changes shown in Fig. 3a and 3b.

The sensing film of the self heated sensor was cycled to temperatures up to 200 degree Celsius (55 V) without difficulties. However, beyond this voltage at around 60 V the glass substrate cracked up and split into several pieces. A possible explanation of splitting the Pyrex glass would be the different thermal expansion coefficient of the Pyrex substrate and the Sn02 material. Thus, the voltage of more than 50 V seems to be the most critical parameter to be controlled in order to get the highest sensitivity. Fig. 4 shows the heating characteristics of the sensing film. It is observed that the sample is heated with an efficiency of around 87 degree Celsius per Watt. Fig 5 shows the variation of the sensitivity of the self heated gas sensor against ac voltage and temperature towards 1000 ppm CO gas. The sensitivity profile obtained here shows the highest value of 37% at a particular ac voltage of 50 V. Converting the voltage into temperature using the temperature controller it can be seen (upper axis of Fig. 5) that as the temperature increases the sensitivity rises rapidly to 37% at 50 V (175 degree Celsius). The sensitivity shows one order of magnitude higher value than that obtained by similar Sn02 gas sensor operated with an external heater as shown in Fig. 6. It is shown in Fig 6 that when the sensor is operated with external heater a considerably lower sensitivity peak of around 3.6% at a higher operating temperature of 200 degree Celsius will be obtained. It should be added that both samples (self heated and with external heater) were tested in multiple cycles and showed repeatable response to CO gas. Moreover, the rate of response and recovery times of the self heated gas senor was compared to that with external heater underneath the sensor substrate. The operating temperature of the self heated senor was degree Celsius (50V) and that of the sensor with external heater was at 200 degree Celsius. h view of response transient, the optimum operating condition is the self heating. The typical response times for self heated sensor and for the sensor with external heater, shown in Fig.7a and b, have been found to be 5 and 20 seconds, respectively. The faster response of the self heated gas sensor is explained by the fact that self heating condition creates high porous sensing film which was confirmed by X-Ray Diffraction (XRD) measurements and Scanning Electron Microscopic (SEM) pictures.

Claims

Claims 1. A self heated sensor means that the sensor does not need any heater fitted externally.
2. A self heated sensor is deposited directly on a substrate such as Pyrex glass substrate without any heater.
3. A self heated sensor is a thin film sensor used simultaneously as gas sensor in which the heater is provided by the thin film self.
4. A self heated sensor exhibits much more sensitivity towards carbon monoxide.
5. A self heated sensor exhibits low power consumption, lower cost and ease of implementation
6. A self heated sensor can be implemented in micro electronic systems