CA2058695C - Sintered ceramic body and a method of making a spark plug insulator therefrom - Google Patents

Abstract

A sintered ceramic body for a spark plug insulator has nitride or oxinite-based ceramic powder with AlN or AlON as a main component. The insulator contains magnesium (Mg), an amount of which ranges from 0.01 wt%
to 5.0 wt% when the amount the magnesium (Mg) is calculated by reducing the magnesium (Mg) to its oxidized compound (MgO). The insulator further contains a sintering additive up to 10 wt% selected from the group consisting of alkali-based earth metals and rare-earth metals in which the weight percentage of the sintering additive is calculated by reducing the sintering additive to its oxidized compound.

Description

TITLE OF THE INVENTION

A Sintered Ceramic Body and a Method of Making a Spark Plug Insulator Therefrom BACKGROUND OF THB INVENTION

Field of the Invention This invention relates to a sintered ceramic body well-suited for a spark plug insulator which needs an elevated insulation property at high ambient temperature while maintaining good thermal conductivity.

Description of Prior Art In a spark plug insulator for an internal combustion engine, a nitride-based sintered ceramic body has been employed since the sintered ceramic body has good thermal conductivity.
The nitride-based sintered ceramic body, however, decreases its electrical insulation when exposed to the high ambient temperature, and grows dendritic crystals treeing over the surface of the sintered ceramic body due to Joule's heat caused from corona discharge creeping over the surface of the sintered ceramic body upon applying high voltage.
Therefore, it is an object of the invention to provide a sintered ceramic body for a spark plug insulator which are capable of maintaining an elevated insulation property at the high ambient temperature while ensuring good thermal conductivity, thus preventing generation of Joule's heat to avoid growth of the dendritic crystals treeing over the surface of the sintered ceramic body when high voltage is applied.
SUMMARY OF THE INVENTION
According to the invention, there is provided a sintered ceramic body comprising nitride or oxinite-based ceramic powder with AlN or AlON as a main component, grain size of which is 1.5 ~m while oxygen-laden rate of the powder is less than 2 weight percentage; and magnesium (Mg), an amount of which ranges from 0.01 wt% to 5.0 wt% inclusive when the amount the magnesium (Mg) is calculated by reducing the magnesium (Mg) to its oxidized compound (MgO).
Further, the sintered ceramic body contains sintering additive up to 10 weight percent selected from the group consisting of alkali-based earth metals and rare-earth metals in which the weight percentage of the sintering additive is calculated by reducing the additive to its oxidized component.
An addition of the magesia (MgO) causes to form grain boundaries among crystal lattices during a process in which the ceramic body is sintered. This significantly contributes to elevating electrical insulation of the B

ceramic body at the high ambient temperature.
When the sintered ceramic body is employed to a spark plug insulator, the elevated insulation enables to prevent corona discharge creeping over the surface of the sintered ceramic body, thus avoiding generation of Joule's heat to prevent growth of dendritic crystals treeing over the surface of the sintered ceramic body when high voltage is applied.
The magnesia (MgO) of less than O.Ol wt% has almost no affect on elevating the electrical insulation of the ceramic body at the high ambient temperature.
The magnesia ~MgO) exceeding 5.0 wt% induces voids in the ceramic body while sintering the ceramic body, thus reducing relative density of the ceramic body so as to give it a moisture absorbing property.
Further, adding the sintering additive up to 10 weight percent leads to improving sintering property of the sintered ceramic body.
Addition of the sintering additive exceeding 10 weight percent causes to significantly impair good thermal conductivity intrinsically provided with the nitride-based ceramic body.
No addition of the sintering additive, however, works to reduce the sintering property, and thus requiring an increased amount of magnesia (MgO) to ensure sufficient insulation property of the sintered ceramic body.

- 20 588 95 ~7.ti With the sintered ceramic body employed to the spark plug insulator, there is provided a spark plug which is capable of preventing growth of dendritic crystals treeing over the surface of the sintered ceramic body upon applying high voltage, thus maintaining both heat-resistant and anti-fouling property.
These and other objects and advantages of the invention will be apparent upon reference to the following specification, attendant claims and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic plan view showing a device to measure electrical insulation of various test pieces at high temperature.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to Fig. 1 and Table 1, aluminum nitride (AlN) powder is prepared as a nitride-based ceramic, grain size of which measures 1.5 ~m in average (sedimentation analysis) with an oxygen-laden rate as 1.0 weight percent. It is necessary to keep the oxygen-laden rate below 2.0 wt% in maintaining good sintering property and good thermal conductivity.
Each of sintering additives employed herein are all 99.9 % purity selected from the group consisting of 205BB 95 ~

yttrium oxide (Y203), calcium oxide (CaO), barium oxide(8aO~, strontium oxide (SrO), scandium oxide (SCzO3), europium oxide (Euz03) and lanthanum oxide (La203).
The test pieces (Nos. 1 ~ 15) of sintered ceramic body according to the invention are manufactured as follows:
(1) A mixture of the sintering additive (except for the case of Nos. 1 ~ 2), aluminum nitrite (AlN) powder, magnesia (MgO) and ethanol are kneaded overnight according to Tabel 1.

(2) After desiccating the mixture to degrease it, the mixture is pressed within a metallic die to form a compact plate which measures 50 mm in diameter and 3 mm in thickness for the purpose of measuring its electrical insulation.

(3) The compact plate is calcined in an atmospheric environment at the temperature of about 500 C for approximately 2 hours, and is pressed under the pressure of about 1.0 ton/cm2 by means of cold isostatic press (C.I.P.).

(4) Then, the compact plate is sintered at 1750 ~ 1900 C in nitrogen atmosphere for 2 ~ 5 hours as shown in Table 1.

(5) The compact plate is lapped to measure 40 mm in diameter and 1 mm in thickness.
According to Table 2, counterpart test pieces (Nos. 16 - 28) are substantially sintered in the same ~0 58 6 9 5 manner as descr ibed above .

_ d ~, ' ~ o O o o o o o o o o o o In~, ~; O O u~ o~ O ~ O O O O O o o o --o ~ ~ ~ ~ ~ ~ U~ O O ~ In o ~ _4 --~ O `D ~O O ~ ~ O U~ C~ O ~ ~ ~ ~ O

~ 8 ~ o o In L~ o o ~ ~ ~ ~ o ~ ~
~ . ~

b'O C~-- N 1~ ~ ~ ~ ~ l~ 1~ l~ N ~J N ~ N ll~
~: o U~ X X X X X X X X X X X X X X X
'~'~ ~ O O O o O O O O O O O O O O
~ ~ X O 1~ 0 0 0 0 l~ If~ O IS~ o o lf~

_.
~ ~ u~ O o oo o o O
O O O U~ O O C`l o U~~ N O O
. O ~ 15~ C`l O O ~ O O ~ O N ~ ~ ~ ~
q~
O bO
~, _ ~ o o o o o o o o o o In O
i ~ V
b~ V ~ 3 1 1 0 ~ ~ O ~ m ._ ~ _ ,,~

~o C V I I O O O O O O O
~ ~ " ,~ ' L ~ :~
",~ ~ ~ ~ ~ v v a~

V o o o o o~ O ~ ~ O o o o o o o i _ x o o m LO 0~ o t~ ~ ~ N m IJ~ o z ~ N ~ ~ ~ O ~ N ~ ~( Table 2 t t pewcxnt I percent f percert 5lnditiO3s den5~ty co~ducel~lcy it57 16 97.00 Yl 03 3.000 - 1800 X 2 99.5 160 5 17 94.00 ~rl 03 6.000 - 1750 x 5 99.0 155 3 18 95.00 CaO 5.000 - 1850 x 5 99.0 120 45 19 92.00 SrO 8.000 - 1750 X 2 99.5 105 25 97.00 Yz 03 2.995 0.005 1750 X 2 99.5 155 ~ 0 21 g 7- 00 Sl-O 2.998 0- 002 1800 X 5 99.5 130 30 22 86.00 Y2 03 12.000 2.000 1700 X 2 98.0 75 1500 23 83.00 Sl O 15.000 2.000 1700 X 2 99.0 60 2000 24 80.00 E u, 03 18.000 2.000 1650 X 2 97.5 45 600 SX~
88.00 Y2 03 5- 000 7.000 1750 x 2 93.0 50 1050 26 85.00 Yl 3 5.00010.000 1750 X 2 90.0 35 2000 ~a 27 88.00 SrO 4. 0008. 000 1650 x 2 92.0 35 4500 28 89.50 CaO 4. 0006.500 1700 x 2 91.0 45 6500 ~o5sB 95 -In Tables 1 and 2, each relative density of test pieces (Nos. 1 - 28) is obtained as a ratio of apparent density/theoretical density by using Archimedean method.
A device shown in Fig. 1 is used to measure each electrical insulation of the test pieces (Nos. 1 - 28) at 700 'C. The device has brass-made electrodes 100, 200, a coil heater 300 and a 500-volt digital resistance meter 400. Regarding to measurement of thermal conductivity, a laser flash method is used. Each amount of magnesia (MgO) and the sintering additive is measured on the basis of fluorescent-sensitive X-ray detection.
Among the test pieces (Nos. 1 - 28), the Nos. 1 -2 are substantially acceptable as a spark plug insulator, considering that the spark plug insulator needs the thermal conductivity of more than ~6 W/m-k in heat-dissipating viewpoint and the electrical insulation of more than 50 MQ at ~00 'C from treeing-preventing viewpoint, while requiring the relative density of more than 95 % in curbing growth of dendritic crystal treeing.
It is found that the test piece Nos. 3 - 15 are better-suited for a spark plug insulator from the point of the sintering property, the relative density, the thermal conductivity and the electrical insulation.
The counterpart test piece Nos. 16 - 19 contain no magnesia (MgO) so that each of their electrical insulation is less than 50 MQ at 700 C. The counterpart test piece Nos. 22 - 24 contain the sintering additive ~o 586 9g exceeding 10 wt% so that each of their thermalconductivity is less than 75 W/m-k. The counterpart test piece Nos. 25 - 28 contain magnesia (MgO) of more than 5 wt% so that each of their relative density is less than 95 %.
A spark plug insulator is made in accordance with the test piece Nos. 1 ~ 15. In an axial bore of the insulator, a center electrode, a resistor and a terminal electrode are placed through a conductive glass sealant.
Then, the insulator is placed within a metallic shell to form a spark plug which is found to be capable of avoiding Joule's heat generation caused from corona discharge creeping over the surface of the insulator so as to prevent growth of dendritic crystals treeing over the surface of the insulator upon applying high voltage, thus maintaining both heat-resistant and anti-fouling property.
It is noted that the nitride-based ceramic categorically includes sialon (Trademark) and aluminum oxinite (AlON).
It is noted that sintering additives may be selected in an appropriate combination from the group consisting of yttrium oxide (YzO3), calcium oxide (CaO), barium oxide (BaO), strontium oxide (SrO), scandium oxide (SCzO3), europium oxide (Euz03) and lanthanum oxide (LazO3), as long as an amount of the combination remains up to 10 wt%.
_ g _ 2n 5â ~ 9 5 It is further noted that the sintering additive may be an oxidized compound of a metal selected from the group consisting of neodymium ~Nd), dysprosium (Dy) and cerium (Ce~.
It is also appreciated that the sintering additive may be a metallic compound selected from the group consisting of chloride, hydroxide, fluoride, carbide, sulfide, carbonate, nitrate, acetate or phosphate.
While the invention has been described with reference to the specific embodiments, it is understood that this description is not to be construed in a limiting sense in as much as various modifications and additions to the specific embodiments may be made by skilled artisan without departing from the spirit and scope of the invention.

Claims (5)

1. A sintered ceramic body comprising nitride or oxinite-based ceramic powder with AlN or AlON as a main component, grain size of which is 1.5 µm while oxygen-laden rate of the powder is less than 2 weight percentage; and magnesium (Mg), an amount of which ranges from 0.01 wt% to 5.0 wt% inclusive when the amount the magnesium (Mg) is calculated by reducing the magnesium (Mg) to its oxidized compound (MgO).

2. A sintered ceramic body as recited in claim 1 further comprising a sintering additive up to 10 wt% selected from the group consisting of alkali-based earth metals and rare-earth metals, the weight percentage of the sintering additive being calculated by reducing the sintering additive to its oxidized compound.

3. A sintered ceramic body as recited in claim 2 wherein, the sintering additive is selected from the group consisting of yttrium oxide (Y2O3), calcium oxide (CaO), barium oxide (BaO), strontium oxide (Sro), scandium oxide (SC2O3), europium oxide (Eu2O3) and lanthanum oxide (La2O3).

4. A method of sintering a ceramic body comprising steps of:
preparing a mixture of nitride-based ceramic powder with AlN or AlON as a main component, magnesium (Mg) and a sintering additive selected from the group consisting of alkali-based earth metals and rare-earth metals, an amount of the magnesium (Mg) ranging from 0.01 wt% to 5.0 wt% inclusive when the amount the magnesium (Mg) is calculated by reducing the magnesium (Mg) to its oxidized compound (MgO), while the weight percentage of the sintering additive is up to 10 wt% when calculated by reducing the sintering additive to its oxidized compound.
calcining the mixture in an atmospheric environment at approximately 500 °C for about 2 hours;
pressing the mixture approximately under 1.0 ton/cm2 by means of cold isostatic press to form a compact body; and sintering the compact body in nitrogen atmosphere at temperature ranging from 1750 °C to 1900 °C for 2 ? 5 hours.

5. A sintered ceramic body as recited in claim 4 wherein, the sintering additive is selected from the group consisting of yttrium oxide (Y2O3), calcium oxide (CaO), barium oxide (BaO), strontium oxide (SrO), scandium oxide (SC2O3), europium oxide (Eu2O3) and lanthanum oxide (La2O3).