목 차

1. 목 차 • 4.5 Solid Fast-Ion Conductors • 4.5.1 Cubic stabilized zirconia (CSZ) • 4.5.2 The β-alumina • 4.6 Humidity and Gas Sensors • 4.6.1 humidity sensors • 4.6.2 Gas sensors

2. 4.5 Solid fast-ion conductors 1 • Inoic transport comprises the movement of ions into vacant sites over substantial energy barrier that result in very low mobilities • Fast-ion conductors, optimal ionic conductors, superionic conductors • there clearly must be a supply of sites into which the mobile ion can move. lattice vacancies ,features of lattice structure such as tunnels, underpopulated planes within which specific ions can move which a low activation energy • Important factors of movement :size and charge of mobile ions ,size of the spaces within a lattice . • Small singly charged cations(Li+,La+) are mobile in a number of environments, in contrast with more highly charged and smaller cations.  this is the polarization of the anions by the smaller more highly charged cations. This interaction constitutes a high-energy barrier to the movement of cations(chap 2.6.3). However, there are factors other than geometry and size that have important effects

3. 4.5 Solid fast-ion conductors 2 • ex)on a rigid sphere model, the interstices between the anions(radius rA) in a face-centred cubic lattice would only allow penetration by a cation(0.15rA). however, in the case of I- ions(radius 220pm), interstices offer an aperture of only 34pm radius, the transport of Li+ (r3=55pm) is significant and the room temperature ionic Conductivity of LiI is 5×10-5 Sm-1 • This can be attributed to a number of factors 1. ions are not rigid spheres. 2. Simple concepts relating to the interaction of massive bodies are not applicable In detail to the interactions of ions. 3.Thermal energy will lead to random enlargements of the interstices and to the creation of anion and cation vacancies facilitating the passage of ions • LiI melts at 450℃ so that at room temperature it is only 430K below its melting point and the concentration of vacant sites will be high.

4. 4.5 Solid fast-ion conductors 3 • RbAg4I5 ,a fast-ion conductor (in polycrystalline form) has an ionic conductivity due to Ag+ ions of 21Sm-1 at 25 ℃ • even though silver has a large radius(r6=115pm), the RbAg4I5 structure is such as to offer apertures approaching this diameter for the Ag+ ions • Ionically conducting ceramic oxides are not as conductive as iodides and sulphides with their larger anions. • However, the oxygen conductor-zironia and the sodium-conductor and β’- and β”-alumina are sufficiently conductive, particularly at elevated temperature to have been adopted for a number of applications. • In particular they can form the electrolytes in electrolytic cells which can be used both to measure the concentration of specific elements and as a means of storing electrical power.

5. 4.5 Solid fast-ion conductors 4 • e.m.f. E of cell which has a source of ions at a high concentration (activity a1) on one side of an electrolyte and at a low concentration (activity a2) on the other side is given by ΔGOT = -zFE • In terms of the activity differences the change in free energy is ΔGOT =-RTln(a1/a2)  • To obtain efficient conversion between chemical and electrical energy, the resistance of the electrolyte must be low, which is assisted by operation at high temperatures. ΔGOT : standard free-energy change of the cell reaction at temperature F : Faraday constant(9.649×104Cmol-1) z : charge number of the ion •R: gas constant(8.314JK-1mol-1) •a1/a2: by the ratio of gas pressures at the active electrodes on each side of the electrolyte or by the rate of the concentrations of ions in solution.

6. Fig.4.26 The ideal fluorite structure showing half a unit cell including a dopant cation and a charge-compensating oxygen vacancy. 4.5.1 Cubic stabilized zirconia(CSZ) 1 • Pure zirconia(ZrO2): purified from mineral zircon(ZrSiO4), baddeleyite • Monoclinic(1170℃)tetragonal(2370℃)cubic(2680 ℃:melting point) Contraction in volume of 5% which can cause mechanical failure in ceramic piece • This difficulty is overcome in CSZ by stabilizing the cubic form down to room temperature by substituting lower-valency cations for the Zironia. • Cubic ZrO2 has the fluorite structure with the O2- ions arranged in simple cubic packing and half the interstices in this lattice occupied by Zr4+ ions as indicated. (Fig 4.26) • The vacancies that stabilize the structure also lead to high mobility in the oxygen sublattice and to be haviour as a fast-ion conductor.

7. Fig 4.27 Electrical conductivity of Zr0.85Ca0.15O1.85 4.5.1 Cubic stabilized zirconia(CSZ) 2 • conductivity-temperature characteristic(fig4.27) • Elements (stabilize the cubic fluorite structure in zi) : lanthanides, scandium(Sc),yttrium (Y), magnesium (Mg), calcium(Ca), manganese(Mn), indium(In) • The main qualification would appear to be an ionic radius close to that of Zr4+ (r8=84pm) • Ce3+(r8=114pm) : one of the largest ions fulfilling this function • Ca2+(r8=112pm) :the most commonly used substituent, 15mol.% • Yttrium(r8=101pm) :stabilizes the cubic phase,13-68mol.% but maximum conductivity (7-8mol.%) • Calcium : resulting in a partially stabilized zi ,7-8mol.% • Scandium : give a material with a higher conductivity, which is particularly valuable at lower temperature. • Thorium : yttria-containing bodies as a grain growth inhibitor

8. Fig. 4.28 Schematic diagram of a solid electrolyte oxygen probe suitable for the measurement of the oxygen content of gases. 4.5.1 Cubic stabilized zirconia(CSZ) 3 • CSZ finds important applications in the control of the oxygen content of gases and in solution in molten metal • For this purpose electrodes of platinum gauze are pressed into the surface of the CSZ compact before firing. • Gauzes have platinum wires welded to them which are connected to a high-impedance voltmeter • Platinum catalyses the dissociation and recombination of oxygen molecules so that O2- ions can be formed at one electrode & converted into O2 molecules at the other. • z=4 for O2 (4.29) so that, if the oxygen pressures at the electrodes are p1 and p2, the e.m.f is • Air is used as a reference material so that p2=0.209 atm or 21.2kPa

9. Fig. 4.29 Section through a ZrO2 oxygen sensor for use in a internal combustion engine 4.5.1 Cubic stabilized zirconia(CSZ) 4 • CSZ oxygen sensors are used to monitor automobile exhaust gas emissions. • Variations in the cell temperature can be estimated from a measurement of the cell resistance. • Check the oxygen content of steels • Sintering of high-quality ferrite inductor cores • Manufacture of some thick-film microcircuits

10. Fig. 4.30 Structure of β’’-alumina;(a) alternating spinel blocks and conduction planes; (b) Migration pathway of Na+ ions indicating paths of concerted motion 4.5.2 The β-aluminas 1 • Na2O·11Al2O3(β-alumina),Na2O·8Al2O3(β’-alumina),Na2O·5Al2O3(β’’-alumina) • conductivity is high within these plane but negaligible in the perpendicular direction. • Conductivity parallel to planes is 1Sm-1:room temperature,30Sm-1:300℃ • Anisotropy is a disadvantage in ceramic because of the random orientation of the crystallites, but nevertheless useful levels of conduction are obtainable. • nasicon (Na superionic conductor) :Na3Zr2Si2PO12 :Na+ ion conduction is high and isotropic • higher conductivites can be obtained by treating β’’-Al2O3 in molten LiCl :La+ Li+

11. Fig 4.31 Schematic diagram of an Na-S cell (Courtesy of chloride Silent Power) 4.5.2 The β-aluminas 2 • Produced by heating the mixture of sodium carbonate and alumina (additive,1250℃) • Resulting calcine is milled to a fine powder, and the required shape is formed • Volatilization of sodium during sintering is suppressed either by surrounding the piece with a β-alumina buffer powder or by ‘zone sintering’ which involves passing the article through a very hot zone(1700℃),10mm/min-1. Intrinsic conductivity anisotropy of β-alumina demands that careful consideration is given to controlling the texture of the ceramic • Na-S battery (Na(l)/β-alumina(s)/S(l)) 2Na+5S <-> Na2S5 • Cell runs at about 300℃ with both sodium and sulphur in the liquid state

12. 4.6.1 Humidity sensors 1 • Essentially thin discs of porous material that change in resistivity when exposed to atmospheres containing certain gases. • Structure of interconnected pores and a high specific surface area( chap 3.9). • fall in resistivity : exposed to humid atmospheres • Water is adsorbed from low-humidity atmospheres by most oxide surfaces when it dissociates, with OH- (attaching itself to a surface cation) and H30+ (attaching itself to a surface O2- ion). • Movement of the ions to alternative sites under the influence of a field give initial reduction in resistivity. • With increased humidities liquid water may condense in the pores as a result of the curvature of the liquid surface induced in the confined space by surface tension.

13. 4.6.1 Humidity sensors 2 • formation of liquid : pore diameter, surface tension ,contact angle between the solid, liquid and gas phases. • contact angle = zero, the vapour pressure (p) in equilibrium with a concave liquid meniscus relative to the vapour pressure (p0) in equilibrium with a flat surface is given by • humidity (h) = 100p/p0 ,diameter (d) of a pore in which water will condense is given by d=2r, the relation between d and h is • To speed of response a humidity element , element must be thin in section and may contain a system of larger pores to reduce the length of the finer pore pathways into which gas must penetrate. Electrodes covering major surfaces must be porous. • The response of the surfaces of the element to moisture must remain constant  teat the sensor to a temperature of ~500℃ so that the surface is restored to a reproducible state. γ: surface tension Vm: molar volume of the liquid R: gas constant T: temperature r: radius of curvature of the meniscus.

14. 4.6.1 Humidity sensors 3 • Sensing element comprises a solid solution of TiO2 in MgCr2O4 with a minor phase of MgTi2O5. • It is formed by mixing the constituent oxides, pressing them into compacts 0.25 mm thick at the relatively low pressure of about 70MPa(about 104 p.s.i) • Firing for several hours at about 1350oC to yield a sintered body (35% porosity). • Electrodes are RuO2 with the minimum amount of added glaze for adhesion so that the layer is porous. • MgCr2O4 is a normal spinel with Mg2+ (tetrahedral site),Cr3+ (octahedral site). 30mol.%TiO2 can be incorporated without losing the spinel structure. • Ti4+ and Mg2+ ions occupy beighbouring octahedral sites and the excess charge is compensated by the formation of Cr2+ ions which occupy tetrahedral sites. • It is a p-type semiconductor, in contrast with its behaviour as a gas sensor discussed below.

15. Fig 4.32 Resistance versus humidity for MgCr2O4+30mol.%TiO2 4.6.1 Humidity sensors 4 • Typical resistance-humidity characteristics. (fig 4.32) • High charge on the Cr3+ ions exposed in the pore surfaces promotes the adsorption and dissociation of water molecules according to 2H2O<-> H3O++OH- leading to the conduction mechanism outlined above. • In practice, the sensor is a plate 4mm square mounted within a heater coil, periodically by heating to 450oC for a few seconds

16. Fig. 4.33 (a) Schematic diagram of a section through a porous SnO2 compact with a porous electrode; (b) detail of the ‘neck’ region between the grains showing the effect of adsorbed oxygen 4.6.2 Gas sensors 1 • A number of semiconducting oxides have been found to show changes in electrical resistivity in the presence of small concentrations of certain gases. ( n-type SnO2, ZnO, γ-Fe2O3, TiO2 and Ag2O) • Used at temperatures in excess of about 350℃ • The sensing behaviour (n-type) :adsorption of oxygen in the neck regions between grains.

17. Fig 4.34 Tin oxide gas sensor; (a) schematic diagram; (b) commercial unit 4.6.2 Gas sensors 2 • a thin layer of porous SnO2 ceramic carried on a small alumina tube (1.0 mm outside diameter ×0.5mm inside diameter ×4mm long). • Two metallized gold electrodes are deposited in the form of bands so that the doped SnO2 makes contact with them. • A coiled heater located in the tube is capable of raising the temperature of the SnO2 layer to 300-400℃.

18. 4.6.2 Gas sensors 3 • Typical operational characteristics are shown in Figs 4.35 and 4.36 • p02 is the equilibrium oxygen pressure in pascals for the metal oxide system at 1000k Fig 4.36 Resistance of a typical SnO2 gas sensor at 400℃ in various gas-air mixtures Fig. 4.35 Effect of pO2 on the resistance of an SnO2 gas sensor.

19. Oxygen affinity is taken as –log pO2 ,where p02 is the equilibrium oxygen pressure in pascals for the metal oxide system at 1000k 4.6.2 Gas sensors 4 • Surface states that are essential to the functioning of sensors must be destroyed at the interface with the electrode. high affinity for oxygen • Table 4.2 Work functions and oxygen affinities for electrode metals (after Kulwichi[10])