Fiber Optic Oxygen Sensors and How Fiber Optic Oxygen Sensors Work by Ocean Optics

Ocean Optics Fiber Optic Oxygen Sensors use the fluorescence of a chemical complex in a sol-gel to measure the partial pressure of oxygen. The pulsed blue LED sends light, at ~475 nm, to an optical fiber. The optical fiber carries the light to the probe. The distal end of the probe tip consists of a thin layer of a hydrophobic sol-gel material.
A sensor formulation is trapped in the sol-gel matrix, effectively immobilized and protected from water. The light from the LED excites the formulation complex at the probe tip. The excited complex fluoresces, emitting energy at ~600 nm.
If the excited complex encounters an oxygen molecule, the excess energy is transferred to the oxygen molecule in a non-radiative transfer, decreasing or quenching the fluorescence signal (see Fluorescence Quenching below). The degree of quenching correlates to the level of oxygen concentration or to oxygen partial pressure in the film, which is in dynamic equilibrium with oxygen in the sample. The energy is collected by the probe and carried through the optical fiber to the spectrometer. This data is then displayed in your OOISensors Software.
Fluorescence Quenching
Oxygen as a triplet molecule is able to quench efficiently the fluorescence and phosphorescence of certain luminophores. This effect (first described by Kautsky in 1939) is called "dynamic fluorescence quenching." Collision of an oxygen molecule with a fluorophore in its excited state leads to a non-radiative transfer of energy. The degree of fluorescence quenching relates to the frequency of collisions, and therefore to the concentration, pressure and temperature of the oxygen-containing media.
Calibration Procedure for Oxygen Sensor System
In order to make accurate oxygen measurements of your sample, you must first perform a calibration procedure with your Oxygen Sensor system. Two major factors affect the calibration procedure of your system.
• First, decide if you are going to compensate for changes in temperature in your sample. If you are working with a sample where there are no fluctuations in temperature, you do not need to compensate for temperature. Temperature affects the fluorescence decay time, fluorescence intensity, collisional frequency of the oxygen molecules with the fluorophore, and the diffusion coefficient of oxygen. The sample should be maintained at a constant temperature (± 3°C) for best results.
• Next, choose the algorithm you wish to use for your calibration procedure. The Linear (Stern-Volmer) algorithm requires at least two standards of known oxygen concentration while the Second Order Polynomial algorithm requires at least three standard of known oxygen concentration.
Calibration curves are generated from your standards and the algorithms to calculate concentration values for unknown samples. The Second Order Polynomial algorithm provides a better curve fit and therefore more accurate data during oxygen measurements, especially when working in a broad oxygen concentration range.
Linear (Stern-Volmer) Algorithm
The output (voltage or fluorescent intensity) of our Fiber Optic Oxygen Sensors can be expressed in terms of the Stern-Volmer algorithm. The Stern-Volmer algorithm requires at least two standards of known oxygen concentration. The first standard must have 0% oxygen concentration and the last standard must have a concentration in the high end of the concentration range in which you will be working. The fluorescence intensity can be expressed in terms of the Stern-Volmer equation where the fluorescence is related quantitatively to the partial pressure of oxygen:

Io is the intensity of fluorescence at zero pressure of oxygen,
I is the intensity of fluorescence at a pressure p of oxygen,
k is the Stern-Volmer constant
For a given media, and at a constant total pressure and temperature, the partial pressure of oxygen is proportional to oxygen mole fraction.
The Stern-Volmer constant (k) is primarily dependent on the chemical composition of the sensor formulation. Our probes have shown excellent stability over time, and this value should be largely independent of the other parts of the measurement system. However, the Stern-Volmer constant (k) does vary among probes, and it is temperature dependent. All measurements should be made at the same temperature as the calibration experiments or temperature monitoring devices should be used.
If you decide to compensate for temperature, the relationship between the Stern-Volmer values and temperature is defined as:
Io = ao + bo * T + co * T2
k = a + b * T + c * T2
The intensity of fluorescence at zero pressure of oxygen (Io) depends on details of the optical setup: the power of the LED, the optical fibers, loss of light at the probe due to fiber coupling, and backscattering from the sample. It is important to measure the intensity of fluorescence at zero pressure of oxygen (Io) for each experimental setup.
It is evident from the equation that the sensor will be most sensitive to low levels of oxygen. The photometric signal-to-noise ratio is roughly proportional to the square root of the signal intensity. The rate of change of signal intensity with oxygen concentration is greatest at low levels. Deviations from the Stern-Volmer relationship occur primarily at higher oxygen concentration levels. Using the Second Order Polynomial algorithm when calibrating corrects these deviations.
Backscattering in the media can increase the collection efficiency of the probe, increasing the observed fluorescence. It is important to perform calibration procedures in the media of interest for highly scattering substances. For optically clear fluids and gases, this is unnecessary.

Figure 1. Fluorescence quenching
Second Order Polynomial Algorithm
The Second Order Polynomial algorithm requires at least three standards of known oxygen concentration. The first standard must have 0% oxygen concentration and the last standard must have a concentration in the high end of the concentration range in which you will be working. The Second Order Polynomial algorithm is considered to provide more accurate data because it requires at least three known concentration standards while the Linear (Stern-Volmer) algorithm requires a minimum of two known concentration standards. The Second Order Polynomial algorithm is defined as:
Io/I = 1 + K1 * [O] + K2 * [O]2
Io is the fluorescence intensity at zero concentration
I is the intensity of fluorescence at a pressure p of oxygen,
K1 is the first coefficient
K2 is the second coefficient
If you decide to compensate for temperature, the relationship between the Second Order Polynomial algorithm and temperature are defined as:
Io = ao + bo * T + co * T2
K1 = a1 + b1 * T + c1 * T2
K2 = a2 + b2 * T + c2 * T2
Henry's Law
It is possible to calibrate the system in gas and then use the probe in liquid or vice versa. In theory, your sensor probe detects the partial pressure of oxygen. In order to convert partial pressure to concentration, you can use Henry's Law. When the temperature is constant, the weight of a gas that dissolves in a liquid is proportional to the pressure exerted by the gas on the liquid. Therefore, the pressure of the gas above a solution is proportional to the concentration of the gas in the solution. The concentration (mole %) can be calculated if the absolute pressure is known:
Oxygen mole fraction = oxygen partial pressure / absolute pressure
Since the sensor detects partial pressure of oxygen, the response in a gas environment is similar to a liquid environment in equilibrium with gas. Therefore, it is possible to calibrate the sensor in gas and then use the system with liquid samples and vice versa if you utilize Henry's Law.
However, Henry's Law does not apply to gases that are extremely soluble in water. The following information illustrates the solubility of oxygen in water at different temperatures.
ln(X) = a + b/T* + cln(T*)
Temperature range: 0°C - 75°C
X = mole fraction
T* = T/100 in Kelvin
a -66.7354
b 87.4755
c 24.4526
Table 1. Solubility of oxygen in water at different temperatures
T (C) T* (T/100K) Mole Fraction of oxygen in water at 1 atmosphere p02 Mole Weight Fraction (ppm) at 1 atmosphere p02 (pure 02) Weight Fraction (ppm) at 0.209476 atmospheres p02 (air)
5 2.7815 3.46024E-05 61.46203583 12.87482142
10 2.8313 3.06991E-05 54.52891411 11.42249881
15 2.8815 2.75552E-05 48.94460474 10.25272002
20 2.9315 2.50049E-05 44.41468119 9.303809756
25 2.9815 2.29245E-05 40.71933198 8.529722785
30 3.0315 2.12205E-05 37.69265242 7.895706058
35 3.0815 1.98218E-05 35.20817214 7.375267068
40 3.1315 1.86735E-05 33.16861329 6.948028438
Scattering Media
Florescence emissions from the sensor formulation propagate in all directions. In clear media, only those emissions propagating toward the fiber within the acceptance angle of the probe are detected. If the probe tip is held near a reflecting surface, or immersed in a highly scattering media, the fluorescence signal will increase.
The increase will be proportional for both the intensity of the fluorescence at a pressure of oxygen and the intensity of fluorescence at zero pressure of oxygen, but will not affect the Stern-Volmer constant.
For this reason, it is necessary to measure the intensity of fluorescence at zero pressure of oxygen in the sample. Also, if you are measuring oxygen in highly scattering media, then the standards you use for your calibration procedure should be in the same media as your sample for the most accurate results.
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IX International workshop on semiconductor gas sensors

You are cordially welcome to attend the IX Workshop on Semiconductor Gas Sensors – SGS 2015, a cyclic interdisciplinary conferences devoted to all aspects of semiconductor gas sensors and systems.

Following on from the success of eight recent events starting in 1998 and organized in bi-annual period this meeting will also be devoted to reviewing the latest achievements and trends in science, technology and application of semiconductor gas sensors with special emphasis to the problem of nanostructured systems, as well as to identifying emerging and future areas of development in this exciting field.

List of topics/sessions:
• novel trends in semiconductor gas sensors: preparation and characterization
• nanostructured metal oxide gas sensors and devices
• nanostructured organic gas sensors and devices
• theory and modeling of nanostructured gas sensors

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Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring

 Metal oxide semiconductor gas sensors are utilised in a variety of different roles and industries. They are relatively inexpensive compared to other sensing technologies, robust, lightweight, long lasting and benefit from high material sensitivity and quick response times.

They have been used extensively to measure and monitor trace amounts of environmentally important gases such as carbon monoxide and nitrogen dioxide. In this review the nature of the gas response and how it is fundamentally linked to surface structure is explored. Synthetic routes to metal oxide semiconductor gas sensors are also discussed and related to their affect on surface structure.

An overview of important contributions and recent advances are discussed for the use of metal oxide semiconductor sensors for the detection of a variety of gases—CO, NOx, NH3 and the particularly challenging case of CO2. Finally a description of recent advances in work completed at University College London is presented including the use of selective zeolites layers, new perovskite type materials and an innovative chemical vapour deposition approach to film deposition.

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Cal Sensors: Infrared Carbon Dioxide Sensor

Designed to meet the challenging accuracy and reliability requirements of demand controlled ventilation systems (DCVs), the Infrared Carbon Dioxide Sensor is a CO2 sensor for HVAC applications.

CO2 concentrations are an indictor of human occupancy levels, providing an effective way to reduce energy costs by tying the control of HVAC and lighting systems to the level of CO2 in a room, and regulating CO2 content within a building.

The sensor can also support better health and higher productivity, the company said. The sensor applies the latest in nondispersive infrared (NDIR) technologies. NDIR sensors generate a signal passively, by measuring the absorption of infrared light through the gas.

The infrared system eliminates degradation concerns, reduces maintenance, and provides accurate measurements more reliably, said the manufacturer. The IRCO2 uses a two-channel detector. The sensor offers low power consumption with an accuracy of 25 parts per million (ppm).

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The semiconductor gas sensing technique

Semiconductor gas sensors rely on a gas coming into contact with a metal oxide surface and then undergoing either oxidation or reduction. The absorption or desorption of the gas on the metal oxide changes either the conductivity or resistivity from a known baseline value. This change in conductivity or resistivity can be measured with electronic circuitry. Usually the change in conductivity or resistivity is a linear and proportional relationship with gas concentration. Therefore, a simple calibration equation can be established between resistivity/conductivity change and gas concentration.
An example of a calibration equation for a change in resistivity is given by Fine et al (2010) for carbon monoxide:
R/Ro = 1+A[CO]

where R is the resistance following contact with a gas, Ro is the baseline resistance, A is a constant, and CO is carbon monoxide.

The metal oxide surface is usually a thin film of a transition or heavy metal. The exact metal that is used will depend on the application and example metals include tin dioxide (SnO2) or tungsten oxide (WO3). The film overlies a layer of silicon and is heated to a temperature between 200 and 400C, again depending on the application. In this way, the chemical processes is hastened and the effects of fluctuating external temperatures is minimised.

Semiconductor sensors work best when they have a large surface area. Such a sensor can absorb as much of the target gas as possible particularly at low concentrations.

example CO sensor calibration output

The figure above is an example calibration output from a CO sensor taken from the ESCOD-200 Carbon Monoxide Detector Manual. The graph on the left is a sensor with a voltage output and the graph on the right is a sensor with a 4..20mA output. Both graphs show a simple, linear relationship. At 2V, CO level is 0ppm where as at 10V the CO level is 300ppm. Similarly, for the current sensor, at 4mA the CO level is 0ppm and at 20mA the CO level is 300ppm. A simple algebraic extraction will determine what the CO levels are between 0 and 300ppm for a given voltage or mA output.

advantages of semiconductor sensors
Semiconductor sensors are relatively inexpensive to manufacture due to their simplicity and scalability.
Specific sensors can be designed for particular applications. For example, a sensor can be designed for low concentration applications whereas an alternative sensor can be designed for high concentration applications.

limitations of semiconductor sensors
Non-target gases may absorb on the oxide surface and provide false measurements. Important disrupting gases include ozone, water and volatile organic compounds.
One of the advantages of semiconductor sensors is their flexibility in sensor design for specific applications (see above in Advantages section). However, this can also be a limitation. A user will need to purchase multiple sensors depending on the application.
The relationship between R/Ro and CO2 is not necessarily linear leading to issues with consistent calibration across sensors. Individual calibration of sensors may be necessary, leading to increased sensor costs associated with resources and labour time.
Fine et al (2010) state that semiconductor CO2 sensors perform best in the range between 2,000 and 10,000 ppm. This limited range means the semiconductor CO2 sensors will not be suitable for a number of applications. For example, in plant physiology, atmospheric CO2 concentrations are most important in the range between 280ppm and 800 ppm. For indoor air quality applications, an ideal monitoring range is in the order between 500 and 1,000 ppm.
Semiconductor CO2 sensors can also be unreliable at high relative humidity and varying temperatures.

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Receptor Function and Response of Semiconductor Gas Sensor

 Theoretical approaches to receptor function and response of semiconductor gas sensor are described, following the illustrations of some relevant key issues such as tunneling transport. Depletion in small semiconductor crystals is characterized by the occurrence of new type depletion (volume depletion) after conventional one (regional depletion), and inclusion of both types makes it possible to formulate the receptor function and response to oxygen (air base), oxidizing gas (nitrogen dioxide), and reducing gas (hydrogen). The equations derived theoretically using physical parameters of the semiconductor side and chemical parameters of the gases side appear to reproduce satisfactorily the sensing behavior to the aforementioned gases as well as the influence of changes in physical parameters such as grain size and donor density. Extension to the semiconductor crystals dispersed with surface electron-traps shows that the traps act as a sensitizer to promote sensor response.

1. Introduction

A semiconductor gas sensor (called device hereafter) possesses an electrical resistance made with a porous assembly of tiny crystals of an n-type metal oxide semiconductor, typically SnO2, In2O3, or WO3. The crystals are often loaded with a small amount of foreign substance (noble metals or their oxides) called a sensitizer. When operated at adequate temperature in air, the resistor changes its resistance sharply on contact with a small concentration of reducing gas or oxidizing gas, enabling us to know the concentration from the resistance change. For its inauguration with a report by Seiyama et al. and a patent by Taguchi, this group of sensors has been subjected to a tremendous amount of R&D efforts world wide aiming at improvements of sensing performances and extensions to new applications. Thanks to these researches, the group not only has grown to provide important tools to detect and/or control gases in places in modern society but also has pioneered to founding a new technology field where the devices are called chemical sensors. Speaking more exactly, semiconductor gas sensors have been classified into two subtypes, that is, surface-sensitive type operating at temperatures below 500∘C and bulk-sensitive one operating at high temperature (typically at 800∘C) . This article is concerned with those of the former type only.

Apart from such remarkable achievements in practical applications, basic understandings of this group of sensors have hardly been satisfactory, despite tremendous efforts of so many researchers as summarized in reviews [5–8]. This is partly because there are many complex factors which affect sensing properties. Not only the selection of a proper oxide semiconductor is important but also the methods and conditions for fabricating sensor devices exert profound influences on gas sensing properties through changes in donor density, crystallite size, contacting geometry between crystals, packing density (or porosity), packing thickness, and so on. In addition, the sensing properties are often modified largely with loading with foreign substances such as sensitizers. Understandings of these phenomena indeed have required interdisciplinary knowledge among semiconductor physics, surface chemistry, solid-state chemistry, and so on. In order to facilitate the understandings, we have proposed to assume that the sensing properties are determined by three main factors, that is, receptor function, transducer function, and utility factor, as schematically shown in Figure 1. The first factor is concerned with how each constituent crystal responds to the surrounding atmosphere containing oxygen and target gases (intraparticle issue).

It is unanimous that oxygen is adsorbed on the crystals as negatively charged species, accompanied by the formation of a depletion layer inside the crystals. The target gases disturb the equilibrium through being adsorbed competitively or reacting with the adsorbed oxygen. The foreign substances like sensitizers dispersed on the crystals are assumed to affect these processes anyhow. The second factor is concerned with how the response of each particle is transformed into that of the whole device, and apparently this is related with the mechanism of electron transport between adjacent crystals (inter-particle issue). For a long time a double Schottky barrier model [9], which assumes migration transport of electrons over the barrier as shown, has been advocated for this process without critical check. The third one is concerned with the attenuation of the response due to the effect of diffusion and reaction of reactive target gases through the pores of the assembly of crystals (assembly issue) . The above scheme has explained rather well qualitative nature of semiconductor gas sensors in several respects. However, it has failed to give quantitative understandings and, most importantly, to give new insights leading to innovations of this group of sensors. There should have been some serious defaults included in the scheme, particularly regarding the receptor and transducer functions.

Figure 1: Three factors determining the response of semiconductor gas sensors.

Fortunately, we encountered an interesting finding several years ago that thin film devices fabricated from hydrothermally prepared colloidal suspensions of SnO2 by a spin-coating technique showed temperature—almost independent resistances in air in the temperature range 150–400∘C, as shown in Figure 2. Such thermal behavior of resistances is hardly consistent with the double Schottky barrier model mentioned above. Instead, tunneling transport of electrons across the contacts (or gaps) between adjacent crystals is strongly suggested. In addition, this transport mechanism has made much easier the theoretical modeling of receptor and transducer functions recently carried out [14–16], because the constituent crystals can now be treated independently from each other. As revealed during this process, depletion in small crystals easily goes beyond conventional one (regional depletion) to enter new type one (volume depletion). Obviously, it is a lack of such information that has delayed fundamental understandings of this group of sensors, for most of their valuable gas sensing properties show up in the stage of volume depletion or nearby.

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Semiconductor gas sensor

 A semiconductor gas sensor for use in equipment for detecting small amounts of H2 S. The method of sensor fabrication comprises spray deposition of a mixture of metal oxides mixed together with various metal and non-metal materials which serve in the finished product as activators, dopants, and/or film binder materials, and including in suspension a molecular sieve material, for enhancing and defining porosity on a scale of molecular dimensions in the finished sensor.

All of the foregoing materials are suspended in a suitable solution and preferably sprayed onto a heated insulating substrate to form the finished product. The example sensor, capable of selective detection of H2 S in air and a sensitivity of less than 1 PPM (part per million), is comprised of a platinum activated alumina, tin oxide, and zeolite molecular sieve material.

FIELD OF THE INVENTION
This invention relates to semiconductor gas sensors and to methods of fabrication thereof, and more particularly to a unique spray deposition method wherein an improved semiconductor sensor is fabricated which comprises preselected gas sensor components in combination with a molecular sieve material to enhance and define porosity in the final semiconducting film. A specific example of a sensor and its method of fabrication is described which is capable of a selective detection of H2 S by changes in the conductivity of the sensor relative to the concentration of H2 S in the gas sample.

BACKGROUND OF THE INVENTION
Various semiconducting metal oxides have been used in conjunction with a variety of metal and non-metal additives in the fabrication of gas sensitive films suitable for use in gas detection apparatus. Exposure of such gas sensitive films to the gas of interest generally is detected as a change in conductivity of the film. In general, these prior devices exhibited inherent deficiencies in sensitivity, selectivity, response and recovery times, and/or calibration stability.

The electrical characteristics and subsequent gas response characteristics of such materials when employed as gas sensors in previous gas sensing equipment have been found to be highly dependent upon film properties such as thickness, uniformity of composition, purity, film porosity, and density. Since it has previously been difficult to adequately control the foregoing factors this art has been seeking a technique of fabrication which would be capable of producing films with the above mentioned and other properties well controlled.

In addition it is of course desireable that any new technique should be reproducible and cost effective. Further, the previous sensors were sometimes of limited utility if they were not capable of low temperature operation. This property is advantageous when sensing flammable gases in that there would be a reduced hazard of flammable gas ignition by the operating sensor, as well as an increased realiability and sensor life, reduced sensor power requirements, and better compatibility with on-chip integrated signal processing circuitry.

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lobal Semiconductor gas sensor Markets Size, Analysis, Share, Growth and Forecasts 2015

2015 Global Semiconductor gas sensor Industry Report is a professional and in-depth research report on the world’s major regional market conditions of the Semiconductor gas sensor industry, focusing on the main regions (North America, Europe and Asia) and the main countries (United States, Germany, Japan and China).

The report firstly introduced the Semiconductor gas sensor basics: definitions, classifications, applications and industry chain overview; industry policies and plans; product specifications; manufacturing processes; cost structures and so on. Then it analyzed the world’s main region market conditions, including the product price, profit, capacity, production, capacity utilization, supply, demand and industry growth rate etc. In the end, the report introduced new project SWOT analysis, investment feasibility analysis, and investment return analysis.

The report includes six parts, dealing with: 1.) basic information; 2.) the Asia Semiconductor gas sensor industry; 3.) the North American Semiconductor gas sensor industry; 4.) the European Semiconductor gas sensor industry; 5.) market entry and investment feasibility; and 6.) the report conclusion.

Part I Semiconductor gas sensor Industry Overview
Chapter One Semiconductor gas sensor Industry Overview
1.1 Semiconductor gas sensor Definition
1.2 Semiconductor gas sensor Classification Analysis
1.2.1 Semiconductor gas sensor Main Classification Analysis
1.2.2 Semiconductor gas sensor Main Classification Share Analysis
1.3 Semiconductor gas sensor Application Analysis
1.3.1 Semiconductor gas sensor Main Application Analysis
1.3.2 Semiconductor gas sensor Main Application Share Analysis
1.4 Semiconductor gas sensor Industry Chain Structure Analysis
1.5 Semiconductor gas sensor Industry Development Overview
1.5.1 Semiconductor gas sensor Product History Development Overview
1.5.1 Semiconductor gas sensor Product Market Development Overview
1.6 Semiconductor gas sensor Global Market Comparison Analysis
1.6.1 Semiconductor gas sensor Global Import Market Analysis
1.6.2 Semiconductor gas sensor Global Export Market Analysis
1.6.3 Semiconductor gas sensor Global Main Region Market Analysis
1.6.4 Semiconductor gas sensor Global Market Comparison Analysis
1.6.5 Semiconductor gas sensor Global Market Development Trend Analysis
Chapter Two Semiconductor gas sensor Up and Down Stream Industry Analysis
2.1 Upstream Raw Materials Analysis
2.1.1 Upstream Raw Materials Price Analysis
2.1.2 Upstream Raw Materials Market Analysis
2.1.3 Upstream Raw Materials Market Trend
2.2 Down Stream Market Analysis
2.1.1 Down Stream Market Analysis
2.2.2 Down Stream Demand Analysis
2.2.3 Down Stream Market Trend Analysis

Part II Asia Semiconductor gas sensor Industry (The Report Company Including the Below Listed But Not All)
Chapter Three Asia Semiconductor gas sensor Market Analysis
3.1 Asia Semiconductor gas sensor Product Development History
3.2 Asia Semiconductor gas sensor Process Development History
3.3 Asia Semiconductor gas sensor Industry Policy and Plan Analysis
3.4 Asia Semiconductor gas sensor Competitive Landscape Analysis
3.5 Asia Semiconductor gas sensor Market Development Trend

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