Exercises
225
2 the first is an oxide ion O
2−
(YSZ) conductor,
2 the second is a silver ion Ag
+
(AgCl) conductor.
The intermediate zone separating the two electrolytes contains the following
mix of powders (by volume): 0.5 of metallic silver Ag, 0.25 of the niobium
oxide Nb 2 O 4 , and 0.25 of the niobium oxide Nb 2 O 5 . We assume that this mix
makes a very good electronic conductor and obtain the following electrochemical chain:
Pt,O 2 / YSZ / Nb 2 O 4 + Nb 2 O 5 + Ag / AgCl / Ag 2 SO 4 + Pt,SO 3 ,O 2
(reference)
(measurement)
The operating temperature of the chain is fixed at 500 °C. The two electrodes
are in the same dry gas. We assume that, at this temperature,
2 the sulfur is entirely in the form of SO 3 at the measurement electrode (Mes)
and
2 the reference electrode (Ref) is not sensitive to SO 3 .
1. Derive the literal expression for the emf ΔE delivered to the terminals of
this chain as a function of the thermodynamic quantities related to the reactions involving Nb 2 O 5 and Ag 2 SO 4 (see data below). Comment on the
particularities of this sensor.
2. Determine
a. the emf for P SO 3 ,Mes = 10
−7
bar,
b. the variation in the emf when P SO 3 ,Mes goes to 2 # 10
−6
bar.
3. Given that the pellets of solid electrolyte are identically sized with 5 mm
diameter and 1 mm thickness, calculate the resistance of the device for
analyzing air (P O 2 ,Mes = 0.21 bar). Neglect the resistance of the interfaces.
4. Measurements of the emf at the sensor terminals are made by using a voltmeter with a 1 MΩ input impedance.
a. Qualitatively, what displacement of substance could this sensor produce?
b. For these conditions, calculate the lifetime of each consumable substance
in the sensor given a 1-mm-thick powder layer mix and that the deposited
Ag 2 SO 4 covers 1/10 of the surface area with a 10-μm-thick coating.
c. Which material limits the lifetime?
d. Which advantage permits the use of a voltage shifter?
225
2 the first is an oxide ion O
2−
(YSZ) conductor,
2 the second is a silver ion Ag
+
(AgCl) conductor.
The intermediate zone separating the two electrolytes contains the following
mix of powders (by volume): 0.5 of metallic silver Ag, 0.25 of the niobium
oxide Nb 2 O 4 , and 0.25 of the niobium oxide Nb 2 O 5 . We assume that this mix
makes a very good electronic conductor and obtain the following electrochemical chain:
Pt,O 2 / YSZ / Nb 2 O 4 + Nb 2 O 5 + Ag / AgCl / Ag 2 SO 4 + Pt,SO 3 ,O 2
(reference)
(measurement)
The operating temperature of the chain is fixed at 500 °C. The two electrodes
are in the same dry gas. We assume that, at this temperature,
2 the sulfur is entirely in the form of SO 3 at the measurement electrode (Mes)
and
2 the reference electrode (Ref) is not sensitive to SO 3 .
1. Derive the literal expression for the emf ΔE delivered to the terminals of
this chain as a function of the thermodynamic quantities related to the reactions involving Nb 2 O 5 and Ag 2 SO 4 (see data below). Comment on the
particularities of this sensor.
2. Determine
a. the emf for P SO 3 ,Mes = 10
−7
bar,
b. the variation in the emf when P SO 3 ,Mes goes to 2 # 10
−6
bar.
3. Given that the pellets of solid electrolyte are identically sized with 5 mm
diameter and 1 mm thickness, calculate the resistance of the device for
analyzing air (P O 2 ,Mes = 0.21 bar). Neglect the resistance of the interfaces.
4. Measurements of the emf at the sensor terminals are made by using a voltmeter with a 1 MΩ input impedance.
a. Qualitatively, what displacement of substance could this sensor produce?
b. For these conditions, calculate the lifetime of each consumable substance
in the sensor given a 1-mm-thick powder layer mix and that the deposited
Ag 2 SO 4 covers 1/10 of the surface area with a 10-μm-thick coating.
c. Which material limits the lifetime?
d. Which advantage permits the use of a voltage shifter?