Note: Descriptions are shown in the official language in which they were submitted.
L335~. .
Zirconia stabilized with Y203, CaO, MgO, etc.
i5 widely used for oxygen sensors in a variety of industrial
a nd automot iv e ~ppl i cat i ons . St abi 1 i zed z i rcon i a, be i ng a
solid state oxyQen ion conductor preferentially transports
oxygen i~n~ ~ram a ga~ ~tream having a higher oxygen part ial
pressure tc> ~ gas ~tream having ~ wer oxygen partial
pressure, if the t~o ~aas ~treams ~re i~;ol~ted 9 c~f cc)ur~e .
The transport rate I resp~n~e time3 i~ ~overned by the
operating temperature. The E.M.lF . of thi~ oxygen cell is
given by the following Nernst Equation:
E r NP ln pO
where R is gas constant, T is temper~ture in K, N is charge,
F is the Faraday constant, P~2(I) and P02(II) are oxygen
partial pressure in the two side6 respectively. Several
models ~re commercially ~vailable today. ~hey a~l can be
classified in two ~ollowing categories:
1) In situ type
2) Sampling Type
In the in itu type the ~ensor i~ placed into
the furnace and senses the atmospheric oxygen~ The sensor
is ~eated by the furnace heat or, if insufficient, by
external pl~tinum or nichrome heators wrapped around it to
provide the temper~ture for adequate operation. The heating
elements have to be protected from t~e furnace atmosphere
3~
which ~dds e~mplexity and weight~ The complexity o~ the in
si~u sensors ~s ~v~ided by ~ sampling ~ype of device wherein
probe is inserted into the ~urn~ce or ~ther atmosphere t~
be measured and a ~ample is dr~wn out which i~ then passed
thro~gh an external ~ensor maintained in at desirea
temperatur~, This type of device is al~ useful as a
portable unit. However, the problem of condensation of ~he
gases through the line from the furnace to be measured ~o
the sensor unit comes into play. One has to heat the line to
prevent condensation and also worry about the calibra~ion of
the sensor to the ~perating temperature of the furnaee rather
~han of ~he sensor to obtain the correct PO~O Lo~king at
the above situation, there exists a need f~r an efficient in
situ device with an inert heating element which is stable
against atm~sphere and temperature and also is inexpensive
for real time atmosphere monitoring. S~me vf the
~pplications fDr ~uch a device are in diffusion ~urnaces, gas
analysis, sintering and bra2ing furnaces, glass mel~ing,
combus~ion and heat treatment furnaces, nitriding furnaces 7
etc.
BRIEF SUMMARY OF_THE INVENTI~
An in situ oxygen &ensing apparatus ~cc~mplishing
the objec~ives ~f the invention is proYided by the use of a
ceramic (preferably ~ilicon carbide) rPsist~nce heating
element surrounding the solid electrolyte and ~eating ~he
electrolyte ~ensor by radiation end convection,
IN THE DRAWINGS
~igure 1 is a perspective view of a sensor of the
invention,
Figure 2 is a cross sectional view of the sensor
element.
Figure 3 is a top view of one of the heating
elements.
~igure 4 is ~ side view of a heating element.
L33~
DETAI LED DESCRIP'rIC~N 0~ TH~ INVEI~TI~l~
Silicon corbide igniters have been commercially
used for igniting ç1ases and c~perate 6-ucce6sfully in ~he
typical atmosphere~ as mentioned above. These iç;niters have
~urvived the most ~tringent requirement of thermal ~nd gas
cycling for extended period~ of time. So ~uch ~o that they
have been accepted by the home ~ppliance ~arket. ~eedless to
~ay these markets ~re very con~erv~tive in product ~election
because of reliability 3nd cost consciousness. Typical
igniters are de~cribed in U.S. Patent 3,875,477.
A working unit which uses two planoconcave SiC
heating elements, 11, 12, ~urrounding ~rO2 13 sensor tube
is ~hown in Figure 1. This whole assembly is mounted on an
insulating ceramic disk 14 which butts ~gainst a furnace port
and seals the furnace atmosphere ~ompletely. The electr~nies
is controlied ~rom a separate unit which pro~esses the E~,.F~
from connec~ors 15 5 16, and correlates ~hat to the oxygen
partial pressure in the furnace. The heaters are powered
through connec~or~ 17, 18, 19, and 200
Pigure 2 shows in more detail a longitudinal cross
~ection through the 6ensor element 13 and as~ocia~ed annular
~lsctrodes 21 and 22, with leads 23 and 24 to the connections
in the base 14. The leads may be protected by ~ flame
sprayed coatiny ~3~ and 24'. Similarly, porous electrodes 21
~nd 22 may be pro~ected by a plasm~ or flame ~prayed coating
21' ~ 22', of- ~ ~aterial of the same composition as the ~olid
electrolyte, or a porous coating of a refractory material
~uch a~ cordierite or spinel.
The connectors from the electrodes ~ay be connected
to a hiyh impedance voltmeter or the other measuring and
control devices~ not part of the present inventi~n, but well
known in the art.
Figure 3 ~hows a top view of one o the heat ing
elements 11 or 12, ~nd Figure 4 sh~ws ~ left ~i~e view of the
heating element of Figure 3. The element i5 provided with
slots 31, 32, ~nd 33, so arranged that the element
~2~-~3~
- 4 -
ef~ectively has outer legs 35 and 36 which function as
opposite electrical ends of a conductor, whereby a voltage
drop applied across the ends 35 and 36 produces a heating
current in the silicon carbide body. The surface direc~ed
toward the sensor 13 is shown as parabolic at 50 in Figure 3
~o direct the heat on to the sensor with maximum efficiency.
Other concave shapes such as circular may be usedO
While the preferred sensor solid electrolyte is
doped zirconia, the particular chemistry of the sensor is not
part of this invention and the sensor may be made of any
suitable material which can conduct oxygen ions and produce a
voltage across its electrodes in response to an oxygen
partial pressure differential.
Referring to the drawing, it should be noted that
the geometry of the heating elements of the heater is such
that the most resistant (smallest conductive cross section)
of the heater is interior of the sides and ends o the
elements. Thus the highest temperature is directed at the
sensor. S~ch contr~l of the heating location, by adjusting
the geometry of the unit, is possible because of the use of
conductive ceramic material in the heater having a relatively
high resistivity as compared to metallic conductor resistance
h~ating elements. In cases where battery power is used or
the power supply is limited, the incre~sed efficiency of the
heater produced by the illustrated geometry is an added
benefit.
When the heater surrounds the outside of the
sensor, as in Figures 2 and 3 of the drawing, a chamber is
formed around the sensor which acts as a buffer to prevent
immediate direct access of the ambient gas outside the
heater. In addition, since gas accessing the space between
the heater and the sensor must flow close to the hot surfaces
of the heating el~ment, excess oxygen will tend to react with
any uncombusted products, thereby insuring an e~uilibrium
oxygen partial pressure condition for the sensed gas. In
addition the buffer chamber formed by the heater protects the
sensor against fouling by solid combustion products.
Cvnventi~nally prep~r~d SiC heating element~ have
been f~nd to be inoperative in that fal~e readings of oxygen
content are obtained after 3 to 6 ~onths of use. This is
opp~ren~ly caused by ~low oxid~ti~n of free ~ilicon or free
carbon in t~e element~. While ~imple heating of the ~lements
in an ~xidizing atmosphere ~t 12~0~C for 10 t~ 15 hours
~voids thi~ problem, added protection of the element~ can be
achieved by filling the ~urface pores of the SiC heaters with
fine ceramic powder ~uch as Si3M~, and heating to oxidize
any m~terials which would interfere with the accuracy. ~he
Si3N4 is pre~erably applied in the form of a ~lurry.
~ n a c~mpari~on test of an atmosphere at a pressure
of one atmosphere containing an ~xygen partial pressure of
about 10 4 atmosp~eres of oxygen, ~n un~reated SiC heater
and a ceramic coated but unvented heater bcth gave erroneous
readings ~f the ~xygen partial pressure as 10-16
atmosphere. When an SiC element which had been treated a t:
1200~C for 1~ to 1~ hours was employed~ the correct pressure
of 10-4 a~mosphere was vb~ained, as was the c~se when the
~ensor was employed in a temperature c~ntrolled a~mosphere
with n~ SiC el~ment~ ~hus~ treatment of the 5iC to remove
~11 materi~l~ oxidizable at t~e temperature range involved
(ilround 70~C~ and~r tre~nent to prevent access s~f the
~xidizeabl~ ~aterial to the atmosphere being tested i~
required.
~ or added pro~ection of the heating el~ment against
oxidation, the pore~ may be filled with a.mixture of fine
silicon carbide and ~odium silica~e, fired to a glassy dry
state. Other pore filling material ~ch as fine silicon
nitride ma~ also be used as taught in ~DS. Patent 4,187,344.
~ nother ~ype of ceramic heating element would be
th~t ~escribed in sopending ~S. P~tent application S~N.
669,399 fil~d November 8, 1984, in which ~tructures with
controlled electric~l characteristics ~re created with
mixtures of aluminum nitride, molybdenum disilicide, and
* corresponding to Canadian Application Serlal No.
492,998 filed October 15, 1985.
3~i~
-- 6
silicon carbideO In accordance with the teachings of that
patent specification, silicon nitride or boron nitride may be
used as the nitride phase~ ~.S. patents 3,890,250;
3,649,310; and 3,875,476 also disclose ceramic heating
elements.
