R. Lalauze

Tin dioxide thin-film gas sensor prepared by chemical vapour deposition: Influence of grain size and thickness on the electrical properties

Tin dioxide films are elaborated by a chemical vapour deposition (CVD) method. An accurate control of deposition parameters (temperature, total pressure, duration) so that appropriate annealing conditions (duration, temperature) can be used to modify the structural properties of the films: grain size, thickness, and stoichiometry. Important modifications of electrical performances in tin dioxide films for gas-sensing applications are observed. A correlation between structural properties of CVD films and their electrical behaviour is proposed. The main results are: (i) a sharp increase in the electrical conductance under pure air G0 from a critical value of the grain size D=2L, due to the apparition of a conduction channel between adjacent grains; the depletion layer L is evaluated to 35 A; (ii) a dependence of the electrical conductance G0 with stoichiometry observed for various deposition temperatures and various annealing conditions; the predominant effect of stoichiometry variations for films deposited at high temperature (100–300 A grain size range) is responsible for the decrease of G0, and (iii) a strong influence of film thickness e, with a maximum of sensitivity for the thinnest films, in which tin dioxide is more discontinuous and disordered, and an increase in G0 with e due to the increase of the number of percolation paths up to 3000 A corresponding to a percolation threshold.

Thin films for gas sensors

Abstract Thin tin dioxide films are used as sensitive elements for gas sensors. The physical properties of such materials and their electrical performance largely depend on their methods of production. This work presents two methods used to produce thin dioxide films, i.e., chemical vapour deposition and reactive evaporation. Optimization of the deposition parameters enables us to process highly sensitive materials and even materials able to discriminate between certain gases, such as alcohol, benzene or hydrogen sulphide.

High-sensitivity materials for gas detection

Presently, many test results tend to prove that the effects of a gas on the electrical properties of tin dioxide are rather complex. Some aspects, such as the presence of a peak on the conductance curves as a function of temperature, may be explained by the combination of several regulating effects such as potential barriers at grain boundaries or the materials intrinsic resistance. This enabled us to state precisely the part played by some morphological aspects, such as grain size, on the electrical performance of the material. Sintered materials obtained through the compression of a commercial powder, as well as materials produced in the form of thin films through the CVD technique (Chemical Vapour Deposition), are considered in this study. The results obtained demonstrate that the preparation and annealing conditions play a major part in the nature and amplitude of the effects observed with methane and alcohol. It should be noted, in particular, that neither the amplitude nor the position of the conductivity peak as a function of the temperature can be varied. Through the differences observed between methane and alcohol, it is possible to conclude that surface chemical reactions are more particularly located at grain boundaries.

SnO2 gas sensor. Effect of SO2 treatment on the electrical properties of SnO2

Abstract The influence of a SO 2 treatment on the electrical properties of SnO 2 has been studied. It is shown that some gaseous compounds induce an important maximum on the curve of the electrical conductance as a function of the temperature of SnO 2 treated with SO 2 . The temperature at the maximum conductance is connected with the nature of the electrical contacts (gold or platinum) and an important part is associated with the metal-oxide interface process. The selective aspect of the electrical response can be exploited to develop a benzene detector.

Experimental study of NiO electrical conductivity changes under low oxygen pressures

The electrical conductivity of NiO was measured at 740°C in an oxygen pressure range of 10−2 −1.3 Torr. By means of continuous recording, longtime experiments were performed. The results show that for any admittance of oxygen, the electrical conductivity initially increased and then decreased to its initial value. For pressures higher than 0.1 Torr the decrease of the signal was reduced and the time required to attain the initial value sometimes reached several days. These results suggest that the electrical conductivity changes may be considered as a transitory phenomenon connected to attaining gassolid equilibrium.

A new type of mixed potential sensor using a thick film of beta-alumina

A particular device configuration for a mixed potential sensor has been previously described [l]. It consists in using electrodes with different catalytic properties, one in platinum, the other in gold: they are both in contact with the same gas mixture. The potential that appears at each electrode depends on the reactions which occur and their kinetics. The electrolyte used may be stabilized zirconia or sodium beta-alumina. These satisfying electrical performances have led us to develop a sensor elaborated from a thick film coating using the same sol-gel processing as used for powder synthesis. This type of sensor should be more suited to industrial requirements as miniaturization.

An electrical percolation model for tin dioxide polycrystalline thin films

The physical properties of tin dioxide and its electrical performance largely depend on its morphological properties. As mentioned by several authors, the size of crystallites relative to the Debye length appears to be a determining factor for the sensitivity to gas. In a previous paper concerning thin films elaborated by reactive evaporation, it has been established that the electrical conductivity value depends on the relative density of the film.

Dual response of tin dioxide gas sensors characteristic of gaseous carbon tetrachloride

Abstract A unique dual response induced by gaseous carbon tetrachloride (CCl4) in the conductance of tin dioxide (SnO2) sensors has been tested to establish a basis for developing a specific sensor for detecting this gas in industrial environments. The electrical conductance of the sensors appeared to increase or decrease when exposed to gaseous CCl4, depending on the operation temperature. This is considered opportune for the specific detection of gaseous CCl4. Different types of SnO2 samples have been tested. One batch is prepared by die-pressing commercial powder at 200 MPa. These samples, thickness 1 mm and diameter 13 mm, are sintered at 1000 or 1300 °C. Thick-film sensors of pure SnO2 or containing palladium (Pd) as a catalytic additive are sintered at 850 °C. Electrical measurements are performed with synthetic air or dried ambient air circulating through the measuring chamber, which is a quartz tube placed in a temperature-controlled oven. An atmosphere containing CCl4 is produced by introducing a small bottle provided with a permeation cap filled with liquid CCl4 into the tubing of the gas-circulation system. Based on the weight loss depending on the permeation membrane, this is calculated to produce a CCl4 concentration of 200, 500 or 1000 ppm. Thick-film sensors with integrated heater resistors are also tested in pairs in a glass chamber connected to the gas circulation system. One sensor in each pair is maintained at the operation temperature of 170 °C, while the other is maintained at 450 °C. This arrangement serves as the proto-type for a series sensor containing two units operating at different temperatures. The results indicate that this combination of two sensor elements operating at different temperatures can form a good basis for the specific detection of gaseous CCl4. Due to the high initial rate of the change in conductance induced by gaseous CCl4, a response time of the order of a minute can be obtained. The concentration can be deduced from either the magnitude of the response or the initial rate of the response.

Theoretical study of heterogeneous reaction kinetics: A comparison between microcalorimetric and thermogravimetric curves

In the kinetic theories of Bodenstein or Semenov the expression for the rate of a chemical reaction with several elementary stages can be expressed by different physical parameters. If two experimental methods are used, one method of necessity being microcalorimetry to measure the thermal flux produced by the reaction, it is possible to distinguish a pure kinetics case from a mixed one. The two-method technique has been verified by a study of the oxidation of niobium.

Influence de l'oxygène adsorbé sur la sulfatation de l'oxyde de nickel par le dioxyde de soufre

The interactions between gaseous sulfur dioxide and nickel oxide at 250° have been investigated by means of temperature-programmed desorption (TPD) and microcalorimetric techniques. Depending on the thermal treatment of the oxide, different NiO samples were prepared containing chemisorbed oxygen species, O1, O2, O3 and O4, with different energies. The calorimetric data indicated that the weakly-bound species O1 is the most reactive of the oxygen forms towards sulfur dioxide. In this case, the TPD curves (m/e=48 andm/e=32) show a new peak at 710°; this may be correlated with the desorption of a sulfur-containing compound, probably NiSO4, created by oxidation of sulfur according to the reaction SIV→SVI. The regeneration of the species O1 has been studied as a function of the temperature of oxygen adsorption; the most favourable temperature for the sulfation of nickel oxide appears to be about 400°.ZusammenfassungDie Wechselwirkung zwischen gasförmigen Schwefeldioxid und Nickeloxid bei 250° wurde mittels temperaturprogrammierter Desorption (TPD) und mikrokalorimetrischer Techniken untersucht. Abhängig von der thermischen Behandlung der Proben wurden verschiedene, die Sauerstoffspecies O1, O2, O3 und O4 mit unterschiedlichen Energien enthaltenden NiO-Proben hergestellt. Die kalorimetrischen Daten weisen darauf hin, dass die schwach gebundene Species O1 gegenüber Schwefeldioxid die reaktivste von diesen Sauerstoffarmen ist. In diesem Falle zeigt die TPD-Kurve (m/e=48 undm/e=32) einen neuen Peak bei 710°; das kann mit der Desorption einer schwefelhaltigen Verbindung, wahrscheinlich NiSO4, erklärt werden, die durch Oxydation von Schwefel entsprechend der Reaktion SIV→SVI gebildet wird. Die Regeneration der Species O1 wurde in Abhängigkeit von der Temperatur der Sauerstoffadsorption untersucht; die günstigste Temperatur für die Überführung von Nickeloxid in das Sulfat scheint etwa 400° zu sein.РезюмеМетодом температурн о-программированной десорбции и микрокал ориметрии исследовано при темп ературе 250° взаимодейс твие между газообразной двуоки сью серы и окисью никеля. В зави симости от термическ ой обработки окисла, пол учены различные образцы NiO, содержащие х емисорбционные част ицы кислорода O1, O2, O3 и O4 с раз личными энергиами. Калоримет рические данные пока зали, что из всех форм кисло рода, слабосвязанные част ицы O1 являются наибол ее реакционно-способны ми по отношению к двуокиси серы. Для эт ой формы кислорода, кр ивые температурно-програ ммированной десорбции (м/е=48 и 32) показ али новый пик при 710°, чт о может быть связано с десорб цией серосодержащего сое динения, возможно NiSO4 образующегося вслед ствии реакции окисления че тырехвалентной серы до шести валентной. Регенерац ия O1 была изучена в зависимости от темп ературы адсорбции кислорода. Установле но, что наиболее благоприятной темпе ратурой сульфирован ия никеля является температур а 400°.