Humidity sensors based on SnO2-Co3O4-Nb2O5-Cr2O3semiconductor varistor ceramics

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Humidity sensors based on SnO2-Co3O4-Nb2O5-Cr2O3semiconductor varistor ceramics

The varistor effect in SnO2-Co3O4-Nb2O5-Cr2O3 ceramics with different additions is accompanied by a strong humidity-sensitivity effect in the low-field electrical conductivity. The low-field electrical conductivity and dielectric permittivity of ceramics are increased when air relative humidity rises from 10 to 93%. Such effects are observed both in the samples prepared by solid-phase sintering without or with CaO addition and in the samples prepared by liquid-phase sintering with CuO or V2O5 additions. The lowest humidity-sensitivity effect was found in the liquid-phase sintering samples due to the more solid structures and the secondary phases which are located between the SnO2 grains. For the studied samples the values of humidity sensitivity coefficient 1.8103-3.2105 at low electric fields and the nonlinearity coefficient 30-57 at high electric fields (Ej = 5.3-7 kVcm-1) were calculated. The properties of oxide ceramics are explained by electrical conductivity which is controlled by the grain-boundary potential barriers. The barrier height decreases with the increase both of air relative humidity at low electric fields and of voltage at high electric fields. Strong humidity sensitivity is a key factor for sensor application of SnO2- based ceramics.

Tin dioxide is well known as a material for the manufacture of gas sensors [1-4], humidity sensors [3-7] and varistors [8-14]. The tin oxide based ceramics has non-Ohmic electrical con-ductivity which is caused by the grain-boundary potential barriers [8 - 14]. It is the two Schottky barriers at the SnO₂ grain boundaries which are connected in opposite directions.

One of the important properties of SnO₂- based varistor ceramics is the sensitivity of its electrical conductivity to the air relative humidi-ty [3-8]. The humidity-sensitive or varistor prop-erties can prevail depending upon the additions to the ceramic samples. The combining of both properties is due to the grain-boundary nature of both effects [15-18]. Therefore the devices with combined properties of a relative-humidity sensor and a varistor can be used. For example, such ceramics were investigated earlier in SnO₂- ZnO-CoO system with SiO₂, GeO₂ or Bi₂O₃ ad-ditions [8] and in SnO₂-Co₃O₄-Nb₂O₅-Cr₂O₃ system with Bi₂O₃ [15, 16], V₂O₅ [17] or CuO

[18] additions. The GeO₂, Bi₂O₃, V₂O₅ and CuO oxides have low melting temperatures which are less than burning temperatures of samples. Therefore ceramics with these additions have liquid phases in the process of sintering.

Recently we have found that CaO and BaO additions to tin oxide based ceramics can pro-vide varistor effect [19]. Such ceramics have no liquid phases at sintering but have intergranular inclusions. The behaviour of these ceramic ma-terials in air with different relative humidity is very important for sensor application.

In order to study the influenceof different oxides on the humidity sensitivity and other electrical properties of SnO₂-based ceramics we investigated SnO₂-Co₃0₄-M>₂0₅-Cr₂0₃ oaristpr system with several additionsp-epfr edoyomlid- phase sintering (without additions or with CaO addition) and liquid-phase sinterinn (wine CuO or V₂O₅ additions). The electrical properties of ceramics which were sintered differently have not been thoroughly investigated before. The re-ceived results are presentedhi this papee below.

Experimental details

The samples (99.4-x) SnO₂ - 0.5 Co₃O₄ -

0. 05 Nb₂O₅ - 0.05 Cr₂O₃ - x A (mol. %), where x = 0 or 0.5, A = CaCO₃, CuO or V₂O₅, were prepared by a conventional mixture method. Mixture of powders was wet-milled with dis-tilled water, dried and pressed in tablets 12 mm in diameter and about 0.7 mm thick under axial pressure 45 MPa. Pressed tablets were sintered in air at the temperatures 1250°C during 1 hour. While heating the tablet with carbonate addi-tion, the decomposition CaCO₃ with CO₂ emis-sion took place and this compound changed into CaO [19]. The microstructure of ceramics was studied by the scanning electron microscope Zeiss Supra 35VP. The shrinkage of the samples y was calculated by the expression

D_n where D₀ and D are the diameters of the sample before and after sintering respectively.

Electrical characteristics were obtained after storage of the samples in the room conditions during four months after the sintering in order to achieve the stability of their electrical prop-erties [17]. In-Ga-eutectic electrodes were used in tested samples. Current-voltage character-istics were recorded by applying dc voltage in air with fixed relative humidity w and measur-ing the steady-state current. The possible self-heating of samples during the measurement was detected as the increase of current at a fixed voltene. The resuitswere ebtainedavoming this self-heating of the samples. They are presented as the dependence of the current density j on the elee2ric fieM es tocompahe dopoerties oe0зefer- pn2matee alf .

The nonlinearity coefficient was calculated by

The fixed values of air relative humidity w in the 10-93% range w eve vetainedbe puttm) sam-ples into a closed chamber above the surface vf water solution ofa proper salt for abovtl liaur 0?ifoi^e stvr²ingmcasuremeats. Current-vfltaae characteristics were recorded at w₁ = 10% and then at higher relative humidity. Before thenext mcrease vV the relativehurmdity fie semples were kept again at d₁ = 10% and current-vo^h^- agachvracteristiet w erereearded a^aain.Tho ie- reoersiblicІlauvesoУ conefctiv-ty toeee not 0I2- served. The humidity sensitivity coefficient

was calculated at electric field 0.5 E1 and rela-tive humidity w1 = 10% and w2 = 93%.

The temperature dependence of dc electri-cal conductivity у(T) was obtained at heating of samples in air. It allows estimating the grain-boundary potential barrier height ц0 in high-temperature part of у(T) dependences. The val-ues of ц0 is only slightly less than the activation energy of electrical conduction Eу [20] found by formula

where у is the electrical conductivity and T is the absolute temperature. Low-temperature part of у(T) dependences contains an anomalous area related to non-zero relative humidity [15 - 18].

The dielectric permittivity of the ceramics at different values of air relative humidity was cal-culated by formula

where s i s ths eamplethlcknest, S Is the det- trode area and s0 is the electric constant. The ca- padtance Catarequency 1 kHz wvo mo)^tured by LCRG meter Tesla BM 591.

RESULTS AND DISCUSSION

Microstructure

The micrographs of all studied ceramic mate-rials are shown in Fig. 1. The obtained parameters for these samples are presented in Table 1. The average grain size of SnO2-Co3O4-Nb2O5-Cr2O3 s ampl e i s 3.5 4 p m.This material was sintered enough but its porosity is significant (Fig. 1a). For that reason this ceramics has the environ- mentalsensitiveeffeets. The sample with CaO addition has the less ahrhbs stmttbre (Fig. 1b) and slightly less value of average grain size (2.58 pm) than the SnO₂-Co₃O₄-Nb₂O₅-Cr₂O₃ sample, his probably connected with the seg- regationof the ion Ca²⁺ with large fon radius (^c04 pm) on fod grain boundariia of S n0₂ (ion aadnll for mьn^ mm pm)ll9r. TaLІe lgads to S-he unfavorablc aandrtions for SnOa gfaingrowth and ^tlee worsening of bumind. Thtly condusCon e s provdd el]₃^r o smalite vasam of lineaa hhrilJ^agf for the s ample wonli additron(6.f5%) as com-pared to thaп; fodthe tample withfutCan addh tion (j^.^оSTfo). rtOl_li Sn0₂, <:a<_beca₄, Nb₂o_l5 Cr₂o₃ and CaO oxides have melting temaerrtbres waita are considerably higher than the burning temperature of ceramics (1250°C). Therefore the smtering of tampleswhh and without CaO addlition ^ts a solid-^f Incase i^i-^^.

In the micrographs of ceramics with CuO and V₂O₅ additions the tin oxide grains and the intergranular layers are seen (Figs. 1c and 1d). These layers occur rather inhomogeneously throughout such samples. They locate between the SnO₂ grains at CuO addition (Fig. 1c) and even covered the surface of SnO₂ grains at V₂O₅ addition (Fig. 1d). These are CuO-pure [18] and V₂O₅-pure [17] secondary phases which are liq-uid at the high burning temperature and solidify during cooling at the sintering. The CuO-rich liquid phases foster the grain growth at high temperatures in the process of burning [18]. Therefore the average grain size in such ceram-ics (5.8 pm) is more than that in the ceramics without addition. The V₂O₅-rich intergranular layers are randomly distributed throughout the sample and inhibit the grain growth [17]. The SnO₂ grains rise only to 3.23 pm at sintering of ceramics with V₂O₅ addition. The linear shrink-age of SnO₂-Co₃O₄-Nb₂O₅-Cr₂O₃ samples with CuO and V₂O₅ additions is large enough (10.92 and 12.17% respectively). The great values of the linear shrinkage confirm the presence of a liquid component at burning and the forma-tion of materials with the strong solid structure (Figs. 1c and 1d). Thus, the sintering of ceram-ics with CuO and V₂O₅ additions is a liquid-phase one.

The structural peculiarities of the studied samples must correlate with their electrical prop-erties. The appearance of different intergranular inclusions and layers at the sintering can change the conditions of grain-boundary potential bar-rier formation. So, the electrical characteristics of examined ceramics are investigated and pre-sented below.

Electrical characteristics

Current-voltage characteristics of SnO₂- based ceramics with different additions mea-sured in air at room conditions are presented in coordinate j(E) in Fig. 2. The obtained electrical parameters of this ceramics are presented in Ta-ble 1. j(E) characteristic of SnCoNbCr sample is highly nonlinear (Fig. 2, curve 1) with values of nonlinearity coefficient p ~ 50 and breakdown electric field E₁ = 5630 Vcm^-1. The relative di-electric permittivity s = 274 of this sample is quite high due to the formation of the sufficient-ly large grains (see Fig. 1a) and the existence of thin depletion layers at the grain boundaries [10, 12].

The CaO addition leads to the some increase of electric field E₁ up to 7000 V cm^-1 (Table 1) though the value of nonlinearity coefficient re-mains quite high (p ~ 30). The observed increase of the electric field E_x in the sample with CaO addition (Fig. 2, curve 2) is due to the decrease of the average grain size (see Fig. 1b) and to the increase of the number of grain boundaries per unit length correspondingly. The relative dielec-tric permittivity of the sample is also correlated with average grain size [17]. Therefore the value of dielectric permittivity of the ceramics with CaO addition s = 78 is less than that for SnO₂- Co₃O₄-Nb₂O₅-Cr₂O₃ sample.

sensory ceramic oxide

In ceramics with CuO addition the average grain size is larger than for the ceramics without addition. So, the electric field E₁ is less (Fig. 2, curve 3), but the dielectric permittivity is larger (Table 1) than that of SnO₂-Co₃O₄-Nb₂O₅-Cr₂O₃ ceramics. The sample with CuO addition exhib-its quite large value of nonlinearity coefficient P - 57. Probably, the tin oxide grains are wet sufficiently by the CuO-pure liquid phases at the sintering (see Fig. 1c) and therefore the Co₃O₄ and Cr₂O₃ oxides are homogeneously distrib-uted on the sample. These additions are respon-sible for the high nonlinearity of current-voltage characteristics of the ceramics [10].

The V-O addition to the SnO-Co.O-Nb.O -

Cr₂O₃ ceramics causes the significant decrease of the nonlinearity coefficient and the increase of breakdown electric field (Fig. 2, curve 4). The vanadium oxide forms quite conductive phas-es which cover SnO₂ grains (see Fig. 1d) and working as an electric shunt to the grain bound-aries [17]. Therefore the nonlinearity of current- voltage characteristics became considerable less and it became impossible to reach the break-down electric field E₁ (at j = 1 mAcm^-2) in our experiment. Besides, the sample with V₂O₅ ad-dition exhibits lower value s - 14 (Table 1) due to the influence of vanadium oxide phase with not high relative dielectric permittivity [21, 22].

The high nonlinearity of current-voltage characteristics in SnO₂-Co₃O₄-Nb₂O₅-Cr₂O₃ ce-ramics and in ceramics with CaO and CuO ad-ditions (P = 30 - 57) is attributed to the grain-boundary nature of conductivity for different tin oxide based varistors [8-19]. In these samples the grains are quite conductive but grain-bound-ary areas are resistive due to the formation of grain-boundary potential barriers during sinter-ing of ceramics in air atmosphere. Therefore the SnO₂-Co₃O₄-Nb₂O₅-Cr₂O₃ sample has low-field electrical conductivity o = 9.110^-13 Ohm^-1cm^-1. The values of electrical conductivity became larger with the addition of CuO and V₂O₅ oxides to the ceramics (Table 1). Such samples have quite conductive intergranular phases and, con-sequently, the electrical conductivity of ceram-ics increases.

The decrease of low-field conductivity o up to 3.510^-13 Ohm^-1cm^-1 is observed in the case of CaO addition (Table 1). This effect is con-nected with the decrease of the grain size (as a result, the significant decrease of the grain boundary cross-section and the increase of grain neck resistivity) and also to the increase of the grain-boundary potential barrier height (see lat-er about this). The effect with lowered low-field conductivity is reproducible if air relative hu-midity increases. In such conditions the sample with CaO addition has the lowest electrical con-ductivity.

To explain the observed alteration of the low- field conductivity in the ceramics with different additions the temperature dependences of elec-trical conductivity were investigated (Fig. 3). The obtained values of the activation energy of electrical conduction E_o of studied samples are presented in

All o(T) dependences are quite complicated: they cannot be approximated by a straight line in the whole studied temperature range 20 - 150°C (Fig. 3). At low temperatures in the range about 20 - 50°C the decrease of low-field conductiv-ity at heating is caused by desorption of water molecules [15 - 18]. The humid air at room con-ditions can penetrate inside the ceramics and reach grain boundary areas. At heating water molecules evaporate and electrical conductivity of samples decreases (Fig. 3). Further tempera-ture growth (50 - 150°C) gives thermally-acti-vated increase of low-field conductivity.

The obtained value E = 0.97 eV for SnO -

Co₃O₄-Nb₂O₅-Cr₂O₃ sample gives quite correct estimation of the barrier height [20]. The ad-dition of different oxides leads to some varia-tion of o(T) dependences (Fig. 3). Commonly, the values of activation energy are quite high (Table 1). The weak decrease of E_o up to 0.91 and 0.94 eV is found for ceramics with CuO

Thus, in the SnO₂-Co₃O₄-Nb₂O₅-Cr₂O₃ sam-ple and in the ceramics with different additions the electrical conductivity is controlled by the grain-boundary potential barriers. The therm-ionic emission across the barrier is the most probable conduction mechanism near about 20 - 150°C.

Effect of humidity on the electrical properties

Low-field electrical conductivity of various tin oxide based ceramics is increased with the growth of air relative humidity [3 - 8, 15 - 18]. However, the humidity sensitivity of solid-phase sintering and liquid-phase sintering ceramic ma-terials is studied insufficiently. Therefore we de-cided to study current-voltage characteristics of all obtained samples at different humidity of air.

For example, the j(E) dependences of SnO₂- Co₃O₄-Nb₂O₅-Cr₂O₃-CuO ceramics measured in air with various relative humidity are shown in

Fig. 4. They are close to linear ones at low elec-tric fields but they are highly nonlinear at higher electric fields. The rise of relative humidity from 10 to 93% causes the strong shift of low-field part of j(E) characteristics to higher current but high-field part of these characteristics changes weakly (Fig. 4). This effect is reversible and reproducible. If such samples are placed in dry air (with relative humidity 10%) after humid air (with relative humidity 34 - 93%), then their characteristics return to the initial state.

For comparison the humidity sensitivity of tin oxide based ceramics with different additions, we presented in Fig. 5 the j(E) characteristics of all investigated samples in air with relative humidity 10 and 93%. The rise of relative hu-midity causes the increase of low-field electrical conductivity for all ceramics. The sample with CuO addition showed the lowest increase than the other samples (Fig. 5). The intergranular layers of CuO-pure secondary phases prevent the access of the humid air into the electrical ac-tive areas of grain boundaries and diminish the role of barrier-related sensitivity mechanism.

In order to estimate the influence of humidity on the characteristics of studied samples the val-ues of humidity sensitivity coefficient S are pre-sented in Table 1. For the solid-phase sintering ceramics the addition of CaO oxide causes the decrease of the value S. It can be explained by the segregation of the secondary solid phases at the grain boundaries [19] and the decrease of the environmental influence on the electrical prop-erties of ceramics. For the liquid-phase sintering samples with CuO and V₂O₅ additions the hu-midity sensitivity is less (Table 1) because such ceramics are more solid and less porous. Be-sides, solidified at sintering the CuO and V₂O₅ secondary liquid phases located at grain bound-ary areas and even cover surface of SnO₂ grains (see Figs. 1c and 1d) and partially prevent the penetration of humid air into the grain bound-aries. For that reason the electrical conductiv-ity increases less at higher humidity. Therefore humidity sensitivity of liquid-phase sintering samples is lower than that of the solid-phase sintering ceramics.

The high humidity sensitivity of studied ma-terial is explained by the grain boundary con-trolled conduction mechanism [8, 15 - 18]. The observed increase of the low-field electrical conductivity with the rise of air relative humid-ity (Figs. 4 and 5) can be caused by the decrease of the grain-boundary barrier height. It was

shown earlier that the potential barrier height decreases only by nearly 20% if air relative hu-midity reaches about 80% [16]. Therefore the grain-boundary potential bawiers athighhumid- ity are still sufficiently high to be decreased at high voltages. Thus, highly-nonlinear j(E) de-pendents s are shillobserved at highvfluesof relativenumiditj (Figs.daneS).

The rise of low-field electrical conductivity at the increase of air relative humidity is con--field electrical conductivity at the increase of at the near-surface areas of studied ceramics.

Tbion molecula s mon cligseciateacaarding fo the eouatien

The quite mobile proton H+ can penetrate to the grain-boundary areas and interact with ox-ygen which has been chemisorbed there at the sintering of ceramics in oxidizing atmosphere. For that reason the total negative charge at the near-surface grain boundary can become lower in absolute value. This leads to the decrease of potential barrier heights. Therefore in air with higher relative humidity the grain-boundary barriers are lower at the near-surface areas of sample and the low-field electrical conductivity of ceramics is larger than in air with lower rela-tive humidity.

If the samples were placed into the dry air (with relative humidity 10%) after recording of current-voltage characteristics, desorption of water molecules began. The grain-boundary potential barrier heights are increasing and the low-field electrical conductivity of samples is decreasing. The j(E) characteristics return to the initial state.

The existence of the grain-boundary poten-tial barriers in studied ceramics is confirmed by the following observed facts: strong non-Ohmic conduction at relatively low electric fields (see Fig. 2), thermally-activated electrical conductiv-ity (see Fig. 3) and reversible increase of dielec-tric permittivity with the rise of relative humid-ity. The last statement is related to the decrease of grain-boundary barrier height as well as the width of these barriers at higher air humidity. The less the grain-boundary barrier width is the larger dielectric permittivity of sample is. This conclusion is proved by the values of dielectric permittivity for all samples with different addi- otential barrier height ectric permittivity of ce-ramics is increasing with the rise of air relative humidity.

Table 2

Thus, the discussed humidity-sensitive effect uctivity of samples is ics leads to the increase of low-field electrical conductivity in humid air in comparison with dry air. The obtained results for SbOe-bsscOhumidity-sensitive ceramics alely low electric fields ors that are the devices for measuring the air relative humidity.

The humidity-sensitive properties and varistor charscrtristiasare observed in solid-phase andliquid-fhvse binterind of UnO2-based ceramics with different additions. These peculiari- tiebare eepramsv by the decrease of the grain- doendarya otentiзl barrier height and width on relative humidity (at low electric fields) and on voltage (at high electric fields). The estimated values of barrier height of studied samples are 0.91 - 1.03 eV. Low-field electrical conductiv- iiyand dieleetria peroritiviry of investidated ceramics are increased with the rise of air relative humidity. The lowest humidity sensitivity coefficient S = 1830 and highest nonlinearity coefficient = 57 at electric field E1 = 5280 Vcm-1 were obtained for SnO2-Co3O4-Nb2O5-Cr2O3- CuO sample. The values S is decreased with CaO addition (solid-phase sintering) and greatly decreased with CuO and V2O5 additions (liquidphase sintering). The CaO addition provides the lowest value of low-field electrical conductivity for studied samples. The observed decrease of the conductivity at low field correlates with the experimentally found increase of the activation energy of electrical conduction and the decrease of the grain size. The studied ceramics are prospective materials for humidity sensors and varistor production.

ACKNOWLEDGMENTS

The author is very grateful to Dr. A. B. Glot (Universidad Tecnologica de la Mixteca) and Dr. R. Bulpett (Experimental Techniques Centre, Brunel University) for their help with the scanning electron microscopy.

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sensory ceramic oxide

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