PRINCIPLES AND APPLICATIONS OF CERAMIC HUMIDITY SENSORS

This paper presents a brief review of the state of the art in humidity and gas ceramic sensors RD it 
also describes the principle, fabrication and application of the humidity-sensitive Si-doped a-hematite 
(α-Fe2O3) sintered compacts. 
The humidity signal response is characterized by volt-amperometric and impedance spectroscopy 
techniques in the 0-97% relative humidity (RH) range. The response time of the sensor is evaluated 
by 0-60% RH variations. Experimental techniques used to measure the hysteresis, drift, and aging of 
the humidity setasor are presented and the results are discussed. The CO sensitivity is investigated by 
activating the sensor in the 350-450℃ temperature range. 
A humidity-meter prototype, based on the α-Fe2O3 ceramic compact is described, and its electronics 
and features are presented in this paper.


I. INTRODUCTION
Sensors are key elements in the rapidly evolving fields of measurements, instru- mentations, and automated systems.The recent progresses made in improving the reliability and lowering the cost of microprocessors and interface circuits has re- sulted in a higher demand for sensors, which convert physical or chemical quantities in various environments into electrical signal.Different functions and materials have been investigated, and several devices have been put on the market or have become part of sophisticated instrumentations1.Among these materials, functional ceramics have played a major role because of their intrinsic characteristics: they are superior in mechanical strength and chem- ical resistence in most environments and in the reproducibility of the electrical properties.They have also been widely used to satisfy diverse needs for sensing devices, and consistent results have been obtained in the field of atmospheric sensors, i.e., temperature , humidity3-8, and gas sensors9--.
It is well known bulk ceramics are prepared by a relatively simple process consisting of mixing raw materials, forming the part, and sintering.Ceramics are characterized by their unique structure consisting of crystal grains, grain boundaries and, when they are porous, by large surface intra-and inter-granular pores.Both high density and porous ceramics are easily produced by controlling the compact forming, and sintering conditions.Furthermore, in ceramic materials, solid solution or doping can be easily formed and improved properties are relatively easy to obtain 3.4.
Sensitive thick ceramic films can be obtained by the screen printing and firing technique5-16, while the radio frequency sputtering7, chemical vapor deposition 8 and the sol-gel processes 9 enable one to deposit thin ceramic films on different substrates.
In Table I, several ceramic materials with their sensor property and function are reported.The humidity sensitivity property is not included in the table because it will be extensively discussed in the next section.
In porous ceramics, the surface and open pores tend to collect water vapor and gases through chemical and physical adsorption and through condensation.Especially in semiconductor ceramics, electrical properties are largely related to the grain size and the pore size distribution of the open pores.The pore surface conductivity changes with even small variations in the humidity or with the ad- sorption of various gases.Porosity-controlled ceramics are suitable for atmospheric sensing such as humidity and gas sensors.
As noted above, humidity or gas sensors are usually exposed to atmospheric conditions containing various other components.They tend to lose their inherent sensitive properties during use because of a number of complicated physical and chemical processes that occur between these components and the material.Major Thermoluminescence research efforts are directed towards developing highly reliable functional ceramic materials.In addition, most ceramics are not single functional but multifunctional.Therefore, the key to success in developing a single functional material is the technology of masking undesired functions.The absence of selectivity is considered limiting for the employment of some ceramics as sensors.Today electronic technology is making full use of the intrinsic multiple function of the sensor and multifunctional devices are now being devel- oped.These devices can perform various functions with a single element.As an example, in atmospheric-type sensors, there is an increasing demand for multi- functional sensors that can simultaneously and independently measure both the humidity and another gas, or the humidity and the temperature.
Thus, attempts to make multifunctional devices are directed towards making active use of the intrinsic multiple function of the materials.In developing multi- functional devices, it is important to establish the technology that can discriminate signals without cross-talk, in place of the conventional masking technology.Recently, the technology of sensor arrays through by multicomponent analysis has been developed.This technique makes use of the pattern recognition method in which selectivity can be obtained by comparing the measured signal vector with corresponding vectors of group of sensors, which are obtained by independent calibration-5.
Pressure, chemical and gas sensors will have the highest ratio of growth (about 2.5) by the year 2000.Increasing fields of application of the sensor technology, in Europe, will be the plant construction and process technology, cars and transpor- tation vehicles and the construction and safety technology6.
We have developed a humidity sensor based on microporous a-Fe_O3 and a humidity-meter prototype has been assembled and tested.The multifunctional sensitivity of a-Fe203 to humidity and carbon monoxide is now being used to produce a multifunctional device able to detect these gases in the environmental conditions.
In this report, state-of-the-art ceramic humidity sensors are discussed and ex- perimental results summarizing our efforts on a-Fe_O3 humidity and CO sensitivity are presented.

CERAMIC HUMIDITY SENSORS
As humidity is a permanent environmental factor, its control and measurement are particularly important not only for human comfort but also for many industries and technologies.In recent years, major domestic applications of humidity sensors have been the automatic humidity control in air conditioners and the automatic cooking in microwave ovens.In chemical and alimentary industries and in the production of electronic devices, rapid and accurate humidity control is of critical importance.Citation deserves the wide development of greenhouse agriculture that requires accurate sensors for simultaneous control of temperature and humidity.
Among the possible humidity measurements, Relative Humidity (RH) in per- centage, that is the ratio of the actual water pressure to the saturated pressure, is the most widely used.As highlighted by several Authors4'7'8, a humidity sensor needs to satisfy several requirements to fulfill a wide range of applications: 1) sensitivity in a wide RH range, 2) rapid response to the variation of RH and good reproducibility of the electrical signal, 3) good mechanical properties and durability, 4) resistence to contaminants, 5) irisignificant or predictable dependence on tem- perature, 6) possibility of masking undesired functions, 7) fitness to circuitry, and 8) simple structure and low cost.
To fulfill these characteristics in response to the various demands, several types of humidity sensors have been investigated and developed functioning by means of many physical and chemical phenomena.They are usually cl:,ilit:d into elec- trolytes, 'organic polymers and porous ceramics.
Since water evaporates or condenses in an electrolyte depending on the RH of the environment, the electrolyte-type sensors 27 utilize the variation of ionic con- ductivity as a function of RH.The polymeric and ceramic sensors work utilizing the mechanism of the chemical and physical adsorption of water vapor on the surface of the material in a quantity that is proportional to the RH of the sur- rounding environment.The adsorption generates a consequent variation of the capacitance or impedance.
The polymeric type hygrometer consists of a capacitor with a hygroscopic polymer dielectric.The difference in relative dielectric constants between the polymer (between 3 to 6) and the water (er 78.54 at 25C) generates a considerable variation in capacitance when water is absorbed28.
In ceramic sensors, the type of conduction mechanism can be ionic or electronic: in the ionic mechanism, the impedance of the sensor element decreases with n increase in the RH due to the physisorption and capillary condensation of water molecules on the surface of the material2'24.In the electronic type, the logarithm of the conductivity of the element changes linearly with the logarithm of the water vapor concentration.Water molecules act as an electron donating gas and their chemisorption increases or decreases the electronic conductivity depending on whether the material is a.n or p-type semiconductor29.In general, it can be asserted that most sensors, in spite of the differences in materials, utilize a common phenomenon that is the chemical and physical adsorption of water molecules on the surface and capillary condensation in the open pores of the microstructure.
In the next section, the mechanism of water interaction with solid surface, metal oxides in particular, is explained through the most significant interaction theories.In Table II, the materials used to realize the humidity sensors, the principle, the range of application and temperature are summarized.

Mechanism of Water Interaction With Solid Surfaces
The mechanism of interaction of water vapor molecules with a porous metal oxide is explained by Morimoto 2 and Zettlemoyer21.It is proposed that the water vapor chemisorbs on the surface of a metal-oxide to form hydroxyl groups: the hydroxyl group adsorbed on the surface metal cation and the proton forming a second hydroxyl group with an adjacent surface O2-ion.
As highlighted by IR investigation, the first layer of physisorbed water results in doubly hydrogen bonded to the chemisorbed hydroxyls and is practically im- mobile, having restricted freedom of rotation.The succeeding physisorbed layers are ordered in an ice-like structure enrouted to the formation of liquid-like ad- sorbates that are mobile, free to rotate and to exchange protons.
In ionic-type humidity sensors, the conduction mechanism is mainly due to the displacement of protons between the water molecules.Arai explains that the conduction mechanism depends on the surface coverage of adsorbed water.Hopping of H / is predominant when the physisorbed HO is absent or very little.When water is present but the coverage of the surface is not complete, H3 O+ diffusion dominates.When the H20 is abundant, the proton-transfer process dominates.
Finally, at higher RH, capillary condensation takes place in the pores according to the Klvin equation and the conduction mechanism becomes electrolytic2tr-24; pRT In Ps/P In the expression, rk is the Kelvin radius of the micropores, y, p and M are the surface tension (72, 75 dyn/cm at 20 C), the density, and the molecular weight of water, P is vapor partial pressure, and Ps is the vapor pressure at saturation.The saturation corresponds to the capillary condensation in the pores and, under a constant P and temperature, takes place in all the pores with radii up to rk.The smaller the value of rk, the more easily it takes place.The capillary condensation is an important factor in ionic-type humidity sensors.
By taking into account the mechanism of water interaction with metal oxides, it turns out that the humidity sensitivity depends mainly on the microstructure, i.e., the pore size distribution and surface area of the ceramic.These bulk properties are usually controlled by adjusting the particle dimensions of the starting material and the forming and sintering conditions.Figure 1 shows a drawing representing the typical features of a porous ceramic the grains, intergranular pores, (open or closed) intragranular pores within the structure of each grain, and grain boundary pores.Another typical characteristic of porous ceramic is the specific surface, which can be as high as 600 mE/g.Open intragranular and intergranular pores play an important role in the measurement of humidity, since water condenses in the pores with different radii as a function of the RH according to the Kelvin equation.
Figure 2 shows a SEM micrograph of a typical fracture section of a-Fe203 Si- doped ceramic sintered at 950 C. The ceramic shown in the figure was obtained by isostatically pressing at 100 MPa a 130 mE/g specific area acicular a-Fe203 powder, obtained by topotactic decomposition (in high vacuum) of a-FeOOH, with a prior suspension in 3-amino-propyl-trihydroxy-silane solution to yield a 2 weight % Si.Sintering was performed for 1 hour at 950 C. The detailed experimental procedure is reported elsewhere.  TheM micrograph highlights the intergranular pores linked so that the struc- ture forms a network of capillary tubes.Intragranular pores (i.e., the pores formed in each particle during the decomposition a-FeOOH and remaining after the sin- tering) are not visible in the figure.They were seen by transmission electron mi- croscopy (TEM) and the pore size distribution, before and after the sintering, was measured by BET liquid N2 adsorption technique s.
Figure 3 shows the pore size distribution of the ceramic compact obtained by the liquid N2 adsorption BET method, while Figure 4 shows the pore size distri- bution obtained by the mercury intrusion technique.The first diagram mainly  highlights the intragranular micropores with diameters smaller than 300 A; in the second diagram (Fig. 4) are distributed the intergranular mesopores and macro- pores formed within the a-Fe203 particles during sintering.

Device Preparation
With the aim of evaluating the influence of the sintering conditions on the electrical response of microporous metal oxide, 10 mm diameter and 1 mm thick compacts were sintered at 850,900 and 950 C respectively for 1 hour.In Table 3 are reported the main microstructural characteristics as measured by Carlo Erba series 4000 Hg intrusion and Micromeritics Asap 2000 N2 adsorption computerized porosimetries.
The variation of impedance as function of RH was converted into electrical signals by painting RuO2 conductive paste on both sides of the compact, as electrodes, and firing at 800 C for 15 minutes.After firing, the electrodes were porous and did not disturb the adsorption and desorption of water.
The Electrical Response The electrical response was measured using the Volt-Amperometric technique by means of a Keithley current amplifier working at 300 Hz frequency and 1 Vpp 950C SINTEIED POIE lADIUS(micron) FIGURE 4 Hg intrusion pore size distribution.
voltage.The humidity was varied between 0 and 95% with an accuracy of +-1% by mixing dry and water saturated N2 through a MKS model i47 multi-gas con- trollerBy this system, variations of RH around the sensor were obtained in a few seconds.Figure 5 shows the typical plot of the impedance variation (ohm in logarithmic scale) as a function of RH, evaluated for the compacts sintered at different tem- peratures.
The response of the a-Fe203 to the variation of RH is linear in the whole investigated range, allowing for simple circuit design when converting the electrical signal into the RH value.
The intrinsic impedance (i.e., the impedance measured at 0% RH) is in the order of magnitude of 1 MII and permits simple and accurate determination at very low RH values.It decreases from 850 to 950 C sintering as the grain boundaries extension increases.
The three decades impedance magnitude variation allows accurate determi- nation of the signal variation due to small changes in RH.
Sintering at higher temperature (1000-1100 C)31'32 results in poor humidity sensitivity in the lower RH values (0-40%) because of the coalescence of micropores due to the thermal treatment.The better humidity sensitivity obtained for the 850-950 C sintering is explained in terms of a favorable pore size distribution around the condensation critical radius.
In defining the properties of a humidity sensor, the evaluation of the response time to the variation of RH acquires particular importance.In bulk sensors, the response time is controlled by the diffusion rate of the water vapor through the pores therefore depends on the forming and sintering conditions.Figure 6 depicts the response time of the 950 C sintering during adsorption and desorption cycles in the 0-60% RH range at different water vapor flow rates.The residence times, as calculated by the geometry of the cell, are 6.2, 4.5 and 3.7 seconds respectively.From the figure it turns out that the higher the water vapor flow rate the shorter the response time.While the adsorption is faster than the desorption, both processes are very quick. 4. THE CALIBRATION PROCEDURE USED TO CHARACTERIZE THE a-Fe203 HUMIDITY SENSOR Hysteresis (H) and Drift (d) In evaluating the properties of a humidity sensor, the hysteresis (H) and drift (d) must be carefully characterized, because they represent a consistent part of the final error associated with the RH measurement.Generally, this error is not con- stant but is influenced by the operating conditions of the sensor.The hysteresis can be defined as the difference in the recording signal measured when carrying out a full cycle of adsorption and desorption, for example from 11 to 97 back to 11%RH.The drift is the difference between the asymptotic signal value at satu- ration (holding the sensor in saturation conditions for several hours) and the value measured after a shorted time, for example one hour at saturation.
In order to carry out accurate characterization of these parameters, RH standards are needed.The easiest way is to use saturated salt solutions such as LiCI, NaCI, K2SO4 that, at a certain temperature, have a constant equilibrium water partial  pressure and can furnish a fixed and stable value of RH.Moreover, the dependence on temperature of RH is very small.In this way, 11%, 75% and 97% RH were obtained by using LiC1, NaCI and K2SO4 saturated solid solution at 30 C33.
A typical run, shown in Fig. 7, was carried out by placing the sensor for 1 h at 11% RH and the impedance recorded at 15 min interval; then rapidly moving to the 75% RH and to 97% for one hour respectively.The desorption was carried out by using the solutions in the opposite way, obtaining the evaluation of the hysteresis relative to each solution, H 1% and H3 3%.Leaving the sensor at 97% RH overnight, at 16 h the drift can be evaluated.At 75% RH the sensor shows an hysteresis H2 4% and at 11% RH an Ha 3%.

Aging
The aging of a sensor is caused by its prolonged use at different temperatures and environments.The aging of our sensor was simulated by means of a climatic cham- ber carrying out a 21 day cycle in which the sensor was submitted to daily variations of temperature between 30 and 42 C and RH between 45 and 95%, as described in Fig. 8.After each aging cycle, the impedance was measured at 11%, 75% and 97% by means of the above mentioned saturated solutions.
Eight cycles were carried out on the same sensor, for a total aging time of 168 days.The results of the impedance are reported in Fig. 9.The uncertainties as- sociated to each 11%, 75% and 97% RH are an overall estimation of the error associated with the impedance measurement, considering the hysteresis and drift evaluated according to the previously reported procedure.The average uncertainty on RH measurement is estimated as 4%.

Temperature Dependence
The evaluation of the temperature dependence of the recorded signal is particularly important in ionic humidity sensors since the physisorption and capillary conden- sation of water molecules on the ceramic surface and pores are greatly influenced by this parameter.The sensor was calibrated in a climatic chamber between 25 and 95 C. The results are showed in Fig. 10.

CARBON MONO-OXIDE SENSITIVITY
As previously described, ceramic sensors exhibit high sensitivity to reducing gases when activated at high temperature, usually between 300 and 450 C. At the moment, studies are being carried out in our laboratory to develop a multifunction ceramic sensor based on Si-doped a-Fe203, which can detect humidity in the 0-100 C and CO in the 350-450 C ranges without cross-talk of the electrical signals.
Preliminary tests of CO sensitivity were carried out on three compacts (undoped, 2% and 4% Si-doped) sintered at 900 C for 1 h.The results, plotted as AR/R% as a function of the activation temperature of the sensor, are shown in Fig. 11.
The tests were carried out by heating the sensor at 100 C steps in the 0-500 C range.At each temperature, the resistence, R in ohm, was measured in dry air and in 900 ppm of a CO-air mixture.The Rair Rco/Rair AR/R was plotted as function of the activation temperature.The ce-Fe203 sensor showed the highest sensitivity to CO at 400 C with 60% variation of resistence.The sensitivity to other reducing gases such as SO2, hydrocarbons, and smoke is also investigated to fully characterize the selectivity of the sensor.

REALIZATION OF A HUMIDITY MONITORING DEVICE
The satisfactory characteristics shown in the laboratory tests by the Si-doped a- Fe203 sensor, allows the realization of dedicated, low-cost, electronic circuits for humidity monitoring.An electronic circuit prototype for humidity measurement has been, buildup taking into account the experimentally determined temperature dependency.The electronic equipment was set up with a commercial, low-cost, integrated circuit and based on the direct measurement of the sensor impedance magnitude.
In Fig. 12, a block scheme of the electronic equipment prototype is shown.The impedance magnitude of the sensor is measured by the volt-amperometric method: a Wien bridge harmonic voltage generator (Osc) and a sophisticated current to voltage converter (I/V Conv.) was utilized.The peak value of the output voltage of the I/V Conv.was made proportional to the impedance magnitude of the sensor.A logarithmic amplifier (Log.Amp) was utilized for the linearization of the response when it changed with humidity.The temperature compensation of the humidity measurement was obtained by an electronic integrated temperature sensor with linear response in the 0-100 C range, an amplifier stage for signal adjustment, and an algebraic summing amplifier (E).The algebraic summing amplifier com- pensated the impedance signal of the humidity sensor with the temperature signal, according to the experimental calibration shown in Fig. 10.The environment- measured RH value is shown by a stack of twenty Light Emitter Diodes (LED), which gave the value of the humidity in the range 0-100% RH, with a 5% step.AMP PBAE DBTBCT FIGURE 12 Block scheme of the humidity monitoring prototype.HS: Humidity Sensor; TS: Tem- perature Sensor; OSC: Harmonic Oscillator; I/V CONV."Current to Voltage Converter; LOG AMP: Logarithmic Amplifier; AMP: Amplifier; : Algebraic Summing Amplifier; DISPLAY: Display System.

FIGURE 2
FIGURE 2 SEM micrograph of 950 C sintered compact microstructure.

16 FIGURE 7
FIGURE 7The response of 950 C above saturated salt solution.

FIGURE 8
FIGURE 8 Temperature and RH variation during one day cycle in climatic test chamber. FIGURE9

FIGURE 10
FIGURE10 Temperature dependance of the impedance as a function of the RH.

TABLE Ceramic sensors
: Properties,Materials and Functions.
material.It is composed of grain boundaries formed in the sintering, necks between FIGURESchematic drawing of a typical porous ceramic sensor microstructure.