Note: Descriptions are shown in the official language in which they were submitted.
20081~5
\
ELECTRICAL CONDUCTORS FORMED OF~
SUB-OXIDES OF TITANIUM ;
This invention relates to an electrical
conductor which i8 formed of an electrically
conducting titanium sub-oxide, and to use of such a
titanium sub-oxide as an electrical conductor, for
example as an electri¢al resistive heating element.
Structures formed of titanium sub-oxides
which have a formula TiOI in which x is l-ss than 2
have been described for use in electrochemical
applications. For example, in US patent No. 4 422
917 there is claimed an electrochemical cell
~` incorporating an electr~de in which the electrode~is
formed of a golid coherent bulk titanium oxide
` 15 having the g~eneral formula Tiol where x is a number
in the range 1.67 to 1.9. The electrode is made of
a structure comprising sintered particles of
titanium sub-oxide. In this patent there i8 a}so
claimed an electrode consisting~essentially o a ~ ~
20 solid coherent bulk titanium oxide having the ~ ~ -
general~formula TiOI~where x is a number in the
~; region 1.67 to l.9,~and an eleotrocatalytically
active surface on part at least of the surface of
the electrode. Many~applications of the electrode,
and many types of electrochemical cells are
described in the patent. For~example~, the patent
describes u8e of the eilectrode in a cathodic
protection sy~tem, as a ground bed electrode, as a
~; bipolar electrode in a chlorate or hypochlorite
cell, and it describes use of the electrode as an
anode in metal winning, as a cathode in metal
recovery, as an anode in redox reactions, and as an
anode a*d as a cathode in electroeynthesis. The
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20()8145
-2-
electrochemical cell of the patent may also be an
electrode boiler supplied with AC current in order
to boil liquids or simply to heat a liquid by
passing an electrical current between electrodes of
_ 5 the type described in the patent.
Similarly, Britis~ patent No. 1 443 502
describes an electrode for use in a electrochemical
process which comprises a base structure of a
titanium oxide material having the formuIa TiOI in
lo which x is a number in the range from 0.25 to 1.50,
the base structure being at Ieast partially covered
by a surface coating containing at least one
activating substance effective in reducing the
voltage drop which would occur, in use, in the
absence thereof. In the patent there is described
use of the electrode in the electrolysis of aqueous
alkali metal chloride solution, in the electrolysis
of hydrochloric acid and of water, in cathodic
protection, in carrying out organic oxidation and
reduction processes, and in fuel cells and in
accumulators.
The applications of the titanium sub-oxide
which have been described hitherto have been
electrochemical applications, and in particular use
of the titanium sub-oxide as an electrode. In all
such applications electrical current is passed -
between at least two electrodes at least one of
which is formed of a titanium sub-oxide of defined
formula. In the applications which have been
described hitherto use of a ~tructure formed of a
titanium sub-oxide as an electrical conductor per se
has not been described, and it is with the use of
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2008145
such a structure as an electrical conductor that the
present invention is concerned.
The present invention is based on the
observation that a structure which is for~ed of a
S titanium sub-oxide can be used as an electrical
conductor per se, and that when used as an
electrical conductor the structure possesses a
number of advantages when compared with materials,
for example metals, which have previously been used
as electrical conductors.
i
For example, a structure which is formed
of a titanium sub-oxide possesses a substantial
resistance to chemical attack by a variety of
chemicals, for éxample, acids, alkalis, and organic
solvents, such that the structure can be used as an
electrical conductor in the presence of such
chemicals. The electricaI resistivity of the
structure, and thus the electrical resistance of the
structure, may also be varied, for example, by
suitable choice of the value of x in the titanium
sub-oxide Tio~ such that the structure may be
adapted for a variety of uses, for example as a
resistive heating element. Furthermore, the
titanium sub-oxide has a ~ery low coefficient of
thermal expansion, and in particular a coefficient
of thermal expansion which may be similar to that of
an associated ceramic insulator, such that when used
for example as a resistive heating element the
structure is compatible with the associated ceramic
insulator. On the other hand, a metal will
generally have a coefficient of thermal expansion
substantially greater than that of a ceramic
insulator such that a metal, when used for example
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2008~45
as a resistive heating element, may be incompatible
with an associated ceramic insulator.
The present invention provides an
electrical conductor which comprises a 6tructure
S formed substantially of a titanium sub-oxide of
formula Tio~where x i8 less than 2. ~ ~
~ The invention also provides a structure
which comprises a titanium sub-oxide of formula Tio~
in which x is less than 2, which is in a form
suitable for use as an electrical conductor,~and
which is adapted for connection to a source of
electrical power at at least two positions on said
structure.
In yet a further embodiment the invention
comprises use of a structure which comprises a
titanium sub-oxide of formula Tio~ in which x is
less than 2, as an electrical conductor.
The structure which forms the electrical
conductor of the invention may take any specifically
~ 20 configured shape. For example, it may be in the
;~; form of a plate, or mesh, or it may be in the form
of a wire, rod or tube. The structure may ~e
adapted, for exampIe by means of two or more
suitable terminals, for conne¢tion to a source of
electrical power, which may be a source of
alternating current or direct current electrical
power. The structure may have a porous form which
is particularly suitable ln certain heating
applications of the electrical conductor, as will be
described hereinafter.
The structure may be produced in a number
of different ways. For example, the structure may
be in the form of particulate titanium sub-oxide
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2008~5
contained in a suitable container, for example,
particulate titanium sub-oxide contained into a
tubular container, which may be of an insulating
material. The structure may, for example, be in the
- 5 form of a wire or rod having an outer sleeve of an
insulating material with titanium sub-oxide
contained within the outer sleeve. The titanium
sub-oxide is preferably tightly packed, and also
preferably consists of particles having a suitable
size distribution in order to achieve a highly
packed structure of the particles and consequently
good electrical contact between the particles and
thus a structure of low electrical resistivity.
In an alternative and preferred form the
; 15 structure comprises a solid coherent bulk form of
the titanium sub-oxide, that is a structure in which ~ -
particles of titanium sub-oxide are sintered
~; together.
Particulate titanium sub-oxide may be
~; 20 produced by reduction of particulate titanium
dioxide, for exam`ple, by heating particulate
titanium dioxide in;an atmosphere of a reduc~ng gas
at elevated temperature, e.g. in an atmosphere
~;~ containing hydrogen or carbon monoxide at a
temperature in the exoQss of 1000C, Q.g. at a
temperature in the~range of 1100C to 1500C, and
for a time sufficient to achieve the desired value
of x in TiO~. Alternatively, particulate titanium
~; sub-oxide may be produ¢ed by heating particulate
titanium dioxide in the presence of a solid reducing
` agent and in a non-oxidising atmosphere, or in a
reducing atmosphere. Suitable solid reducing agents
include, for example, titanium and also TiN, TiSi, ,
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20081~5
-6-
carbon, Si, Tio and Ti2o~. After the reduction
process has been compIeted the titanium sub-oxide
may be ground to achieve a required particle size.
When the structure which forms the
electrical condw tor of the invention is a solid
coherent bulk form of titanium sub- oxide in which
particles of titanium sub-oxide are sintered ~
togethér the structure may be produced by forming a
mass of particles of titanium sub-oxide into a
desired shape and heating the mass in a
non-oxidising atmosphere and at elevated
temperature, e.g. at a temperature in excess of
1000C, in order to sinter the particles. The
desired shape may be formed merely by packing the
particles of titanium sub-oxide into a suitably
shaped container prior to the heating, but it is
preferably formed by shaping a composition
`comprising particulate titanium sub-oxide and water,
and preferably an organic material, e.g. an organic
polymeric material, heating the shaped structure to
remove the water, and the organic material if
present, from the structure, and furthermorè heating
the structure as described above to sinter together
the particles of titanium sub-oxide in the
structure. The use of an organic material in the
composition enables a variety of shaping techniques
to be used, and it enables structures in a variety
of different shapes to be produced readily. The
shaping method used will depend on the consistency
of the composition.
A fluid composition may be poured into a
suitably shaped mould, whereas a more viscous
oomposition may be moulded into a~desired shape by
2008145
-7-
plastics processing techniques,~for example, by
calendering, injection moulding, compression
moulding, or by extrusion of the composition.
Extrusion is a particularly suitable shaping method
to use in the production of an electrical conductor
in the form of a wire, rod or tube.
The structure which forms the electrical
conductor of the invention may be formed by shaping
a composition comprising Tio~ possibly in admixture
with water and with an organic material, and~heating
the shaped structure to remove the water, and the
organic material if present, from the structure, and
then heating the structure at elevaèed temperature,
e.g. at a temperature in excess of 1000C, in order
to sinter together the particles of Tio2- The Tio2
in the sintered structure may then be reduced to the
desired titanium sub-oxide by heating in an
atmosphere containing a reducing agent, e.g. in an
atmosphere of hydrogen or carbon monoxide, as herein
before described. Alternatively, the composition
which is shaped and sintered may comprise Tio~ and a
solid reducing~agent, as hereinbefore described, and
the reduction to the desired titanium ~ub-oxide may
be effected by heating the shaped sintered structure
in the presence of the solid reducing agent and in a
non-oxidising atmosphere, or in a reducing~
atmosphere.
The appended drawings illustrate some of
the forms which the conductor of the invention may
take.
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20081~5
--8--
IN THE DRAWINGS:
Figure 1 shows a conductive strip 1 of
titanium sub-oxide having wires 2 attached thereto.
Areas 3 indicate an area on strip 1 where metal has
- 5 been plated onto the strip so as to allow attachment
of wires 2 to the strip, for~example, by soldering.
Figure 2 is a cross section of strip 1 of
figure l~taken ~ ~
along line A-A, whereby coating 4~can be s-en.
Figure 3 shows a cut out strip 5, strip 5
also being adapted for conn-ction to a power source
via wires 6. Wires 6 are
electrically connected to strip 5 via pIated areas
7. ~ ~ ;
The conductivity of the eIectrical
conductor of the invention is dependent inter alia
on the value of-x in the titanium sub-oxide Tio2 in
the structure.
In general the electrical conductivity of
the structure formed of titanium sub-oxide decreases
as x in the formula TiOI decreases, although there is
not a progressive increase in electrical
conductivity, and decrease in electrical
resistivity, with decrease in the value of x. For
example, as x decreases the electrical conductivity
of the titanium sub-oxide increases and reaches a
peak at a value of x in the region of 1.7 to 1.85,
and thereafter the electrical conductivity decreases
with decrease in the value of x, reaching a~minimum
at a value of x in the region of 1.5 to 1.6.
Thereafter, as x decreases the electrical
conductivity of the structure increases
progressively. The resistance of the structure to
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Z0081~5
g
chemical attack by a variety of chemicals~, including
liquid and gaseous chemicals, decreases as x in the
titanium sub-oxide TiO, decreases, and, although x
may have a value as low as 0.25, for a good
_ 5 combination of high electrical conductivity and
resistance to chemical attack it is preferred that x
in the titanium sub-oxide Tio2 in the structure
should be in the range~of 1.7 to 1.85, particularly
where the structure is to be used as an electrical
conductor per se. On the other hand, where the
structure is to be used as a resistive heating ;~
element the electrical conductivity may not need to
be at or near the maximum electrical conductivity of
the titanium sub-oxide~in the structure, and in this
case the value of x in the titanium sub-oxide may be
greater than 1.85, provided of course that x is less
than 2. The value of x may be as high as l.98.
.
The electrical conductivity of the
structure formed of titanium sub-oxide is also
dependent on the presence of other components in the
structure additional to the titanium sub-oxide.
Such other components may serve to increase -or
decrease the electrical conductivity of the
structure. For example, the electrical conductivity
of the structure may be increased by the presence in
the structure of metal in particulate or fibrous
form. Other components may be present in the
structure in order to change the physical properties
of the structure. Such other components include,
for example, ceramic materials in particulate or
fibrous form.
In general, by choice of the value of x in
the formuIa Tio2 and by choice of other components in
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20081~5
--10--
the structure, the resistivity of the electrical
conductor of the invention may be varied between
approximately 10 ohm cm and approximately 1 milliohm
cm. For example, where the structure is to be used
_ 5 as an electrical conductor per se,~ the resistivity
of the structure is suitably in the range 1 to 100
milliohms cm, whereas, when the structure is to be
used as a resistive heating element, the resistivity
of the structure is suitably in the range 100 to
1000 milliohms cm.
The electrical resistance of the structure
may be changed by a variety of techniques. For
example, the 6tructure, or a part thereof, may be
heated in an oxidizing atmosphere, e.g., in air,;in
lS order to oxidise some at least of the titanium -
sub-oxide in the structure. The heating may be
effected, for example, by passing a large electrical
current through the structure,~or by application of
a flame to the structure or to a part thereof. The
electrical resistance of the structure may also be
changed by forming cuts in the structure, for
example by means of a saw.
The structure of the invention which is
formed of titanium sub-oxide i6 suitably adapted for
connection to a source of electrical power at at
least two positions on the structure. The
electrical connection may be by any 6uitable means.
For example, the structure may be plated with a
metal and a wire, e.g. of metal, may be soldered or
brazed to the metal plating. The structure may have
a raised projection or projections to which
electrical connection may be made for example, by
means of spring loaded jaw which may be clamped onto
- . ~
20081~5
--11--
the projection, or the projections may be
screw-threaded and the electrical connection may be
made via a wire, e.g. of metal, clamped to the
projection by means of a screw-threaded nut on the
_ 5 projection. Alternatively, the structure may
comprise a screw-threaded hole or holes adapted to
received a corresponding screw-threaded connection.
The structure which forms the electrical
conductor of the invention may be used in
combination with an electrically inaulating
material. For example, the structure may be coated
with an insulating organic plastic material, e.g.,
with a material of known type. Alternatively the
; structure may be used ~n combination with a ceramic
insulating material, and the structure may have an
outer layer of such a material. For example, the
outer part of the structure may comprise titanium
dioxide as an insulating layer. The insulating
layer of titanium dioxide may be formed by oxidation
of the titanium sub-oxide at or near the surface of
the structure, for example, by controlled heating of
the structure in an oxidising atmosphere. A
structure having an outer layer of a ceramic
insulating material is advantageous in that the
temperature range over which the electricaI
conductor of the invention may be used is increased.
Thus, in the absence of such an insulating and
protective layer the conductor may tend to lose
electrical conductivity at about 350C in air;
whereas a conductor which is protected by an outer
layer of a ceramic insulating material maintains its
electrical conductivity at substantially higher
temperatures.
,
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20081~5
-12-
The electrical conductor of the invention
may be used as a conductor of the electricity per
se, particularly where the conductor has a low
electrical resistivity, and the electricity may be
alternating current or direct current. However, the
electrical conductor is widely applicable as an
electrical resistive heating element on accoùnt of
the ability to vary the electrical resistivity of
the conductor and to select a suitable electrical
resistivity for the conductor for use in such an
application. Use of the electrical conductor as an
electrical resistive heating element is particularly
beneficial in view of the resistance of the titanium
sub-oxide to degradation by a wide variety of
chemicals, both liquid and gaseous, such as, for
example, acids, alkalis and organic solvents. As a
consequence of this chemical resistance the
~; electrical conductor is particularly suitable for
use as a resistive heating element even at high
temperatures in such chemicals.
The electrical conductor of the invention
may have a porous structure, e.g. for exampl-e a
porosity as high as 80%, e.g. a porosity in the
range 20% to 80%. Such a porous 6tructure is
particularly suitable for use as a resistive heating
element as it presents a large surface area to the
material with which the element is in contact and
which is to be heated by the element.
The invention is illustrated by the
following Examples.
.
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200a~45
-13-
Examples 1 to 4
A sample of powdered titanium dioxide
havinq a particle size in the range O.Ol micron to 2
microns was mixed with water and charged to a mould
and pressed in the mould in order to expeI excess
watèr from the composition into the;shape of a
sheet.
The sheet was placed in an oven at a
temperature of 120C for 4 hours to dry the sheet
and the~sheet was then heated up to a temperature of
1100C at a rate of increase of temperature of
5C/minute and the temperature was maintained at
1100C for 2 hours to sinter the particles of
titanium dioxide in the sheet. Thereafter heating
of the sheet was continued in an atmosphere of
hydrogen flowing through the oven. The temperature
of the oven was increased at a rate of 5C/minute
until a temperature of 1240C was reached and this
latter temperature was maintained from 6 hours. The
sheet was then allowed to cool to ambient
temperature in the oven in the atmosphere of
hydrogen. The sheet comprised sintered par~icles of
reduced titanium oxide, that i8 titanium sub-oxide.
Samples of titanium sub-oxide removed from the sheet
and sub~ected to chemical analysis showed that the
titanium sub-oxide in the sheet had a composition
Tio~ where x 1.75.
In four separate examples four strips were
cut from this sheet, the strips having the following
dlmensions.
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20081~5
~ -14- ~
Example Width cm. Length cm. Thickness cm.
1 1.42 9.14 0.44
2 1.47 9.14 ~ 0.44
3 1.64 9.14 0.44
4 1.58 9.14 0.44
The ends of each of the strips were plated
with a layer of nickel'from a bath of nickel
su}phamate and copper wires were soldered to each of
the nickel plated ends~of each of the strips. The
length of the current~path through each of the
' ~ strips depended on tbe length of each strip which
i had been nickel-plated at the ends thereof, the~
' length of the current path corresponding to the~
length of each strip which had not been nickel
~, 15 plated.
; Example Current path
~,~' Length cm.
1 7.29
2 8.03
3 '~ 7.42
4 7.62
-~, ' The resistivity,'the resistance of each of the
electr~cal connectlons'to the strlps, and the
resistance of~each of the strips was measured by a
Keithley Model 580~Microohmmeter using an Alessi
C44-67 four point probe head.~
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20081~5
:
--15--
Example Resistivity Resiutance milliohm ~
milliohm End 1 End 2 Total
cm
10.33 7.01 ~6.63 ~ 127
2 ~ ,10.62 1.90 2~.24 136
~, 3 11.48 ~~4.12~ 1.86 ~ 123
4 12.57 4.28 3.~05 '145
It can be seen that the~strips of titanium
~' sub-oxide connected at the ends thereof to wires and
10 thence to a source~of -lectrical power~ are eminently ~;
suitable for use as~'~electrical conductors.
~ . ~ , . . .
Examples 5 to 7 ' ,, ~, ~
In each ~of three separate examplès the
resistance of two of the nickel'-plated ti~anium '-
lS~ sub-oxide~oo~uctor~ strips as; described in Examples
lian~ 2 was modifièd by ,making- cuts with ,a~saw into
the strips at`the 0.44 'cm thick,~edge of each of the
strips.~ Each~of,~the cuts wa- 0.83 Cm'~deep~ The
cuts, which were'~evenly spac~e,d~, weré made~as~
;20; follows.
Example
8~'cuts at same'edge of strip.
6 4'- cuts at one `edge,of Btrip
;~ , alternating with 4 cuts at opposite,
~ édge o,f strip. ,~
' 7 , 8~cuts at one -dgei of, trip
alternating
with 8 cuts at opposite edg- of
strip.
30The measured resistances of the strips of
Examplés 5, 6 and 7 were as follows.
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2008145
--16--
.
Example Resistance
milliohm
223 (127)
6 430 (136)
_ 5 7 583 tl27)
For comparison the resistances of the
unmodified strips are given in parenthesls.
~ , ,
Examples 8 to 10
In an alternative method of modifying the
~,~ 10 resistance of a~ nickel-plated titanium sub-oxide
conductor strip such a strip, having a resistànce of
10~33 milliohm, a width of 1.64 cm, a length of 9.14
~' cm, and a thickness of 0.44 cm, and produced
;~ following the procedur- described in Example l, was
~ 15 heated at the centre of one of the 1.64 cm wide
s~ - faces with the flame from a~ propane torch for a
; period of-5 minutes. The heating was conducted in
air and resulted i~n~ ome re-oxidation of the
titanium sub-ox~ide~in the strip. The resistance of
thè thus heated strlp was 149~milliohm
(Example 8). r,_
` The strip which had been heated as
described above i~n Example 8 was then heated on the
face opposite to that at which the above~heating had
been~effected. This additional~ heating was effected
in air for 5 minutes with a propane flame at two
points either sld- of the Centre of the strip. The
resistance of the;~thus heated Strip was 210
milliohm. (Example 9).
The strip which had been heated as
described above in Example 9 was then heated in air
~ for 5 minutes with a propane flame at a point at the
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20081G~5
centre of the strip and between the points at which
the heating of Example 9 was effected. The
resistance of the thus heated strip was 230
milliohm.
Examples 11 to 14
- The resistance of a single nickel-plated
titanium sub-oxide conductor strip, having a
resistance of 12.57 milliohm, a width of 1.58 cm, a
lenqth of 9.14 cm and a thickness of 0.44 cm was
modified by progressive heatinq in air to re-oxidise
some of the titanium sub-oxide in the strip
followinq the procedure described in Examples 8 to
10, except that each of the proqressive periods of
heating was of 15 minutes duration and the locations
of the heatinq were~as follows.
~` ~ Example ;Location
;I~ 11 Centre of 1.58 cm wide face.
~, "
12 ~Centre of opposite 1.58 cm wide
~;~ face.
13 Centrè of both 0.44 cm.
thick edges ;~
14 Centre of 1.58 cm wide face
oriqinally heated ~in Example 11).
The resistance of the strip increased`
progressively~with the extent of the re-oxidation of
the titanium sub-oxide effected at each staqe of the
heating, as follows.
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20081~5
-18-
Example Resistance
milliohm
11 185
12 228
13 278
14 438
Exam~les 15 and 16
In order to demonstrate that re-oxidation
of a nickel-plated titanium sub-oxide conductor
strip, and thus modification of the strip, may be
effected by means of electrical heating of the strip
in air such a strip, having a resistance of 7.62
milliohm, a width of 1.58 cm a length of 9.14 cm,
and a thickness of 0.44 cm, was connected to a
source of electrical power and a current of 20 amps
was passed through the strip. ;The centre of the
strip glowed red.~ The current was passed for 1
minute and then the strip~was~allowed to cool to
ambient temperature. The resistance of the strip
was 712 milliohm~(~Example 15).
In a second examplc~the above procedure
was repeated on the strip used in Example 15. The
resistance of the strip was then 2.99 ohm. ~Example
~- 16).
It should be noted that the conductor of
this invention may consist of titanium oxide having
the general formula TiO, where x is less than two.
~; However, the conductor will u6ually include dopants
used for various purposes and will thus comprise a
structure formed substantialIy of Tiol where x is
less than two. Such dopants may include tantalum
oxide, niobium oxide, zirconium oxlde, lead oxid-s,
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2008145
antimony oxides or bismuth oxide where appropriate
to lower the electrical resistance.~ Thus it is seen
that the invention lies in a conductor of Tio~, where
x is less than two, whether said Tiol is doped or not
doped, and regardless of the shape of the structure
into which th- conductor is formed. ~ ~
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