Production process for alumina based insulator

Details Production process for alumina based insulator Production process for alumina based insulator

2001-08-31

2003-10-14

Fiorilla, Christopher A. (Department: 1731)

Plastic and nonmetallic article shaping or treating: processes

Formation of solid particulate material directly from molten...

By impinging or atomizing with gaseous jet or blast

C264S669000, C264S681000

Reexamination Certificate

active

06632381

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a spark plug, alumina based insulator used therein and a production process for the same insulator.
BACKGROUND ART
In a spark plug that is used for an internal combustion engine such as an automobile engine, alumina (Al
2
O
3
) based material excellent in heat resistance has been used as insulator therein from old times. Alumina as raw material for such insulator as described above has generally been produced by Bayer process (hereinafter referred to as Bayer alumina).
Bayer process is a process in which alumina is extracted from bauxite in a wet condition, which is naturally occurring aluminum ore, wherein an aqueous solution of caustic soda (NaOH) in a comparatively high concentration is employed as an extracting reagent. Therefore, since thus obtained Bayer alumina includes a Na component (soda component) to a considerable level, a soda removal treatment has often been applied before use on as-needed basis. Alumina is classified according to levels of soda removal into three classes which are shown as follows with respective common names: low alumina soda of a Na component content less than 0.1 wt %, medium soda alumina of a Na component content of the order of 0.1 to 0.2 wt % and regular soda alumina of a Na component content more than the medium soda level, for example of the order of 0.2 wt % or more.
Since a Na component included in alumina shows a nature of high ionic conductance, there arise inconvenience when its content is in excess: insulation resistance, especially insulation resistance at a high temperature of 500° C. or higher is reduced or a mechanical strength at a high temperature is deteriorated. Therefore, alumina based insulator used in a spark plug, which has a Na component at as small a content as possible, has conventionally been considered to be the best of its kind and it has been common sense that a content of the Na component is set at a value as small as 0.05 wt % or less.
It is, as described above, indispensable to use alumina, as raw material, of a low soda family in order to produce alumina based insulator low in Na component content. Alumina of a low soda family is expensive because of a cost increase corresponding to a soda removal process step conducted out of necessity and has an aspect that the alumina is not necessarily desirable from the viewpoint of a raw material cost. In recent years, however, in company with development toward a high powered automobile engine, even insulator for a spark plug with a higher withstand voltage and higher heat resistance has come to be demanded. As a result, the common sense in regard to a low content of a Na component in insulator has increasingly been firmer and the present situation is such that there is established a notion that a cost increase to some extent caused by use of low soda alumina has to be unavoidably accepted.
It is an object of the present invention to provide a spark plug, in which insulator is used that is available at as low a cost as beyond a so far established common sense associated with a Na component content while no inferiority is found in its performance compared with existing insulators, the insulator and a production process for the insulator.
DISCLOSURE OF INVENTION
A spark plug of the present invention comprises: a metal shell arranged outside a center electrode; a ground electrode arranged so as to be opposed to the center electrode, an end of the ground electrode being connected to the metal shell; and an insulator, which resides between the center electrode and the metal shell, and which insulator surrounds the outside of the center electrode, wherein
the insulator includes alumina as a main component and further includes a Na component in the range of 0.07 to 0.5 wt % as a value converted into Na
2
O, and
insulation resistance that is measured by passing a current through the insulator between a terminal metal member and the metal shell while the entire spark plug is held at about 500° C. is 200 M&OHgr; or higher.
A production process for alumina based insulator for a spark plug of the present invention in order to produce an insulator used in the above described spark plug is such that alumina powder in use includes a Na component in the range of 0.07 to 0.65 wt % as a value converted into Na
2
O and a Na component in the surface layers of particles of the powder is in the range of 0.01 to 0.2 wt % as a value converted into Na
2
O; the raw material powder including such alumina powder as a main component is formed into a green (a powder compact) prescribed shape of an insulator; and then the green is sintered to obtain an insulator including alumina as a main component and a Na component in the range of 0.07 to 0.5 wt %. It should be noted that the term a Na component content hereinafter means a content of Na
2
O equivalent to the Na component content in the following description unless otherwise specified.
The present inventors have conducted a serious study on an Na component content in insulator and have discovered facts that, by adoption of a production process specific to the present, with alumina powder, as raw material, in a high compositional range of a Na component content that has been regarded to be of no common sense: in a concrete manner of description, with alumina powder containing a Na component content in the range of 0.07 to 0.65 wt % as a value converted into Na
2
O and a Na component content in the range of 0.01 to 0.2 wt % as a value converted into Na
2
O in the surface layers of particles of the powder, there can be obtained insulator whose performance is comparable to a conventional insulator containing a lower Na component content than the above described range of the insulator, while insulation resistance and mechanical strength at high temperature is not reduced, unexpectedly. In a detailed manner of description about the performance, a spark plug constituted of the insulator can secure insulation resistance of 200 M&OHgr; or more which has conventionally been regarded to be impossible to acquire for the Na component content described above, wherein the insulating resistance is measured in such a manner that the entire spark plug is kept at about 500° C. and a current is made to pass through the insulator between a terminal metal member and a metal shell.
FIG. 10
shows an example of a measuring system. Not only a DC constant-voltage power supply (for example, a power supply voltage 100V) is connected to a terminal metal member
13
side of a spark plug
100
, but a metal shell
1
side is grounded. The spark plug
100
thus outfitted is placed into a heating furnace to heat the spark plug
100
at 500° C. and a current is made to pass through the spark plug
100
. For example, a case is considered where a current measurement resistor (a resistance value is Rm) is used to measure a value of an applied current Im, then an insulation resistance Rx to be measured can be obtained by a formula (VS/Im)−Rm, where VS is an applied voltage to pass the current (in the figure, the applied current value Im is measured from an output of a differential amplifier that amplifies a voltage difference between both ends of the current measurement resistor).
A first constitution of insulator for a spark plug of the present invention is such that the insulator includes alumina as a main component, further includes a Na component in the range of 0.07 to 0.5 wt % as a value converted into Na
2
O and has an insulation withstand voltage of 35 kV/mm or higher at 20° C. When alumina powder including a Na component in the range of 0.07 to 0.65 wt % as a value converted into Na
2
O and a Na component in the range of 0.01 to 0.2 wt % as a value converted into Na
2
O in the surface layers of particles of the powder is used, there can be realized insulator having an insulation withstand voltage comparable with that of a conventional insulator whose Na component content is lower than the above described range in alumina powder of the present invention.
An insulation withstand voltage of the insulator assembled in

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