Super System T4401 User Manual

ZIRCONIA SENSOR

THEORY

T4401

Issue 1

by Thomas H. Lotze

The zirconia carbon sensor is really
an oxygen sensor. The primary
mechanism for electrical current
flow in many ceramic electrolytes
is ionic conduction, in this case via
oxygen ions. T ypical construction,
basic theory, opening equations
and applications are covered.

INTRODUCTION

The purpose of this paper is to describe the typical zirconia carbon sensor in general terms, and
provide a thorough grounding in the mathematical equations governing it’s practical use.

COMMERCIAL SENSORS

The basic elements of all commercial carbon sensors are shown in Fig. 1:

Technical Data

The tubular zirconia sensing element illustrated here is the preferred form for the SSi Gold Probe™.
Slip cast and fired at exceptionally high temperatures to provide a dense, non-porous body, this
component is not prone to develop the leakage paths that are common in probes using a cemented
zirconia plug in a tubular alumina body .

The inner, reference electrode, is spring loaded to hold it in intimate contact with the inner zirconia
surface. It consists of a specially formed extension of the lead wire that connects it to the probe
terminal block. The inner reference components are especially resistant to the normally oxidizing
reference atmosphere.

The outer, measuring electrode for most commercial probes is mechanically and electrically part of
the alloy protective sheath, and the zirconia substrate is spring loaded to make intimate contact with
this electrode. The SSi outer electrode, by virtue of premium quality, heat resistant alloy and a
proprietary surface treatment, is designed to survive the rigors of the harsh furnace atmosphere.

SUPER SYSTEMS TECHNICAL DATA SHEET

Printed in U.S.A.

SSi Super System Inc CINCINNATI, OH 45215

T4401

Issue 1

ZIRCONIA SENSOR

THEORY

Technical Data

While probe manufacturers use a variety of materials and geometries, all properly designed sensors
will display precisely the same output when at equilibrium in a furnace atmosphere. Despite this
fact, many control instrument manufacturers will provide different algorithms for probes from
different manufacturers. This is primarily because manufacturers have derived their algorithms
using different sources for data that are not in complete agreement.

PRINCIPLES OF OPERATION

Pure zirconium oxide is a monoclinic crystalline material that transforms reversibly to a tetragonal
form at 1832°F with a large change in volume. This makes it unsuitable for normal refractory use.
If placed in solid solution, however, with 4% to 12% MgO, CaO or Y2O3, it is held in the stable
isometric (cubic) form which has no transformation in the range of heat treating atmospheres. By
virtue of the addition of these stabilizing oxides, oxygen ion vacancies are established in the crystal
lattice. The mobility of O- ions is greatly enhanced, and under certain conditions of temperature and
composition, the conductivity is entirely due to oxygen ions. This condition coincides with the
existence of the pure cubic crystalline phase, and is responsible for the oxygen sensing capability of
stabilized zirconia which will be discussed later.

A minimum quantity of the stabilizing oxides will ensure the existence of the pure cubic crystalline
phase of zirconia. When this amount is present, the zirconia is said to be fully stabilized. The
commercially available zirconia for oxygen sensors will have somewhat less than this minimum
amount, resulting in a “partially stabilized” electrolyte, having a better resistance to thermal fracture.
The zirconia in SSi sensors contains about 6 mole % (10.5 weight %) of Y2O3. The cell construction
of Fig. 1 demonstrates a characteristic typical of electrolytes having unity transference numbers for
an ionic species; there is an electromotive force displayed at it’s terminals that can be precisely
related to the corresponding molecular concentration at the two surfaces. In the case of cubic zirconia,
the cell e.m.f. is given by a form of the familiar Nernst equation,

EC=-.0275TR log10(p0/p1) millivolts (1)

where TR is the absolute temperature in degrees Rankine (= °F + 459.67), p0 and p1 are the oxygen
concentrations at the inner and outer electrodes respectively, stated in any units (usually in
atmospheres of partial pressure). Although there are applications where the oxygen present is the
only critical parameter, the heat treater is concerned with two other variables he wishes to control;
dew point and carbon potential. Fortunately, both parameters can be calculated directly from the
oxygen measurement.

SUPER SYSTEMS TECHNICAL DATA SHEET

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