2.4.4 Summary to: Ionic Conductors

Electrical current can conducted by ions in
  • Liquid electrolytes (like H2SO4 in your "lead - acid" car battery); including gels
  • Solid electrolytes (= ion-conducting crystals). Mandatory for fuel cells and sensors
  • Ion beams. Used in (expensive) machinery for "nanoprocessing".
Challenge: Find / design a material with a "good" ion conductivity at room temperature
Basic principle  
Diffusion current jdiff driven by concentration gradients grad(c) of the charged particles (= ions here) equilibrates with the
jdiff  =  – D · grad(c)

jfield  = s · E  =  q · c · µ · E
Field current jfield caused by the internal field always associated to concentration gradients of charged particles plus the field coming from the outside  
Diffusion coefficient D and mobility µ are linked via theEinstein relation;
concentration c(x) and potential U(x) or field E(x) = –dU/dxby the Poisson equation.
 
µ = eD/kT

d2U
dx2
 =  dE
dx
 =  e · c(x) 
ee0
         
Immediate results of the equations from above are:  
In equilibrium we find a preserved quantity, i.e. a quantity independent of x - the electrochemical potential Vec:  
Vec   =  const.  =   e · U(x)  +   kT  · ln c(x)
If you rewrite the equaiton for c(x), it simply asserts that the particles are distributed on the energy scale according to the Boltzmann distrubution:
c(x)  = exp –  (Vx) – Vec
kT
Electrical field gradients and concentration gradients at "contacts" are coupled and non-zero on a length scale given by the Debye length dDebye  
dDebye  =   æ
ç
è
e · e0 · kT
e2 · c0
ö
÷
ø
1/2
The Debye length is an extremely important material parameter in "ionics" (akin to the space charge region width in semiconductors); it depends on temperature T and in particular on the (bulk) concentration c0 of the (ionic) carriers.  
The Debye length is not an important material parameter in metals since it is so small that it doesn't matter much.  
 
The potential difference between two materials (her ionic conductors) in close contact thus...    
... extends over a length given (approximately) by :  
dDebye(1) + dDebye(2)
... is directly given by the Boltzmann distribution written for the energy:
(with the ci =equilibrium conc. far away from the contact.
 
c1
c2
 =  exp – e · DU
kT
       Boltz-
mann 

DU  =  –   kT
e
  ·  ln  c1
c2
       Nernst's
equation 
The famous Nernst equation, fundamental to ionics, is thus just the Boltzmann distribution in disguise!  
"Ionic" sensors (most famous the ZrO2 - based O2 sensor in your car exhaust system) produce a voltage according to the Nernst equation because the concentration of ions on the exposed side depends somehow on the concentration of the species to be measured.    
         
   
Questionaire
Multiple Choice questions to all of 2.4
         

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© H. Föll (Electronic Materials - Script)