Ceramic heater

Details Ceramic heater Ceramic heater

2000-03-29

2001-04-17

Hoang, Tu Ba (Department: 3742)

Electric heating

Heating devices

With heating unit structure

C219S544000, C219S553000

Reexamination Certificate

active

06218648

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a ceramic heater comprising a resistance heating element embedded in ceramics.
BACKGROUND OF THE INVENTION
The ceramic heater comprising a resistance heating element of high-melting metal as embedded between a core and an insulation sheet covering the core is in widespread use as a heating means for the automotive oxygen sensor, glow system, etc. or as a heat source for devices for gassification of petroleum oil, such as a heater for use in semiconductor heating or an oil fan heater.
FIG.
5
(
a
) is a perspective view showing a typical ceramic heater of this type schematically and FIG.
5
(
b
) is a sectional view taken along the line A—A of FIG.
5
(
a
).
This ceramic heater comprises a cylindrical core
31
, an insulation sheet
32
wrapped around said core
31
with an adhesive layer
37
interposed, and a resistance heating element
33
embedded between said core and insulation sheet, with terminal portions of said resistance heating element
33
being connected to external terminals
34
disposed externally of said insulation sheet
32
and lead wires
36
being connected to said external terminals
34
, respectively.
As shown in FIG.
5
(
b
), each terminal portion of said resistance heating element
33
is connected to the corresponding external terminal
34
via a plated-through hole
35
provided under said external terminal
34
in the insulation sheet
32
. In this arrangement, as an electric current is applied between the external terminals
34
through the lead wires
36
, the resistance heating element
33
generates heat and thereby functions as a heater.
The insulation sheet
32
of said ceramic heater generally comprises Al
2
O
3
supplemented with, as sintering aids, SiO
2
, MgO, CaO, etc., and the SiO
2
, MgO, etc. are segregated as glass phases in the grain boundaries of alumina ceramics.
When a ceramic heater of this type is used as a heat source for the oxygen sensor of an automobile, for instance, a 12V direct current is applied between the terminals
34
of the ceramic heater, whereupon the resistance heating element
33
of the heater reaches to a high temperature of about 1000 to 1100° C. at the maximum.
Since the Mg and Ca in the insulation sheet
32
are present chiefly as glass phases in the grain boundaries, prolonged operation of the heater under such high-temperature DC conditions results in attraction of Mg
2+
and Ca
2+
in glass phases toward the negative pole so that the so-called migration, i.e. a shift of said metal ions toward the negative terminal, takes place. As this migration occurs, voids are formed in the grain boundaries of the alumina ceramics.
As the amount of voids in the alumina ceramics increases, the resistance heating element embedded beneath the insulation layer comes into contact with the air infiltrating into the voids, resulting in a progress in oxidation of the resistance heating element, with the result that not only is the resistance value of the heating element increased gradually but the resistance heating element as such expands due to oxidation. As a result, the heating temperature of the resistance heating element varies and the heating element becomes liable to be destroyed and, in extreme cases, develops a disconnection trouble.
SUMMARY OF THE INVENTION
In view of the above state of the art, the present invention has for its object to provide a ceramic heater wherein, even when a direct current is applied to the heater for many hours, the resistance heating element is not easily oxidized so that the resistance change of the resistance heating element due to such oxidation and heater degradation due to aging can be successfully prevented.
The present invention is directed to a ceramic heater which comprises
an insulation sheet comprising 88 to 95 weight % of Al
2
O
3
supplemented with, as sintering aids, 3 to 10 weight % of SiO
2
, 0.4 to 1.0 weight % of MgO and 1.0 to 2.5 weight % of CaO,
a core covered with said insulation sheet,
a resistance heating element of high-melting metal as interposed between said insulation sheet and said core,
an intermediate layer of an alumina ceramic body having a thickness of 5 to 50 &mgr;m, and
said alumina ceramic body containing 0.05 to 4 weight % of SiO
2
, 0.01 to 0.5 weight % of MgO and 0.01 to 1.2 weight % of CaO as interposed between at least a part of said resistance heating element and said core and/or between at least a part of said resistance heating element and said insulation sheet.


REFERENCES:
patent: 5233166 (1993-08-01), Maeda et al.
patent: 5451748 (1995-09-01), Matsuzaki et al.
patent: 5753893 (1998-05-01), Noda et al.
patent: 6084220 (2000-07-01), Suematsu et al.

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