ELEMENT CHAUFFANT A FILM MINCE

THIN FILM HEATING ELEMENT

Abrégé français

L'invention concerne un élément chauffant à film mince, pouvant résister à des densités de puissance de 10-20 watts cm?-2¿ et/ou à des températures pouvant atteindre 650 ·C. Dans sa forme préférée, ledit élément chauffant se compose d'une couche d'oxyde d'étain dopée à l'aide de quantités relativement importantes de cérium et de lanthanum, déposée sur un substrat isolant, par pyrolyse d'une solution de trichlorure de monobutylétain contenant les éléments du groupe des terres rares. La solution et la couche d'oxyde subséquente comprennent, par ailleurs, des éléments donneurs et accepteurs tels que l'antimoine et le zinc, ce qui a pour effet d'augmenter la conductivité dudit élément chauffant.

Abrégé anglais

A thin film heating element capable of withstanding power densities of 10-20
watts cm-2 and/or temperatures up to 650~C is disclosed. The preferred form of
the heating element includes a layer of tin oxide doped with relatively large
quantities of cerium and lanthanum deposited on an insulating substrate by
pyrolysis of a solution of monobutyl tin trichloride containing the above rare
earth elements. The solution and subsequent oxide layer further include donor
and acceptor elements such as antimony and zinc to enhance the conductivity of
the heating element.

Revendications

7
CLAIMS
1. A thin film electrical heating element including a layer of an electrically
conducting metal oxide on an electrically insulating substrate, said metal
oxide layer
being doped with at least one rare earth element and being deposited on said
substrate
from an organometallic base solution.
2. A thin film heating element according to claim 1 wherein said metal oxide
layer includes at least two rare earth elements.
3. A thin film heating element according to claim 2 wherein said two rare
earth
elements are present in said metal oxide layer in substantially equal
concentrations.
4. A heating element according to claim 2 or 3 wherein said at least two rare
earth elements include both cerium and lanthanum.
5. A heating element according to claim 1 wherein said metal oxide is tin
oxide.
6. A heating element according to claim 2 wherein said metal oxide layer
further
includes substantially equal quantities of donor and acceptor elements.
7. A heating element according to claim 6 wherein said donor and acceptor
elements are respectively antimony and zinc.
8. A heating element according to claim 6 wherein said metal oxide layer is
substantially free of fluorine.
9. A heating element according to claim 1 wherein said heating element is
stable
at a power density of 20 watts cm-2.
10. A heating element according to claim 1 wherein said heating element is
stable
at a temperature of 650°C.

8
11. A thin film heating element according to claim 1 wherein the or each rare
earth
element is present in said base solution at a concentration up to 5 mol %.
12. A thin film heating element according to claim 11 wherein said at least
one
rare earth element includes both cerium and lanthanum.
13. A thin film heating element according to claim 12 wherein cerium and
lanthanum are each present in said base solution in the range of approximately
1.25
mol % to approximately 3.75 mol %.
14. A thin film heating element according to claim 13 wherein the
concentration of
each of cerium and lanthanum in said solution is approximately 2.5 mol %.
15. A thin film heating element according to claim 1 wherein said base
solution
further includes substantially equal quantities of donor and acceptor
elements.
16. A thin film heating element according to claim 15 wherein each of said
donor
and acceptor elements are respectively antimony and zinc and are each present
in said
solution at a concentration of approximately 2.8 mol %.
17. A thin film heating element according to claim 1 or 12 wherein said base
solution is monobutyl tin trichloride.
18. A thin film heating element according to Claim 1 wherein said metal oxide
layer is
deposited on said substrate from an organometallic base solution using a spray
pyrolysis process.
19. A method for the manufacture of a thin film heating element including the
step
of depositing a layer of metal oxide onto an electrically insulating substrate
by
pyrolysis of an organometallic base solution containing at least one rare
earth element.

9
20. A method according to claim 19 wherein said solution contains at least two
rare earth elements.
21. A method according to claim 20 wherein said two rare earth elements are
present in said solution in substantially equal concentrations.
22. A method according to claim 19 wherein said at least one rare earth
element is
present in said solution in the range of approximately 1.25 mol % to
approximately
3.75 mol %.
23. A method according to claim 20 wherein said at least two rare earth
element
includes both cerium and lanthanum.
24. A method according to claim 23 wherein said cerium and lanthanum are each
present in said solution in substantially equal concentrations.
25. A method according to claim 19 wherein said base solution is monobutyl tin
trichlorde.
26. A method according to claim 19 wherein said solution further includes
chlorides of at least one donor and at least one acceptor element, said donor
chlorides
and acceptor chlorides being present in said solution in substantially equal
concentrations.
27. A method according to claim 26 wherein said donor chloride is antimony
chloride and said acceptor chloride is zinc chloride.
28. A method according to claim 19 wherein said solution is substantially free
of
fluorine.

10
29. A method according to claim 19 further including the step of annealing
said
metal oxide layer on said substrate for at least one hour at a temperature
higher than
the substrate temperature used during said pyrolysis.

Description

CA 02344486 2001-03-14
THIN FILM HEATING ELEMENT
BACKGROUND OF THE INVENTION
PCT/AU99/00791
Received 4 April 2000
This invention relates to heating elements of the kind including an
electrically
conductive metal oxide film on an electrically insulating substrate.
Such devices are known, and may for example consist of a thin film of tin
oxide
deposited on a glass substrate by means of pyrolitic deposition.
If such thin film heating elements are to be used in electrical appliances
such as
cooktops, it is desirable that they be capable of operating at high
temperatures, up to
650°C. In applications such as electric kettles where the heating
element is small, the
element must be capable of handling high power densities, of the order of 10-
20
Watts cm~'. Prior art devices have not proved satisfactory in these
conditions. It has
been found by the present applicants that tin oxide layers tend to become
unstable
with increasing temperature, due to the tendency for the oxide to change
state. It has
also been found that where fluorine is employed as an electron donor or
conductivity
carrier the properties of the film change irreversibly with increasing
temperature,
apparently due to the fluorine tending to leave the film at temperatures above
400°C.
We have also found that the tin chloride solutions used in the prior art, for
example in
the spray pyrolysis process, are not stable in conditions of high humidity due
to their
hygroscopic properties, and this can lead to lack of uniformity in the oxide
films
produced.
US Patent No. 4,889,974 of Auding, et al. describes thin film elements
intended for
temperatures beyond 600°C, using oxide films doped at high levels with
pairs of
compensating foreign atoms. The metal oxide films are doped with, maximally,
10
mol % of each of the foreign atoms compensating each other in pairs, the
quantity of
said acceptor-forming elements and said donor-forming elements differing
maximally
4MtNDED SHEET
~EA/AU

CA 02344486 2001-03-14
2
PCT/AU99/00791
Received 4 April 2000
by 10%. The Auding patent describes the use of indium, boron, aluminium or
zinc as
the acceptor-forming dopant, and antimony or fluorine as the donor-forming
dopant.
However, these films using stannic chloride have been found to be difficult to
deposit
in humid atmospheres and have been found to be unstable in the power densities
of
approximately 20 Watts per cm2 required for rapid rise-time applications.
To the applicants' knowledge the films described in the Auding patent have not
seen
commercial use and are known only from this document.
SUMMARY OF THE INVENTION
The present applicants have found that a metal oxide layer of satisfactory
stability in
high power density applications may be obtained by doping with at least one
and
preferably two rare earth elements and that stability can be further enhanced
by
depositing the layer from different starting solutions than previously
employed. The
rare earth dopants are preferably cerium and lanthanum. Preferably these two
rare
earths are present in substantially equal concentrations. The presence of the
rare earth
dopants in the thin film layer has been found by the present applicants to
have the
effect of stabilising the oxidation state of the metal.
We have also found that stability at high temperatures may be obtained by
further
doping with equal quantities of donor and acceptor elements, and by avoiding
the use
of fluorine as a dopant. The preferred donor and acceptor elements for this
purpose
are respectively antimony and zinc.
In one aspect, the invention resides in a thin film electrical heating element
including
a layer of an electrically conducting metal oxide on an electrically
insulating substrate,
said metal oxide layer being doped with at least one rare earth element and
being
deposited on said substrate from an organometallic base solution.
AMENO~ED SHEET
1PEAIAU

CA 02344486 2001-03-14
3
PCT/AU99/00791
Received 4 April 2000
Preferably the metal oxide is deposited on the substrate by pyrolysis of an
organometallic base solution containing the at least one rare earth element.
In a preferred form the metal oxide layer is tin oxide and contains two rare
earth
elements such as cerium and lanthanum.
This aspect of the invention provides a thin film heating element which is
capable of
withstanding power densities of up to 10-20 Watts cm~' and/or temperatures in
excess
of 600°C.
In another aspect, the invention resides in a method for the manufacture of a
thin film
heating element including the step of depositing a layer of metal oxide onto
an
electrically insulating substrate by pyrolysis of an organometallic base
solution
containing at least one rare earth element.
Preferably the base solution contains both cerium and lanthanum in
concentrations up
to 5 moi %.
We have found that superior results can be obtained if the film is prepared by
spray
pyrolysis from a solution of monobutyl tin trichloride. The stability of this
material in
high humidity enables consistent results to be obtained across varying
atmospheric
conditions, by reducing premature oxidation.
BRIEF DESCRIPTION OF THE DRAWINGS - -
Fig. 1 is graph showing the power dissipation versus time relationship for a
thin film
heating element made according to the invention.
Fig. 2 shows the relationship between temperature and power at steady state
for five
elements having power ratings between 500 and 1330 watts.
DESCRIPTION OF PREFERRED EMBODIMENTS
AMENDED SHEET
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CA 02344486 2001-03-14
4
PCT/AU99/00791
Received 4 April 2000
While some benefit will be obtained from quite low concentrations of the rare
earth
dopant, minimal effects will be observed with concentrations in the pyrolysis
solution
of 0.01 mol %, preferred concentrations of each of the cerium and lanthanum
are
between approximately 1.25 mol % and approximately 3.75 mol %. Preliminary
tests
have shown that stability of the metal oxide layer is maximised when
substantially
equal concentrations of two rare earth elements, such as cerium and lanthanum,
are
used. Generally speaking the concentration of these rare earths will be chosen
as that
which contributes to film stability at the power densities for which the film
is
intended. Best results for films intended for operation at 20 Watts cm-' have
been
obtained using equal concentrations of approximately 2.5 mol %.
The film is preferably doped with substantially equal quantities of donor and
acceptor
elements, the preferred dopants being antimony and zinc. The concentrations of
both
antimony and zinc will be influenced by the resistivity which is required. We
have
found base solution concentrations for these materials in the region of 2.8
mol % to be
suitable for heating element applications.
A useful characteristic of such films in their application as heating elements
arises
from the positive temperature coefficient resistance of the film. This enables
elements
to be produced which are self regulating, in that they will initially operate
at a higher
wattage and, with increasing temperature, stabilise at the lower design
wattage.
The substrate-material will of course be chosen to suit the application.
Suitable
substrates include glass ceramics, silicon nitrides and other ceramic
substrates as well
as metallic substrates coated with high-temperature stable, electrically-
insulating
materials.
The preferred substrate temperatures for applying the base solution with
dopants range
from 500 to 750°C. Preferably, for application at 500°C, post
annealing at
approximately 600°C for at least one hour is carried out to assist in
stabilising the
film.
AAAEwuED SHEET
1PEA/AU

CA 02344486 2001-03-14
PCT/AU99/00791
Received 4 April 2000
Films according to this invention were manufactured from a solution using the
spray
pyrolysis process. For this purpose, monobutyl tin trichloride was used as a
base
solution, with 2.8 mol % antimony chloride, 2.8 mol % zinc chloride, 2.5 mol
5 cerium and 2.5 mol % lanthanum.
These films were fabricated with effective resistances of 26 ohm, 30 ohm and
45 ohm
to enable heaters of 2.2 kW, 1.8 kW and 1.2 kW respectively to be used,
powered by a
240V mains supply voltage. The films were selectively deposited using high
temperature masking inks which were removed by brushing after deposition of
the
film. The films deposited had a high degree of transparency. The resistive
properties
of the heating elements remained unchanged after 3500 cycles (40 minutes on
and 20
minutes off) at 650°C.
As indicated above, the positive temperature coefficient of resistance of
these
elements enables a self regulating characteristic to be obtained, with an
initially high
power dissipation which may be of advantage in achieving more rapid rise to
operating temperature. Fig. 1 shows the typical behaviour of the elements,
where
power dissipation is plotted against time of operation. As will be observed,
the
dissipation of the element commences at a high level and decreases as the
resistance
of the element increases with temperature, until a steady state condition is
achieved at
the design power consumption. Upon temporary cooling of the element, for
example
through contact with a cooler body to be heated, power dissipation will
temporarily
increase, assisting in achieving rapid heating.
Fig. 2 shows the relationship between temperature and power at steady state
for five
elements having power ratings between 500 and 1330 watts.
Life tests have shown that the films are particularly stable on inert
substrates like
quartz 96% silica in temperatures up to 650°C with power densities in
excess of
15.5W/cm' . The films on lower grades of glass ceramics having alkali
impurities
such as lithium and sodium were stable to 500°C at extremely high power
densities.
AMENDED SHEET
1PEA/AU

CA 02344486 2001-03-14
PC'T/AU99/00791
Received 4 April 2000
6
Sheet resistances varying from around 60 ohms to above 400 ohms have been
fabricated by varying the number of spray passes. The thin film thickness
could be
varied between 2000 Angstrom Units to around 14000 Angstrom Units by varying
the
number of spray passes. The films were deposited on various substrates
including
glass ceramics, alumina, silica glass and silicon nitride.
As well as their suitability in high temperature and/or high rise time
applications,
films made in accordance with the invention may be used in low temperature
applications, such as comfort heating, refrigerating defrost, and general
heating.
Heating elements of tubular shape manufactured using the above technology can
be
used in heat exchangers for flow applications, air-conditioning re-heaters,
hair dryers,
washing and drying appliances, and can also be used as radiating surfaces.
While particular embodiments of this invention have been described, it will be
evident
to those skilled in the art that the present invention may be embodied in
other specific
forms without departing from the essential characteristics thereof. The
present
embodiments and examples are therefore to be considered in all respects as
illustrative
and not restrictive, the scope of the invention being indicated by the
appended claims
rather than the foregoing description, and all changes which come within the
meaning
and range of equivalency of the claims are therefore intended to be embraced
therein.
AMENDED SHEET _
~PEA/AU

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