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Description  |
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BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a ceramic heater excellent in the thermal
shock resistance and the strength at high temperatures, which can be
widely used for ordinary houses, electronic parts, industrial equipments
and automobiles.
(2) Description of the Prior Art
As the heater comprising a ceramic material as the substrate, there has
been mainly used a heater comprising a resistor composed mainly of a metal
such as tungsten (W) or molybdenum (Mo), which is in an alumina (Al.sub.2
O.sub.3) sintered body.
This ceramic heater is advantageous in that it is excellent in the
electrically insulating property chemical resistance and abrasion
resistance. However, alumina is poor in the thermal shock resistance and
the strength at high temperatures. Namely, the thermal shock-resistant
temperature difference is about 200.degree. C. when thrown in water and
quenched, and the strength at high temperatures of up to 800.degree. C. is
about 30 kg/mm.sup.2 as determined by the 4-point bending flexural
strength method.
Accordingly, use of a silicon nitride sintered body, which is excellent
over other ceramics in the thermal shock resistance and the strength at
high temperatures, as the substrate of a heater has attracted attention in
the art. This silicon nitride sintered body is superior to alumina in that
the thermal shock-resistant temperature difference is about 600.degree. C.
and the strength at high temperatures of up to 800.degree. C. (4-point
bending flexural strength) is 60 kg/mm.sup.2.
A ceramic heater comprising this silicon nitride sintered body as the
substrate, in which a heat-generating resistor metal such as tungsten (W)
or molybdenum (Mo) is embedded as in case of the alumina, has already been
proposed, and a heater formed by printing a heat-generating resistor paste
composed of such a metal as tungsten (W) or molybdenum (Mo) on a silicon
nitride green sheet, laminating the green sheet on the printed sheets and
sintering the laminate integrally is proposed in Japanese Patent
Application Laid-Open No. 55-126989.
However, when a metal such as tungsten (W) or molybdenum (Mo) is used as
the heat-generating resistor, at sintering at high temperatures or during
long-time application where elevation and dropping of the temperature are
repeated, the metal such as tungstem (W) or molybdenum (Mo) reacts with
silicon nitride (Si.sub.3 N.sub.4) in the interface between the periphery
of the heat-generating resistor and silicon nitride and a layer of
WSi.sub.2 or MoSi.sub.2 is readily formed. Furthermore, a layer of
WO.sub.3 or MoO.sub.3 is readily formed by reaction with oxygen. Since the
so-formed reaction layers are physically brittle, the dispersion of the
resistance value is large, and especially in case of a high-resistance
heater, cracks are readily formed in the interface where the reaction
layers are formed and breaking of the heat-generating resistor is caused.
Because of these defects, the conventional heaters, especially the heaters
formed by using a heat-generating resistor paste, are hardly put into
practical use. A heat-generating resistor composed of a metal such as
tungsten (W) or molybdenum (Mo) has a relatively high resistance
temperature coefficient (TCR) of about 4.times.10.sup.-3 to about
5.times.10.sup.-3 (0.degree. to 800.degree. C.). Accordingly, even in the
practically used heater having a heat-generating resistor metal such as
tungsten (W) or molybdenum (Mo) embedded in the substrate, the inrush
current is increased at the time of application of the voltage, and an
electricity control apparatus in which the current capacity is large is
necessary for the heater. In a heat-generating resistor composed of a
metal such as tungsten (W) or molybdenum (Mo), the change of the
resistance according to the temperature is not linear and the temperature
is not constantly elevated with rise of the voltage. Accordingly, it is
difficult to perform the temperature control by detecting the resistance
value.
SUMMARY OF THE INVENTION
It is a primary object of the present invention to provide a ceramic heater
excellent in the thermal shock resistance, in which dispersion of the
resistance of the heat-generating resistor or breaking of the
heat-generating resistor is not caused, the change ratio (TCR) of the
resistance according to the temperature is low and the change of the
resistance is linear.
Another object of the present invention is to provide a ceramic heater in
which the temperature at the generation of heat is reduced in a
terminal-attaching portion and the strength durability of this portion is
increased.
In accordance with the aspect of the present invention, there is provided a
ceramic heater comprising a ceramic substrate, a heat-generating resistor
disposed in the interior of the ceramic substrate or on the surface of the
ceramic substrate and terminals connected to both the ends of the
heat-generating resistor, wherein the ceramic substrate is composed of a
sintered body of a nitride of an element selected from the group
consisting of silicon and aluminum and the heat-generating resistor is
composed of a ceramic layer containing titanium nitride (TiN) or tungsten
carbide (WC).
In accordance with another aspect of the present invention, there is
provided a ceramic heater comprising a ceramic substrate, a
heat-generating resistor disposed in the interior of the ceramic substrate
or on the surface of the ceramic substrate and terminals connected to both
the ends of the heat-generating resistor, wherein the ceramic substrate is
composed of a sintered body of a nitride of an element selected from the
group consisting of silicon and aluminum, the heat generating resistor
comprises terminal-attaching portions having a large sectional area and a
heat-generating intermediate portion having a small sectional area, which
is located between the terminal-attaching portions, the terminal-attaching
portions are composed of a ceramic layer containing tungsten carbide, and
the heat-generating intermediate portion is composed of a ceramic layer
containing titanium nitride.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1-A is a graph illustrating the temperature dependency of the
resistance in various resistors.
FIG. 1-B is a graph illustrating the relation between the time and the
current at the current inrush in the resistors shown in FIG. 1-A.
FIG. 2 is a perspective view illustrating the state where a heat-generating
resistor layer of the present invention is formed on a ceramic substrate.
FIG. 3 is a perspective view of a complete ceramic heater obtained by
forming a ceramic covering on the heat-generating resistor layer shown in
FIG. 2.
FIG. 4 is a graph illustrating the relation between the content of TiN in
the TiN resistor layer and the resistance.
FIG. 5 is a perspective view illustrating in the exploded state the
laminate structure in another embodiment of the ceramic heater of the
present invention.
FIG. 6 is an enlarged sectional view illustrating the state where a
terminal is attached to a terminal-attaching exposed portion of the
ceramic heater shown in FIG. 5.
FIG. 7 is a graph illustrating the resistance temperature coefficients
(TCR) of a comparative heater Y comprising a tungsten resistor formed on
an alumina substrate and a heater X of the present invention (Example 2).
FIG. 8 is a graph illustrating in comparison the linearities of the
resistance temperature coefficients (TCR) of an MgO-added resistor and an
MgO-free resistor in Example 4 of the present invention.
FIG. 9 is a graph illustrating the dispersion of the resistance of each
sample in Example 5.
FIG. 10 is a graph illustrating the change of the resistance according to
the temperature in each sample in Example 5.
FIG. 11 and 12 are graphs illustrating the results of the heat cycle test
of each sample in Example 5.
FIG. 13 is a microscope photograph (5000 magnifications) showing the TiN
layer and the portion surrounding the interface thereof in an aluminum
nitride sintered body.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the ceramic heater of the present invention, tungsten carbide (WC) or
titanium nitride (TiN) used as the heat-generating resistor is
thermodynamically more stable at high temperatures than the single element
such as tungsten (W) or molybdenum (Mo), and a brittle reaction layer is
hardly formed by the reaction with the ceramic substrate. Accordingly,
breaking or embrittlement of the resistor by the presence of the brittle
reaction layer is substantially completely prevented.
Furthermore, the resistance is hardly changed at the sintering step or by
long-time repetition of the elevation and dropping of the temperature.
Moreover, since the thermal expansion coefficient of tungsten carbide (WC)
as the heat-generating resistor is close to that of silicon nitride
(Si.sub.3 N.sub.4) as the substrate, peeling is not caused between them
when elevation and dropping of the temperature are repeated, and a tough
heat-generating resistor can be formed. Similarly, as is apparent from the
fact that titanium nitride (TiN) as the heat-generating resistor can act
as a sintering aid for silicon nitride (Si.sub.3 N.sub.4) as the
substrate, TiN and Si.sub.3 N.sub.4 are tightly bonded to each other, and
a ceramic heater excellent in the peeling resistance and thermal shock
resistance can be formed.
The heat-generating resistor composed of tungsten carbide (WC) or titanium
nitride (TiN) is characterized in that the resistance temperature
coefficient (TCR) is 1.times.10.sup.-3 to 2.times.10.sup.-3 (temperature
range of 0.degree. to 800.degree. C.) and much smaller than that of a
heat-generating resistor composed of tungsten (W) or molybdenum (Mo).
More specifically, as shown in FIGS. 1-A and 1-B, when heaters of the same
wattage, that is, a heater Ro.sub.1 comprising WC or TiN as the resistor
and a heater Ro.sub.2 comprising tungsten (W) or molybdenum (Mo), are
prepared (see FIG. 1-A with respect to the case where the resistance at
800.degree. C. is the same), since the heater comprising tungsten (W) or
molybdenum (Mo) as the resistor has a small resistance at room
temperature, the inrush current at the time of application of the voltage
is increased according to the general formula of V=IR (see FIG. 1-B).
On the other hand, since the resistance at room temperature of the heater
comprising tungsten carbide (WC) or titanium nitride (TiN) as the resistor
is large, the inrush current at the time of application of the voltage can
be reduced and the current capacity of the control apparatus for the
heater can be reduced. Moreover, if the resistance temperature coefficient
(TCR) is small, the temperature distribution is uniformalized in the
heater irrespectively of the atmosphere where the heater is used.
More specifically, since the relation of W=I.sup.2 R (I is a constant) is
established according to Ohm's law, it is known that the heat-generating
energy is increased proportionally to the resistance value. Accordingly,
when a part of a heater having a large resistance temperature coefficient
(TCR) is locally cooled, the resistance of the resistor at this part is
drastically reduced and the quantity of heat generated at this part is
drastically reduced. On the other hand, in case of the heater of the
present invention having a small resistance temperature coefficient (TCR),
even if a part of the heater is locally cooled, the resistance of the
resistor at this part is not so reduced and the change of the quantity of
heat generated at this part is small. Namely, the temperature distribution
of the heater hardly undergoes an external influence.
It was experimentally confirmed that in the heat-generating resistor
composed of tungsten carbide (WC) or titanium nitride (TiN) according to
the present invention, the change of the resistance according to the
temperature is substantially linear.
For the production of the ceramic heater, as shown in FIG. 2, a paste layer
containing tungsten carbide or titanium nitride is patterned by screen
printing or the like on a green ceramic sheet 1a to be converted to an
insulator by sintering, which is obtained by press molding or tape
formation of a nitride of silicon or aluminum, whereby a resistance
circuit 2 is formed. A green ceramic sheet 1b (see FIGS. 3 and 5) similar
to the above-mentioned green sheet is laminated on this paste layer 2 and
the laminate is integrally sintered. The sintered body is subjected to the
grinding or surface treatment to expose electrode-attaching portions 6
(see FIG. 5), and terminals 3 are attached thereto, as shown in FIG. 3, if
necessary through a metallized layer (not shown), whereby a plate-shaped
ceramic heater is fabricated.
In the present invention, in order to obtain desired generation of heat
while maintaining the temperature of the terminal or a surrounding portion
thereof, it is preferred that the heat-generating resistor layer 2 should
comprise terminal-attaching portions 5 having a large sectional area and a
heat-generating intermediate portion 4 having a small sectional area,
which is located between the terminal-attaching portions 5, as clearly
shown in FIG. 2.
It is preferred that the thickness of the heat-generating resistor layer be
smaller than 3 mm, especially smaller than 2 mm. It is generally
recommended that the thickness of the heat-generating resistor layer be 1
to 100 .mu.m, especially 3 to 40 .mu.m. It is preferred that the thickness
of the substrate of silicon nitride or aluminum nitride after sintering be
0.1 to 30 mm, especially 0.5 to 10 mm. The sectional area and length of
the heat-generating resistor layer are changed according to the desired
resistance value, the electric input and the like. It is preferred that
the resistance of the heat-generating resistor layer as a whole be 0.1 to
1000.OMEGA., especially 1 to 500.OMEGA., and the quantity of generated
heat be 5 to 3000 W, especially 20 to 2000 W. Furthermore, it is preferred
that the sectional area of the terminal-attaching portion 5 be 1.5 to 100
times, especially 2 to 60 times, as large as the sectional area of the
heat-generating intermediate portion.
In accordance with one preferred embodiment of the present invention, the
ceramic substrate 1a, 1b is composed of a silicon nitride (Si.sub.3
N.sub.4) sintered body, and the heat-generating resistor layer 2 is
composed of a ceramic layer containing titanium nitride (TiN). In this
embodiment, the silicon nitride sintered body is excellent in the thermal
shock resistance and the strength at high temperatures over other
ceramics, and the thermal shock resistance temperature difference of this
silicon nitride sintered body is about 600.degree. C. and the strength at
high temperatures (4-point bonding flexural strength) is 60 kg/mm.sup.2
and higher than that of alumina. As pointed out hereinbefore, the
heat-generating resistor layer of titanium nitride is excellent in the
adhesion to the Si.sub.3 N.sub.4 substrate. The heat-generating generator
layer containing TiN is formed of a sintered body of (a) titanium nitride,
(b) silicon nitride and (c) a sintering aid. As the sintering aid (c),
there are used yttria, magnesia and alumina. An especially preferred
example of the ceramic composition comprises 40 to 85% by weight of
titanium nitride, 20 to 54% by weight of silicon nitride and 1 to 10% by
weight of the sintering aid.
Namely, Si.sub.3 N.sub.4 and other component in the TiN resistor paste
improve the sintering property of the paste and exert the function of
increasing the bonding force between the Si.sub.3 N.sub.4 substrate and
the resistor. However, if the amounts of components other than TiN are too
large, the resistance change ratio is increased. If the TiN content in the
paste exceeds 85% by weight, the sintering property is degraded, and if
the TiN content is lower than 40% by weight, the resistor change ratio per
the TiN content is too large, as shown in FIG. 4, and control of the
resistance value becomes difficult.
Another preferred example of the TiN resistor paste used in the present
invention comprises 20 to 70% by weight, especially 25.degree. to 55% by
weight, of Si.sub.3 N.sub.4, 30 to 75% by weight, especially 42 to 72% by
weight, of TiN and 0.1 to 9% by weight of MgO or a compound to be
converted to MgO under sintering conditions.
Namely, MgO acts as the sintering aid for titanium nitride (TiN) and
promotes densification. Accordingly, the change of the resistance by
formation of voids or cracks is controlled, and as the result, a good
linearity is obtained in the resistance temperature coefficient (TCR). If
the amount of Si.sub.3 N.sub.4 is smaller than 20% by weight and the TiN
content exceeds 75% by weight, the thermal expansion coefficient of the
heat-generating resistor composed mainly of TiN is much larger than that
of the Si.sub.3 N.sub.4 substrate, and the thermal stress is imposed at
the time of generation of heat and cracks are readily formed in the
resistor. Moreover, since the content of the Si.sub.3 N.sub.4 is reduced,
the strength of the resistor per se is reduced. On the other hand, if the
content of Si.sub.3 N.sub.4 exceeds 70% by weight and the TiN content is
lower than 30% by weight, the insulating property and the resistance value
is increased because of the low content of TiN, and the sintered body
cannot be used as a heat-generating resistor. Moreover, if the TiN content
is low, the resistance value is greatly changed by a slight change of the
ratio between TiN and Si.sub.3 N.sub.4, and therefore, control of the
resistance value and maintenance of a stabilized resistance value become
difficult. If the MgO content is lower than 0.1% by weight, since
sintering is not sufficiently promoted in the resistor, the strength is
reduced and cracks are readily formed. Therefore, the linearity is lost in
the resistance temperature coefficient and the resistance is readily
changed. If the MgO content exceeds 9% by weight, the content of the
vitreous component in the resistor is increased and the strength is rather
reduced. Accordingly, cracks are readily formed in the resistor, and the
linearity is lost in the resistance temperature coefficient and the
resistance value si readily changed.
In accordance with another preferred embodiment of the present invention,
the heat-generating resistor layer is a sintered body of a composition
comprising titanium nitride and 0.05 to 8% by weight, based on titanium
nitride, of silicon carbide, this sintered body has a chemical structure
in which at least a part of SiC is solid-dissolved in the TiN lattice, and
this sintered body has a density of at least 4.2 g/cm.sup.3 and a specific
resistance smaller than 40 .mu..OMEGA.-cm.
In this embodiment, if the amount added of SiC is smaller than 0.05% by
weight based on TiN as the main component, no substantial effect is
attained by addition of SiC, and the specific resistance is not
significantly reduced but the density is reduced. If the amount added of
SiC exceeds 8% by weight based on TiN, the specific resistance is abruptly
increased and the density is drastically reduced. In case of a sintered
body composed solely of TiN, which is formed without addition of SiC, a Ti
phase often appears in addition to the TiN phase. When the amount of SiC
added to TiN is gradually increased, an .alpha.-SiC phase is precipitated
if the amount added of SiC exceeds about 10% by weight. If the amount
added of SiC is in the range of from 0.05 to 8% by weight, SiC is
solid-dissolved in the TiN lattice, and in the state of this solid
solution, the specific resistance is lower than 40 .mu..OMEGA.-cm and the
sintered body is densified so that the density is at least 4.2 g/cm.sup.3.
In accordance with still another embodiment of the present invention, the
ceramic substrate 1a, 1b is composed of a sintered body of aluminum
nitride and the heat-generating resistor layer 2 is composed of a ceramic
layer containing titanium nitride. In this embodiment, TiN and AlN are
tightly bonded to each other, but embrittlement or breaking of the
resistor layer is effectively prevented by mutual reaction between them.
The above-mentioned TiN compositions can be similarly used for the
resistor layer.
In accordance with still another embodiment of the present invention, the
ceramic substrate is composed of a sintered body of silicon nitride and
the heat-generating resistor is composed of a tungsten carbide layer. The
heat-generating resistor layer of WC is prepared, for example, by
sintering a paste containing WC alone.
In the examples of the present invention, the heat-generating resistor
paste comprising substantially pure WC, that is, WC having a purity of
99.8%, was used. However, in order to adjust the resistance value of the
heat-generating resistor, improve the denseness of the resistor or enhance
the bondability to the silicon nitride substrate, up to about 40% by
weight of a single substrate, oxide, nitride, carbide or carbonitride of
an element of the group IIIA such as Y or an element of the group IIa such
as Mg, or the same Si.sub.3 N.sub.4 as that of the silicon nitride
substrate, may be added to WC. If such an additive is incorporated, the
effects of the present invention are not degraded.
In accordance with still another embodiment of the present invention, the
terminal-attaching portion having a large sectional area is formed of a
ceramic material composed mainly of WC, and the heat-generating
intermediate portion having a small sectional area is formed of a
TiN-containing ceramic material.
Referring to FIGS. 5 and 6 illustrating this embodiment, a
terminal-attaching portion 5a having a large sectional area is formed of a
ceramic composed mainly of MC, and a heat-generating intermediate portion
4a having a small sectional area is formed of a ceramic material
containing TiN. The terminal-attaching portion 5a having a large sectional
area has a portion 6 exposed to both the ends of the laminate, and a
metallized layer composed of a mixture of a glass and a powder of Ni is
formed on this exposed portion 6, as shown in FIG. 6. A metal cap 9 for
fixing an electrode lead line 8 is secured on the metallized layer 7
through a silver solder 10.
If this structure is adopted, generation of heat is controlled in the
terminal-attaching portion 5a, and the strength of the bonded portion can
be stably maintained without degradation even if the heater is used over a
long period.
Since the melting points of the metallized layer 7 and silver solder 10 for
bonding the metal cap 9 to the exposed surface 6 of the terminal-attaching
portion 5a are generally 600.degree. to 850.degree. C., it is preferred
that the terminal-attaching portion 5a be maintained at a temperature much
lower than the above-mentioned temperature even when an electric current
is supplied to the heater. The specific resistance of TiN used for the
heat-generating intermediate portion 4a in the present invention is
relatively high and about 400 .mu..OMEGA.-cm. Accordingly, if the
temperature of the heat-generating intermediate portion 4a is elevated to
an allowable highest level of about 1300.degree. C., the temperature of
the terminal-attaching portion 5a is likely to rise to about 500.degree.
C. to about 700.degree. C. If the terminal-attaching portion 5a is formed
of WC having a relatively low specific resistance of 100 .mu..OMEGA.-cm
according to this embodiment, the heat-generating temperature of the
terminal-attaching portion 5a can be controlled to a level lower than
300.degree. C., and degradation of the strength of the bonded portion can
be prevented.
A silicon nitride sintered body or aluminum nitride sintered body that is
used as the substrate in the present invention can be prepared by optional
known means. For example, the silicon nitride is formed by adding an
oxide, nitride or oxynitride of an element of the group IIa or IIIa of the
Periodic Table or Al as the sintering aid to silicon nitride powder,
molding the mixture and sintering the molded body at 1600.degree. to
2200.degree. C. in a nitrogen atmosphere according to the pressureless
sintering method, the hot pressing method or the gas pressure sintering
method. The aluminum nitride sintered body can be obtained by using a
sintering aid as described above and carrying out sintering at
1600.degree. to 2000.degree. C. in a nitrogen atmosphere according to the
sintering method described above.
The present invention will now be described in detail with reference to the
following examples that by no means limit the scope of the invention.
EXAMPLE 1
Additives such as Si.sub.3 N.sub.4, Y.sub.2 O.sub.3, MgO and Al.sub.2
O.sub.3 were added in amounts shown in Table 1 to titanium nitride (TiN).
Then, acetone and a binder were added to the mixture, and the mixture was
blended by a shaking mill for 72 hours. Acetone was removed from the
mixture, and the residue was kneaded and the viscosity was adjusted to
form a paste for a heat-generating resistor. Each paste of samples Nos. 1
through 10 shown in Table 1 was subjected to a press-molding or
tape-forming operation to form a green silicon nitride formed body 1a to
be rendered electrically insulating by sintering. As shown in FIG. 2, a
resistance circuit 2 was formed on the formed body 1a by screen printing,
and another green formed body was laminated on the so-formed sheet and the
laminate was integrally sintered according to any of the pressureless
sintering method (PL), the gas pressure sintering method and the hot
pressing method. The sintered body was subjected to a grinding or surface
treatment to expose electrodes. Electrode-attaching fittings 3 were
soldered to the electrodes through a metallized layer to obtain a
plate-shaped ceramic heater of 70 mm.times.5 mm.times.1.2 mm as shown in
FIG. 3.
A voltage (100 to 120 V) which would raise the temperature of the top end
of the heat-generating resistor to 900.degree. C. by 5 seconds'
application was applied to each of the plate-shaped ceramic heaters
corresponding to samples Nos. 1 through 10 for 5 seconds, and the ceramic
heater was forcibly cooled in air for 13 seconds. This operation was
regarded as one cycle, and the initial resistance value and the resistance
value after 20,000 cycles were measured and the resistance change ratio
was examined. During the production of each ceramic heater, the thickness
of the heat-generating resistor at the paste-printing step was measured by
a film thickness meter. The obtained results are shown in Table 1.
Similarly, a resistor paste of W or Mo was printed on the surface of the
green silicon nitride formed body to form a resistance circuit, and
so-prepared sheets were laminated as described above and integrally
sintered under atmospheric pressure to form a ceramic heater as shown in
FIG. 3. The initial resistance value and the resistance value after 20,000
cycles were measured and the resistance change ratio was examined. The
obtained results (comparative samples) are shown in Table 2.
TABLE 1
__________________________________________________________________________
Heat-Generating Resistor Paste
Heat Cycle Test
(% by weight) Sintering
initial
after 20,000
change ratio
Thickness (.mu.m)
Sample No.
TiN Si.sub.3 N.sub.4
additives
Method
value (.OMEGA.)
cycles (.OMEGA.)
(%) of Resistor
__________________________________________________________________________
1 36.2
60.7
Y.sub.2 O.sub.3
2.4
HP 1000 998 0.2 13
MgO 0.7
2 53.1
44.5
Y.sub.2 O.sub.3
1.9
HP 80 81 1.3 14
MgO 0.5
3 53.1
40.8
Al.sub.2 O.sub.3
4.1
HP 78 79 1.3 13
Y.sub.2 O.sub.3
2.0
4 53.1
44.5
Al.sub.2 O.sub.3
1.9
GPS 150 152 1.3 12
Y.sub.2 O.sub.3
0.5
5 53.1
29.2
Y.sub.2 O.sub.3
9.2
HP 75 77 2.6 15
MgO 8.5
6 53.1
44.5
Y.sub.2 O.sub.3
1.9
PL 400 399 0.3 13
MgO 0.5
7 53.1
20.7
Y.sub.2 O.sub.3
15.2
HP 78 85 6.3 13
MgO 11.0
8 53.1
46.9
-- HP 79 82 3.8 14
9 94 2.6
Y.sub.2 O.sub.3
2.8
HP 40 41.6 4.0 12
MgO 0.6
10 100 -- -- HP 35 40.3 15.5 11
__________________________________________________________________________
TABLE 2
__________________________________________________________________________
Heat Cycle Test
Comparative
Heat-generating
Sintering
initial
after 20,000
change ratio
Sample No.
Resistor Paste
Method
value (.OMEGA.)
cycles (.OMEGA.)
(%)
__________________________________________________________________________
1 Mo PL 105 breaking
--
2 W PL 130 190 46.1
__________________________________________________________________________
As is understood from the results shown in Table 1 and 2, in case of the
heat-generating resistor formed by using W or Mo, the change of the
resistance after 20,000 cycles of the cycle test is large or breaking is
caused. On the other hand, in the ceramic heater formed by sintering a
heat-generating resistor paste containing TiN, the change of the
resistance value after 20,000 cycles of the cycle test is extremely small.
It is considered that the reason is that a brittle reaction layer as
described above is not formed in the interface between the resistor and
silicon nitride.
From the results shown in Table 1, it is understood that the resistance
change ratio in samples Nos. 1 through 7 and 9 formed by adding Si.sub.3
N.sub.4 and other additives (Y.sub.2 O.sub.3, MgO and Al.sub.2 O.sub.3) to
TiN is smaller than the resistance change ratio in sample No. 10 formed
without adding Si.sub.3 N.sub.4 and other additives or sample No. 8 formed
without adding only additives. It is considered that the reason is that
Si.sub.3 N.sub.4 and other additives described above in the TiN resistor
paste improve the sintering property at the sintering step and the force
of bonding of the resistor to the substrate by diffusion into the
substrate. However, if other additives (Y.sub.2 O.sub.3, MgO and Al.sub.2
O.sub.3) are added in too large amounts as in sample No. 7, the resistance
change ratio is increased. Accordingly, it is preferred that the amount
added of other additives be 1 to 20% by weight, especially 1 to 10% by
weight. Since the sintering property is degraded if TiN is incorporated in
too large an amount, it is preferred that the amount of TiN be up to 85%
by weight, and if the amount of TiN is too small (for example, smaller
than 40% by weight), the change of the resistance value according to the
TiN content is too large as shown in FIG. 4, control for deciding the
resistance value becomes difficult.
In view of the amounts of TiN and other additives, it is preferred that the
amount added of Si.sub.3 N.sub.4 be up to 54% by weight, especially 20 to
54% by weight.
EXAMPLE 2
With respect to each of sample No. 2 (X) shown in Table 1 and a heater (Y)
prepared in the same manner as described in Example 1 except that alumina
(Al.sub.2 O.sub.3) was used as the ceramic substrate and a tungsten (W)
paste was used for the heat-generating resistor, the voltage was changed
while measuring the top end of the heat-generating resistor of the heater,
and the relation between the temperature and the resistance value was
examined. In FIG. 7, the ratio of the resistance value to the resistance
value at room temperature is plotted on the ordinate and the temperature
was plotted on the abscissa. As is apparent from FIG. 7, it is understood
that although the resistance temperature coefficient (TCR) of the Al.sub.2
O.sub.3 system (Y) is 4.4.times.10.sup.-3, the resistance temperature
coefficient (TCR) of the Si.sub.3 N.sub.4 -TiN system (X) according to the
present invention was smaller and 1.1.times.10.sup.-3. This means that
according to the present invention, the inrush current can be reduced as
pointed out hereinbefore, and the temperature distribution hardly
undergoes the influence of the external atmosphere.
Incidentally, in Examples 1 and 2, the TiN-containing heat-generating
resistor was embedded in the silicon nitride sintered body. However, the
present invention is not limited to this feature. Namely, there may be
adopted a method in which the above-mentioned heat-generating resistor is
arranged on the surface of the silicon nitride sintered body and,
according to need, a covering composed of a ceramic material is formed,
whereby a ceramic heater is formed.
Moreover, a heater may be formed by embedding a linear or plate-shaped
heat-generating resistor in a silicon nitride sintered body. In this
embodiment, since the resistance value of the heating-generating resistor
is r | | |
