Sheikh A. Akbar

Oxygen sensors: Materials, methods, designs and applications

Advancement of gas sensor technology over the past few decades has led to significant progress in pollution control and thereby, to environmental protection. An excellent example is the control of automobile exhaust emissions, made possible by the use of oxygen gas sensors. Since early 1970s there have been sustained studies on oxygen sensors and has led to development of sensors for various applications with varying performance characteristics. Solid electrolyte based potentiometric, amperometric and metal oxide based semiconducting resistive type sensors are used for high temperature applications. For solution-based pollution monitoring, dissolved oxygen sensors based on Clark electrodes have played a major role. More recently, for biological and medical applications, optical oxygen sensors are beginning to have an impact. In this review, we focus on both high temperature as well as dissolved oxygen sensors and compare the different methods of oxygen sensing, discuss underlying principles, and outline the designs and specific applications.

Bismuth oxide-based solid electrolytes for fuel cells

During the last three decades, a large number of investigations has been reported pertaining to the science and technology of solid oxide fuel cells (SOFCs) based mainly on the yttria-stabilized zirconia (YSZ) electrolyte. Because of the problems associated with the high temperature of operation (~ 1000°C) of the YSZ-based cells, there has been a substantial effort to develop alternative electrolytes with ionic conductivity comparable to that of YSZ at relatively lower temperatures. This review presents a systematic evolution in the area of the development of new electrolytes based on bismuth sesquioxide for fuel cell applications at moderate temperatures.

Titanium dioxide based high temperature carbon monoxide selective sensor

The anatase form of TiO2 has been examined for the sensing of CO and CH4 at temperatures of 873 K. Though, there were differences in the sensitivity of the anatase sensor towards CO and CH4, both gases showed considerable resistance changes. However, in the presence of lanthanum oxide and copper oxide (labeled as ALC sensor), the sensor showed minimal response towards CH4, while still exhibiting sensitivity towards CO. The insensitivity towards CH4 was also confirmed by measuring the sensor response in the presence of both gases. In order to understand the basis for selective CO sensing, diffuse reflectance infrared spectroscopy was carried out on the sensor materials at elevated temperatures. Lanthanum oxide was used to inhibit the anatase to rutile transformation. Infrared spectroscopic data strongly suggest that there is a layer of lanthanum oxide on the titania surface, which acts as a trap for the oxidation products of CO and CH4. Upon oxidation of CO on ALC, carbonate species were detected, whereas the reaction of CH4 produced negligible carbonate species. The insensitivity of the ALC sensor towards CH4 is proposed to be due to its rapid oxidation by O2 on the copper oxide. This efficient oxidation was responsible for lack of CH4 reaction on the anatase surface, thus, producing minimal resistance change. CO oxidation also occurred partially on the CuO surface but significant reaction also occurred on the anatase surface and produced a change in resistance. # 2001 Elsevier Science B.V. All rights reserved.

Ceramics for chemical sensing

Many types of sensors have been developed to detect chemical species in the gas phase. These include optical based on color change or fluoresence, surface acoustic wave (SAW) devices, electrochemical, chemoresistive/semiconductive, field effect transistors (FET), metal-insulator-semiconductor (MIS) diode devices, and many other. Among these, resistive type sensors based on ceramic oxides are particularly attractive because of their low cost, wide range of applications and potential for use in electronic nose. This article focuses mainly on the resistive/semiconductive, especially the surface conductive ceramic oxide type gas sensors. The main emphasis is on the basic principles involving gas-solid reactions. Also discussed are selected applications with an emphasis on sensor design issues. Since SnO2 can be used as a model system for oxide-based sensors, most of the discussions focuses on this system, though other systems are occasionally highlighted illustrating recent developments.

Pyrolysis of Negative Photoresists to Fabricate Carbon Structures for Microelectromechanical Systems and Electrochemical Applications

Carbon structures were fabricated by the pyrolysis of photopatterned negative photoresists (SU-8 and photosensitive polyimide) on silicon and fused silica wafers. Results here are compared with those of positive resists published earlier by this group. Negative resist films need exposure to ultraviolet light prior to pyrolysis to produce carbon films. The pyrolysis was carried out in a closed quartz tube furnace in a forming gas (95% N 2 , 5% H 2 ) atmosphere. The pyrolysis process was characterized using a combination of thermogravimetric analysis and differential thermal analysis. The pyrolysis of SU-8 involved gas evolution in a narower range of temperature than polyimide, The adhesion of the carbon film was found to depend on the resist, the substrate, and the heating cycle used. The carbon structures were characterized in terms of their shrinkage during the pyrolysis, the resistivity, the degree of crystallinity and the peak separation in cyclic voltammetry. Carbons derived from pyrolysis of negative resists showed higher resistivity, vertical shrinkage, and peak-to-peak separation voltage than positive resists. Transmission electron microscope results showed a distinct lack of crystallinity even after pyrolysis at 1100°C, unlike the positive resist derived carbon.

Ceramic electrolytes and electrochemical sensors

The electrochemical method involving solid electrolytes has been known as a selective and an accurate way of sensing chemical species in the environment and even in liquid metal for some time. The most successful among the electrochemical sensors are the emission control sensor (λ-sensor) for the automobile engine and the oxygen sensor used in steelmaking, both made of stabilized zirconia. This article presents an overview of basic principles of various types of electrochemical sensors including active (potentiometric) and passive (amperometric) sensors. Recent advances in oxygen (O2), carbon dioxide (CO2) and hydrogen (H2) sensors are also presented.

Electrical properties of high-temperature oxides, borides, carbides, and nitrides

High-temperature materials including oxides, borides, carbides, and nitrides encompass all types of conductors: metallic, semiconducting, and ionic. Their electrical conductivities are generally very sensitive to impurities regardless of the type of conductor. For large band-gap materials, which includes most of the oxides, the conductivities at low temperatures are frequently dominated by impurities or dopants, and intrinsic conduction only becomes significant above a temperature which depends largely on the level of dopant, the band gap and the defect structure of the base material. The borides, carbides, and nitrides of transition metals are metallic conductors with conductivities and temperature coefficients of resistivity comparable to that of their parent metals.

Sensing Mechanism of a Carbon Monoxide Sensor Based on Anatase Titania

This paper reports results on a TiO{sub 2} (anatase)-based CO sensor and proposes a possible surface-controlled sensing mechanism. The complex plane analysis of the ac electrical data provides quantitative evidence supporting the mechanism. The grain-boundary capacitance and conductance, extracted from the impedance plane, were observed to increase with the CO concentration. This effect is attributed to a change in the depletion region thickness and barrier height at the TiO{sub 2} intergranular contact. The analysis of the electrical data combined with x-ray diffraction and x-ray photoelectron spectroscopy observations support the proposed sensing mechanism involving CO adsorption and ionization on the titania surface, and not an oxidation-reduction type reaction as observed in most oxide-based sensors.

Potentiometric CO2 gas sensor with lithium phosphorous oxynitride electrolyte

This work was supported by a grant from the National Science Foundation (EEC-9523358) with matching support from the State of Ohio and an Industrial Consortium.

CERAMIC BASED RESISTIVE SENSORS

This paper reviews the current status and research trends of two types of ceramic based resistive sensors, thermistors and gas sensors. The issues and challenges associated with their development for high temperature applications are examined and discussed. Worldwide research efforts in ceramic based resistive sensors, devoted mostly to resolve the issues of selectivity and stability, are also reviewed. These efforts tend to integrate the results obtained from both empirical and basic science approaches, and focus on various stages of sensor development, including development of new material systems, sensor fabrication and manufacturing techniques, and smart sensor arrays.