Resistive Oxygen Sensors Research Articles

Kinetics of oxygen exchange in strontium titanate

The kinetics of oxygen exchange are of primary importance for the application of titanates as fast resistive oxygen sensors. The sensor’s conductivity is correlated with the oxygen partial pressure pO2 of the surrounding atmosphere: Due to oxygen surface transfer and subsequent diffusion of oxygen vacancies V O ·· , a pO2 change gives rise to a conductivity change of the sample. While bulk diffusion usually occurs very fast, the surface transfer reaction becomes the rate determining step for thin samples and for low temperatures. We have shown that in the case of acceptor doped SrTiO3 the kinetics of the surface transfer reaction can be strongly influenced through stoichiometric changes brought about by thin coatings of alkaline earth metal oxides (e.g. SrO). In contrast to the commonly used jump method (conductivity response to a sudden pO2 change in the time domain), a model is presented which is based on the frequency-domain analysis of amplitude and phase shift of the response signal obtained from a pO2 modulation in a fast kinetic measurement set-up. This method allows not only for measuring response times in the sub-millisecond range but also for distinguishing between behaviour either controlled by volume diffusion or by surface transfer reaction.

Materials for temperature independent resistive oxygen sensors for combustion exhaust gas control

Acceptor and donor doped SrTi 1− x Fe x O 3− δ materials for novel temperature independent resistive oxygen sensors for lean-burn engine exhaust gases were prepared and characterized by X-ray diffraction. Their electrical resistance, R, was investigated in the oxygen partial pressure range from 10 −4 to 1 bar between 700°C and 1000°C. Doped and undoped samples with x=0.3 obey an R∝ pO 2 −1/5 power law. Undoped samples show negligible temperature dependence in a small oxygen partial pressure ( pO 2) range between 10 −2 and 3×10 −2 bar. Acceptor (Ga) doping shifts the pO 2 range of negligible temperature dependence to lower pO 2, whereas donor (La) doping results in a right-hand shift to higher pO 2. Combined acceptor and donor doping leads to an extended pO 2 range of negligible temperature dependence. At pO 2=10 −1 bar the temperature coefficient of resistance (TCR) of this material composition is always below 250 ppm/K in the complete investigated temperature range. At about 725°C and 975°C TCR=0 can be found.

Evaluation of SrTi 1− yNb yO 3+ δ materials for gas sensors

Porous samples were used to evaluate the dependence of electrical conductivity on the oxygen partial pressure. Results obtained with these porous samples revealed that Nb for Ti substitution suppresses the p-type contribution even in oxidizing conditions, and enhances the n-type conductivity. The conductivity of porous SrTi 1− y Nb y O 3+ δ samples can be described by the power law dependence on p −1/4 O 2 , except possibly in very reducing conditions. The response time of porous samples is short, as required for resistive oxygen sensors.

Sr(Ti,Fe)O3: Materials for a Temperature Independent Resistive Oxygen Sensor

AbstractThe metal oxide strontium titanate is a promising material for resistive high temperature oxygen sensors (T > 1000 K), that can be applied to control combustion engines. However, a disadvantage of this and of many other metal oxides is the strong temperature dependence of conductivity. In this work, we show that the temperature dependence of Sr(Ti1-x, Fex)O3-δ can be adjusted by the iron content. For x = 0.35 the thermal activation of the conductivity is near zero for a distinct temperature and oxygen partial pressure range (T = 1000 K - 1200 K, PO2 = 10 to 105 Pa). Short response times in the range of 10 ms can be realized by using sensors in thick film technology

Oxygen Tracer Diffusion in Lanthanum‐Doped Single‐Crystal Strontium Titanate

Strontium titanate (SrTiO3) is known as a good high‐temperature resistive oxygen sensor material; its response time depends on oxygen bulk diffusion and surface exchange processes. In the present work, 18O diffusion has been investigated in lanthanum‐doped SrTiO3, single crystals in the temperature range 700° to 900°C by secondary ion mass spectrometry (SIMS). Oxygen tracer diffusivities between 2 × 10−15 and 1 × 10−13 cm2/s have been calculated from the SIMS results. Low surface enrichment of 18O compared to the 18O concentration in the gas atmosphere gives clear evidence for a surface exchange reaction.

A high-temperature metallic oxide resistive oxygen sensor

The efficacy of a resistive thick film of YBa 2Cu 3O x on an yttria-stabilized zirconia substrate for monitoring the concentration of oxygen in a gas mixture has been experimentally evaluated over the temperature range 600–680 °C and oxygen concentrations corresponding to a partial pressure from 0.009 to 1 atmosphere. The mixed oxide shows a fast oxygen indiffusion but relatively slow outdiffusion. The sensor resistance decreases quantitatively with a decrease in the gas temperature and an increase in the partial pressure of oxygen. The lower resistance with higher oxygen concentration may be attributed to the increase in the concentration of positive holes in the oxide film, and vice versa. Corresponding to a sensor resistance of R ohms, the oxygen partial pressure ( P O 2 atm) in a gas mixture at a temperature T K is given by the correlation ▪

The influence of the surface transfer reaction on the response characteristics of resistive oxygen sensors

The response characteristic of resistive oxygen sensors based on sputtered films and thin single crystals of SrTiO 3 has been investigated at high temperatures and different oxygen partial pressures. The frequency response function has been determined in the range 10 −2–2 × 10 3 Hz. Depending on the temperature of the sensor, the kinetic behaviour is controlled either by the surface transfer reaction of the oxygen or by the diffusion of oxygen vacancies in the bulk. The transfer reaction, which plays a decisive role in the development of fast oxygen sensors, was also investigated on single crystals of different thicknesses. The main influences determining the rate of chemical reaction are outlined.

Kinetic behaviour of resistive oxygen sensors

A novel method to measure the response characteristic of gas sensors is presented. Using single crystals as a model system, their response characteristics are examined and compared with those of sputtered SrTiO 3 thin-film oxygen sensors at temperatures of 800–1000 °C and frequencies of 10 −3-10 2 Hz. The measurement of the kinetic behaviour in this frequency range allows us to distinguish between behaviour controlled by surface transfer reaction or volume diffusion. Whereas the response characteristics of thick single crystals are dominated by diffusion, the oxygen transfer reaction at the solid-gas interface becomes the rate-determining step for thin single crystals and sputtered films of SrTiO 3.

Response times of resistive thick-film oxygen sensors

Resistive oxygen sensors based on SrTiO 3 show a reproducible change in the sensor resistance as a function of the oxygen partial pressure ( pO 2) of the atmosphere surrounding them. The sensor's response behaviour after a step in pO 2 is determined by two consecutive processes, the transfer reactions of the oxygen at the surface and the diffusion of oxygen vacancies in the interior of the solid. Previous investigations showed that the kinetics of (200–1000 μm thick) densely sintered SrTiO 3 ceramics is controlled by diffusion and the surface reactions are negligibly fast. Thus, the response times can be reduced by the square of the sample thickness, which determines the diffusion length. In porous thick films, however, the diffusion length is determined by the grain size (φ ⩽ 1 μm) and is therefore extremely short. A variation in grain size here leads to a linear change in the response time. Thus it can be assumed that the kinetics are no longer controlled by diffusion but by the rate of the surface reactions, and the expected fast response behaviour cannot be achieved. In spite of this rate-limiting effect, response times much less than one second are possible with thick-film sensors.

Ambipolar diffusion phenomena in BaTiO3 and SrTiO3

The electrical conductivity of resistive oxygen sensors based on semiconducting titanates (BaTiO3, SrTiO3) is determined by the amount and the ratio of the different concentrations of atomic defects in the sensor material. At sufficiently high temperatures the sensor exchanges oxygen with the surrounding gas atmosphere resulting in variations of the concentration of defects. The kinetics of these oxygen exchange processes are determined by the diffusion of the defects in the solid. In this paper the diffusion coefficients of the defects, which are decisive for the electrical conductivity, are determined by measurement of the conductivity during the diffusion processes. The evaluation of these results by an analytical model which considers the interaction between all simultaneously diffusing types of defects, allows a survey of the effects influencing the diffusion rate. With this knowledge, it is possible to estimate the influence of grain boundaries, acceptors, temperature and oxygen pressure on the response time of the sensor.