WO1998003857A1

WO1998003857A1 - Thick-film sensor for measuring humidity

Info

Publication number: WO1998003857A1
Application number: PCT/SE1997/001314
Authority: WO
Inventors: Mara Larini
Original assignee: Louis Gibeck Ab
Priority date: 1996-07-23
Filing date: 1997-07-21
Publication date: 1998-01-29
Also published as: ITFI960175A1; IT1286680B1; AU3715097A

Abstract

The invention relates to a sensor which may be used to measure the humidity of air or of other gas mixtures, this sensor consisting of an alumina support on one face of which are printed two metal (preferably, but not necessarily, platinum) or carbon electrodes, covered with a layer of ceramic as a thick-film whose impedance decreases as the partial pressure of water vapour (pH2O) increases and which is prepared, according to the invention, by firing, at a temperature of 600-850 °C, an aqueous-alcoholic mixture consisting of an alkali metal fluoride [e.g. LiF (50-200 mg)], an alkaline-earth metal fluoride [e.g. BaF2 (50-300 mg)], a rare-earth metal oxide [e.g. Nb2O5 (100-200 mg)], an uranium salt [e.g. UO2CO3 (50-200 mg)] and silicon oxide [SiO2 (10-100 mg)], while the other face has a heating element which prevents the formation of condensation, and one or more thermistors for measuring the temperature of the sensor and/or of the gas phase.

Description

THICK-FILM SENSOR FOR MEASURING HUMIDITY

DESCRIPTION

-Prior Art

Knowing the amount of water vapour present in the air (humidity, RH%, pH₂0) is often necessary in scientific research and in many technical fields, from agriculture to aeronautics, from chemical analysis to medi- cine, from the manufacture of fabrics to that of computers etc .

Various methods and various instruments for measuring the humidity have been developed over the years . - Psychrometer: based on measuring the decrease in temperature due to evaporation: slow method, cannot be used at sub-zero temperatures, relies on accurate and continuous maintenance.

Air hygrometer: mechanical type instrument, of poor accuracy.

Poly er: sensor with polymer film which, by adsorbing or deadsorbing water vapour, changes either the capacitance or the electrical conductivity; exhibits serious problems in the long term and at high values of

CONFIRMATION COPY humidity .

Electrolyte: an electrolyte solution is placed in equilibrium with the pH₂0 with subsequent variation of the electrical conductivity. - Sulphuric acid: the acid solution is placed in equilibrium with the pH-O, at the same time giving rise to a temperature variation.

Dew point: uses an optical detector or a capacitative detector; the instrumentation is complex and requires excessive maintenance.

Infrared absorption: based on the absorption band of water in the infrared region; the instrumentation is complex and expensive and is of limited use at high values of humidity and in the presence of fog. Humidity sensors which make use of inorganic ceramic semiconducting (at temperatures > 300°C) and non- semiconducting (at temperatures < 100°C) compounds have recently been proposed in the scientific literature.

These thick-film or thin-film humidity sensors are formed of an inert support of alumina, on one face of which are printed a pair of electrodes made of noble metal, on which a layer of a suitable mixture is spread which, when exposed to suitable temperatures, is converted into an adherent and mechanically strong ceramic layer. The ceramic layer thus formed is placed in equilibrium with the pH₂0, and the water molecules which enter into its structure cause a change in the impedance which is measured between the two metal electrodes.

It is possible to have ceramic layers in which the adsorption of water molecules causes hydrolysis of the hydroxyl groups at the surface and the release of protons or of alkaline ions which produce an increase in the ionic-type conductivity.

The choice and the percentage of the various chemical components of the ceramic compound are determining factors in the functioning of the sensor. Thus, many researchers are studying compounds and various types of mixtures in order to obtain more efficient sensors.

When brought to high temperatures, other types of ceramic systems acquire semiconductor properties and, in this case, their electrical conductivity changes with the pH_jO since the water molecules act as electron donors . - Description of the invention The aim of the present invention is to produce a sensor of ceramic type which responds very quickly -and is extremely stable mechanically even when exposed to high values of pH₂0 for long periods.

Thus, a sensor for detecting humidity has been developed, which consists of an insulating inert support, on one face of which is firstly printed a pair of metal or carbon electrodes which are preferably, but not necessarily, made of platinum, on which is then placed a layer of a mixture which, after firing at a suitable temperature, is converted into a ceramic layer whose impedance changes as a function of the amount of water which is adsorbed on its surface, while the opposite face has a heating element supplied with a voltage of suitable value, this heating element enabling the temperature of the sensitive part to be raised by a few degrees so as to prevent the condensation of water on the sensor, the sensor lastly being supplemented by one or more thermistors for measuring the temperature. According to a preferred solution, the above-mentioned heating element is supplied with direct current at a stabilized voltage.

According to the present invention, a preferred composition for the mixture to be deposited on the electrodes comprises compounds chosen from alkali metal fluorides, alkaline-earth metal fluorides and rare-earth metal oxides, together with uranyl oxide and silicon oxide. According to a mixture example given in Table I, these compounds are mixed with water and ethyl alcohol and the mixture thus obtained is deposited xerographically onto the alumina support so as to cover part of the two electrodes with a layer about 200 microns in thickness . TABLE I

Preferred composition of the mixture for the preparation of the sensitive ceramic layer (amount by weight)

LiF from 50 to 200 mg BaF₂ from 150 to 300 mg

Nb₂0₅ from 100 to 200 mg

U0₂C0₃ from 50 to 200 mg

Si0₂ from 10 to 100 mg

After drying in air, the system is then placed in an oven at 600-850 °C for about 15 minutes in order to partially melt the layer deposited, converting it into a very adherent and mechanically strong compound of oxides

+ fluorides .

With the heating element supplied so as to bring the entire support of the sensor to between 30 and 50 °C, for example, the impedance of the ceramic layer varies with the pH₂0 according to a logarithmic law within the

RH operating range of between 30 and 100%.

With reference to Table A, which gives, by way of non-limiting example, a preferred embodiment of the invention, characteristics and advantages of the invention will now be described.

Fig. 1 is a view of the upper face of a sensor for detecting humidity, produced according to the present invention;

Fig. 2 is a view of the lower face of the sensor of Fig. 1.

Figs. 1 and 2 show the sensor for detecting the humidity of air (relative humidity RH%) . The sensor is prepared on an inert alumina support 10 and comprises two interlaced electrodes made of noble metal and printed on one face of tho support 10, represented by 14 and 15. A sensitive ceramic film 16, obtained by exposing a mixture - preferably of a composition exemplified in Table I - to high temperatures, is spread on the electrodes 14 and 15.

The lower face of the support 10 carries a heating element, represented by 21, supplied with a con- stant preset voltage. The sensor is provided with the two conductors 12 and 13 which are connected to the electrodes 15 and 14, and with the conductors 17 and 20, which are connected to the heating element 21. A thermistor 22 may be placed on the lower face of the support 10, this thermistor being connected to the conductors 18 and 19 for measuring the temperature of the sensor, or for measuring the temperature of the gas phase. The thermistor for measuring the temperature of the gas phase may be placed on the alumina substrate or may be placed (23) close to the sensor.

The sensor is preferably provided with a container, not shown, which may be made, for example, of porous material which is very permeable to air, but is capable of protecting it, especially when the sensor is used in an outdoor environment.

The impedance of the sensor varies with the adsorption and deadsorption, by the ceramic layer 16 of the water contained in the gas mixture; in particular, the impedance decreases as the pH₂0 increases, according to a logarithmic law within the range of humidity values of practical interest, between 30 and 100% RH. A general A.C. bridge for measuring the impedance may be used for measuring the value of the resistance of the sensor as a function of the pH₂0.

The heating element 21, whose purpose is to increase the temperature of the sensor in order to prevent condensation, is, as has been stated, supplied with a constant direct-current voltage of preset value. The value of the voltage is chosen so as to maintain the sensor at a temperature which is a determined amount DT above room temperature.

The sensor may be calibrated in relative humidity (RH%) or, when the temperature is also measured, it may be calibrated in absolute humidity (mg_{H 0}/l) .

Claims

1. Sensor for detecting the humidity of air or of other gas mixtures, this sensor being formed of an insulating support (10) provided, on one face, with a pair of metal or carbon electrodes (14, 15), covered with a thick-film layer which is treated at a high temperature (16), the impedance of this sensor decreasing as the pH₂0 increases, and its initial composition comprising an alkali metal fluoride [e.g. LiF (50-200 mg) ] , an alkaline-earth metal fluoride [e.g. BaF₂ (150-300 mg) ] , a rare-earth metal oxide [e.g. Nb₂0₅ (100-200 mg) ] , a uranium salt [e.g. U0₂C0₃ (50-200 mg) ] and silicon oxide

[Si0₂ (10-100 mg) ] , the sensor being supplemented by a heating element (21) which prevents the formation of condensation thereon.

2. Humidity sensor according to Claim 1, characterized in that the heating element (21) is placed on the opposite face of the substrate to that which carries the sensitive layer (16) .

3. Humidity sensor according to Claim 1, characterized in that a thermistor (22) for measuring and/or controlling the temperature of the sensitive layer is also placed on the opposite face to that which carries the sensitive layer.

4. Humidity sensor according to Claim 1, characterized in that the sensor is supplemented by a thermistor (23) which can measure the temperature of the gas phase, the said thermistor being located on the insulating substrate or alternatively being placed close to the sensitive layer.

Florence, 23 July 1996