Material for high frequency suppression and distributor for combustion engine composed of the same material

A distributor having electrodes made of a sintered body composed of a composite material such that the distributor can suppress radio noises to thereby prevent electric interference. The material for the electrodes, the sintered body, is composed of 45-90 mol % of zinc oxide and 55-10 mol % of ferrite, which can suppress radio noises generated between the electrodes due to spark discharging. At the same time, the energy loss within such composite material is small and consumption of electrode can be restrained. Electrode durability is further improved by including bismuth oxide, manganese oxide and cobalt oxide in the electrode material. An ignition plug and electric discharge machine can also advantageously use the electrodes composed of such materials.

FIELD OF THE INVENTION 
The present invention relates to a material for electrodes or contacts 
wherein spark discharges are generated therebetween. The electrodes or 
contacts may be used in automotive distributors, in automotive ignition 
plugs and in electric discharge apparatus. The present invention also 
relates to materials for high frequency suppression. 
BACKGROUND OF THE INVENTION 
A general description of the prior art follows. 
Electromagnetic interferences from radio noise is generated by a spark 
discharge between electrodes. It is necessary that materials used for 
electrodes have low energy loss characteristics while also providing 
suppression effect for high frequency noise. 
The ignition systems of internal combustion engines frequently generate 
radio noise. Radio noise often adversely affects communications systems 
such as televisions or radios. The interference problem is compounded as 
the radio noise is migratory; internal combustion engines having noise 
generating ignition systems are commonly found in vehicles such as 
automobiles and as the automobile moves the radio noise generated by the 
ignition system moves. 
Following are the primary causes of the radiation of the radio noise in the 
ignition system: 
(1) spark discharge between electrodes of ignition plugs, 
(2) spark discharge between a rotating electrode and the fixed electrodes 
of a distributor, and 
(3) spark discharge at a break-contact of the distributor, accompanied with 
the make and break motion. 
There have been efforts to suppress the radio noise generated by the spark 
discharge between a rotating electrode and the fixed electrodes of a 
distributor. Such efforts are hereinafter explained with respect to method 
(I) through method (V). However, none of these prior methods has achieved 
a satisfactory and effective suppression of the radio noise generated by 
the spark discharged between a rotating electrode and the fixed electrodes 
of a distributor. 
METHOD (I): employing a rotating electrode including resistor 
In METHOD (I) a resistor is embedded in a rotating electrode. However, in 
such an assembly stray capacity exists in the electrode in parallel with 
the resistor. As a consequence, this method has disadvantages. For 
instance, only a small noise suppression effect at high frequency ranges 
above 300 MHz is obtained while a large loss in ignition energy through 
the resistor (about several kilo ohms) occurs. Moreover, the noise 
suppression effect is small, e.g. only 5-6 dB even at a low frequency 
range below 200 MHz. 
METHOD (II): employing a flame spray coating electrode 
In this method, a high resistive film is formed on the surface of the 
electrode by flame spray coating method. This method has following two 
disadvantages: 
(1) a large loss in ignition energy, due to the high resistive film formed 
on the surface, 
(2) a poor noise suppression effect, for example only 5-5 dB in the 
frequency range below 200 MHz. 
METHOD (III): widening the discharging gap 
In this method, each discharging gap between a rotating electrode and fixed 
electrodes is widened to 1.5-6.4 mm. Advantageously, this provides 
superior noise suppression effect, for example 15-20 dB. 
However, widening the discharge gap between a rotating electrode and fixed 
electrodes suffers from disadvantages. The disadvantages include an 
extremely large loss in ignition energy due to a wide discharging gap, and 
electrode corrosion caused by the corrosive gas affecting metals, such as 
NO.sub.x, which is generated by the higher discharging voltage between 
electrodes. 
METHOD (IV): employing boride, silicide, carbide and conductive ceramics 
(with the resistivity of 10.sup.-6 -10.sup.-2 ohm cm) for material of 
electrodes. 
The resistivity of such electrodes is relatively small so that the energy 
loss can be small. However, this method has disadvantages such as a poor 
noise suppression effect, for example only 5-10 dB at the frequency range 
below 300 MHz, and easy consumption of electrodes because the low thermal 
conductivity of the materials causes the local heating of the electrodes. 
METHOD (V): employing conductive ferrite for electrodes 
This method provides a good noise suppression effect, for example 10-15 dB. 
However, the electrode is heated by the large current flowing in inductive 
discharge between the discharging gap. Also the method has such a 
disadvantage as the local consumption of the electrode because of the 
discharge heating. Conversely, employing such ferrite that has a high 
resistivity, both the noise suppression effect and the durability of the 
electrodes are sufficient, but the energy loss is seriously large. 
SUMMARY OF THE INVENTION 
Therefore, the primary object of the present invention is to provide a 
material having a high frequency suppression effect, particularly to 
suppress a generation of a radio noise. 
Another object of the present invention is to provide a material composing 
electrodes to form a discharging gap therebetween, which is endurable 
against the discharge, suppressive for a radio noise generated in the 
discharge and has a small energy loss. 
A further object of the present invention is to provide a material suitable 
for use as an electrode in a distributor, an ignition plug and an electric 
discharge machine. 
Still another object of the present invention is to solve the aforesaid 
problems of the prior arts and to provide a distributor for an ignition 
system having a sufficient noise suppression effect. 
A further object of the present invention is to provide a distributor for 
an ignition system, with a small energy loss. 
Another object of the present invention is to provide a distributor for an 
ignition system employing electrodes at low cost, and the end part of 
which is durable against consumption. 
The material for high frequency suppression of the present invention is 
composed of sintered powdery mixture of zinc oxide in 45-90 mol % and 
ferrite in 55-10 mol %.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 is a cross sectional view showing the structure of the distributor 
according to the embodiment of the present invention. The distributor 
comprises a housing 1 and the distributor cap 2 made of insulating 
material and attached to the housing 1. At the upper bottom of the 
distributor cap 2, are settled fixed electrodes 3 which project from the 
cap. The fixed electrodes 3 are respectively connected to each ignition 
plug through a high-tension wire which is not shown. At the center of the 
upper bottom of the distributor cap 2, a central terminal 4 is placed 
projecting. The central terminal 4 is connected to a secondary winding of 
an ignition coil which are not shown. At the end of the central terminal 
4, a conductive spring 6 is disposed which is provided with a slider 5 
composed of a carbon and movably supported by the distributor cap 2. The 
housing 1 and the distributor cap 2 form an inside hollow space, wherein a 
cam shaft 7 is situated. The cam shaft 7 is rotated interlocking with a 
crank shaft of the internal combustion engine. At the upper end of the cam 
shaft 7, a distributor rotor 8 is placed. The distributor rotor 8 
comprises an insulating substrate 9 and a rotating electrode 10 disposed 
on the upper surface of the insulating substrate 9. One side of the 
rotating electrode 10 is connected with the slider 5 pressurized by the 
conductive spring 6. The rotating electrode 10 is rotated in accordance 
with the rotating of the distributor rotor 8, in the manner where the 
rotating electrode 10 sequentially opposes to each of the plural fixed 
electrodes 3 so as to form a narrow gap with the fixed electrode. When the 
rotating electrode 10 comes to oppose to one of the fixed electrodes 
through a narrow gap therebetween, which is the state as shown in FIG. 1, 
a high voltage generated by the ignition coil is applied to the central 
terminal. A spark discharge occurs at the narrow gap as a result of the 
air ionization. Simultaneously, a discharge occurs at the spark gap within 
the spark plug, which is connected with the aforesaid narrow gap in 
series. Thus the desired ignition operation can be performed. In this 
operation, the spark discharge at the narrow gap in the distributor causes 
radio noises, as well as the spark discharge within the ignition plug. In 
the above-mentioned operation, the high voltage is not applied to the 
discharging gap as a step function, but increases along with a exponential 
curve characterized by a time constant which is determined by the circuit 
constant such as the ignition coil and the high-tension wire. When the 
voltage reaches to the value sufficient to cause a spark discharge, a 
breakdown occurs at the discharging gap accompanied with a spark 
discharge. Because of the abrupt breakdown, the discharge current flows 
suddenly, as narrow pulses (10-100 ampere). This unstable current with 
high peak value of 10-100 amperes, containing many unfavorable 
high-frequency waves, causes the high-tension wire to radiate the radio 
noise. Noise-field intensity is in proportion to the amount of a noise 
current, therefore, the noise current has to be reduced in order to 
suppress the radio noises. 
The discharging current flowing between the rotating electrode and the 
fixed electrode is composed of two kinds of currents which are a 
capacitive discharge and inductive discharge. 
In the capacitive discharge, the electric charge accumulated in capacitors 
abruptly discharges. These capacitors are constructed by the electrodes at 
discharging gap, the high-tension wire and earth, or the electrodes and 
earth. Therefore, high-frequency current momentary and abruptly flows in 
several nanoseconds. In the capacitive discharge, the discharging current 
is the source of the aforesaid noise current. 
On the other hand, the inductive discharge means a low-frequency current 
(10-100 mA) which flows continuously after the capacitive discharge. The 
energy for ignition supplied to the ignition plugs is approximately in 
proportion to the product of the inductive discharge I and the duration of 
the discharge T, (I.times.T). 
Accordingly, it is obvious that only the capacitive discharge may be 
reduced in order to suppress the noise current without reducing the energy 
for ignition. For this noise suppression purpose, various methods have 
been proposed, such as the above-described methods (IV and V), which 
employ conductive ceramic; but the resistivity value for such a ceramic 
should be small in order to minimize the energy loss. As shown by the line 
"a" in FIG. 2, excessive current may flow through the electrodes to heat 
even at the low voltage. Also such ceramics have a low thermal 
conductivity so that the electrodes are worn out because of the local 
heating. 
According to the present invention, zinc oxide alone or mixed with bismuth 
oxide and manganese dioxide is employed as one component for the material 
of electrodes. In this regard, zinc oxide has certain advantageous 
characteristics. More particularly, zinc oxide may act as a varistor such 
that the resistivity at a high voltage decreases while ignition energy 
losses are minimized. 
Zinc oxide is a semi-conductive ceramic and the structure thereof is as 
shown in FIG. 3, which has grain boundary 20. When the noise current flows 
therethrough, the lattice misfit of the grain boundary causes a large 
resistivity only to the high-frequency noise current so that the noise 
current can be suppressed. 
By adding magnetic materials such as Ni-Zn ferrite to zinc oxide, a 
high-frequency magnetic flux is induced by the noise current. Thereby the 
noise current can be suppressed because of the eddy current loss and the 
hysteresis loss which are caused by the high-frequency magnetic flux. The 
higher the permeability of the employed magnetic material is, the larger 
is the eddy current loss and a larger noise suppression effect is obtained 
at the same frequency. However, the excessive permeability is not 
preferable because it causes the large loss in low-frequency current. 
Therefore, there is an adequate range of permeability of the magnetic 
material. For example, a magnetic material with a relative permeability of 
100 can absorb and suppress a noise current in the frequency range of 
50-500 MHz, as long as the energy loss is within a permitted range. 
The present invention is based on the foregoing features. 
In the distributor according to the present invention, a sintered body is 
employed for either or both of the rotating electrode and the fixed 
electrode. The body is produced by blending powders of zinc oxide in 45-90 
mol % and ferrite in 55-10 mol % and sintering the mixture. The 
composition of ferrite over 55 mol % causes the large loss in ignition 
energy above the permitted value, though the noise suppression effect 
becomes higher. Therefore, the composition of ferrite is preferred to be 
under 55 mol %, and is preferred to be above 10 mol % because the 
noise-suppression effect by use of the magnetic characteristics such as an 
eddy current loss and hysteresis loss is small when it is under 10 mol %. 
Various types of ferrite are available and include Ni-Zn ferrite 
[(Ni-Zn)Fe.sub.2 O.sub.4 ], Ni ferrite (NiFe.sub.2 O.sub.4), Ni-Mn ferrite 
(Ni-Mn)(Fe.sub.2 O.sub.4). In general, the other forms of ferrite material 
can be used wherein (a) the ferrite is characterized by the formula 
MFe.sub.2 O.sub.4 where "M" designates a metal selected from a group 
comprising managanese, iron, cobalt, nickel, copper, lithium and the like; 
(b) the ferrite is characterized by the formula MFe.sub.12 O.sub.19 where 
M designates barium, strontium (Sr), lead or the like having a 
magnetoplumbite type crystalline structure; (c) an iron oxide is 
characterized as having a perovskite type of crystalline structures such 
as that the oxide has the formula MFeO.sub.3 where "M" designates rare 
earth elements; and (d) an iron oxide is characterized as having the 
garnet type of crystalline structure such as one having the formula 
M.sub.3 Fe.sub.5 O.sub.12 where M designates earth elements. 
When employing Ni-Zn ferrite, the preferred composition ratio is designated 
by the oblique lines in FIG. 9. Zn ferrite effectively improves the 
permeability. 
As the other component of the present invention, zinc oxide may be included 
along with the ferrite materials or the sintered material (zinc oxide and 
ferrite) may also include bismuth oxide, manganese oxide, cobalt oxide or 
mixtures thereof. The addition of these materials can improve the 
durability of the sintered body. 
Zinc oxide is employed as one component for the material of the electrode 
used for the present invention. Zinc oxide has a characteristic as a 
varistor, the resistivity of which is high at low-voltage and reduced as 
the voltage getting higher. Therefore, the loss in ignition energy at the 
electrodes can be minimized by selecting the range of low resistivity. The 
high-frequency noise current is generated when the voltage at the gap is 
high that is electric field in the electrodes is small. The high-frequency 
current flows within the high resistivity range of zinc oxide so that the 
current can be suppressed. The addition of ferrite to zinc oxide can cause 
an eddy current loss or hypteresis loss due to the high frequency magnetic 
flux which is produced by the capacitive discharging current. Thus, the 
capacitive discharging current can be restrained to flow. Consequently, 
the electrodes composed of the material which comprises zinc oxide and 
ferrite blended in a determined proportion can be effective for minimizing 
the loss in the ignition energy at the electrodes and also effective for 
suppressing the radio noise. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, the preferred embodiments will be described. 
EMBODIMENT 1 
Materials composed of ferric oxide (Fe.sub.2 O.sub.4) in 50 mol %, nickel 
oxide (NiO) in 35 mol % and zinc oxide (ZnO) in 15 mol % were pulverized 
and mixed together by set blending in a ball mill, then they were 
temporarily sintered for two hours at 1100.degree. C. so that Ni-Zn 
ferrite was compounded. The relative permeability of this ferrite was 
1000. The polyvinylalcohol (PVA) in 1 weight % was added as a binder to 
the compounded ferrite in 40 mol % with zinc oxide 60 mol %, thereafter 
the whole materials were formed into a shape of a rotating electrode by a 
dry pressure compacting. This formed body was processed by sintering at 
the speed of temperature up of 100.degree. C. per hour, keeping at 
1400.degree. C. for two hours, and at the speed of temperature down of 
100.degree. C. per hour. The resistivity of the rotating electrode 
obtained by this process was 2.times.10.sup.5 ohm cm at 100 V DC, and the 
relative permeability thereof was about 50. FIG. 4 shows the measuring 
result of the noise-field intensity when this rotating electrode was 
mounted on a distributor and operated in the condition that the rotating 
speed of a crank shaft was 1500 rpm. In FIG. 4, noise-field intensity of 1 
.mu.A/m within 120 KHz band is determined as 0 db. As apparent from this 
figure, the distributor according to the present invention had more 
improved noise suppression effect of 20-30 dB, compared with the 
conventional distributor employing conventional metallic electrodes. 
Also a conventional type of a distributor composed of conductive ferrite 
has a problem in durability though it is effective considering the 
noise-suppression. Compared with this conventional one, the distributor of 
the present invention could be more durable. A car equipped with the 
distributor according to the present invention was operated for 100,000 
km; the distributor of the present invention operated without any 
problems. It is because the electrode used in the present device were made 
of such material that the resistivity thereof is high in the low voltage 
range, and low in the high voltage range as described by the curve "b" in 
FIG. 2, and therefore, the peak value of the discharging current was 
depressed to the low level so that local heating could be prevented. 
Generally, the material of a high resistivity is likely to cause a large 
energy loss. The material of the electrodes according to the present 
invention was organized as shown in FIG. 5, wherein zinc oxide particles 
24 around ferrite particles 22 form a grain boundary. The contact 
resistance between zinc oxide 24 and ferrite 22 and that among each 
particles of zinc oxide 24 within the grain boundary were abruptly reduced 
in a high voltage. Thus, the material had a characteristics of a varistor 
as designated with a curve "b" in FIG. 2 and the energy loss could be 
restrained to such a level as not to affect the normal running by a car. 
Conventionally, as so-called conductive ferrite is composed of ternary 
system Fe.sub.2 O.sub.3 -ZnO-NiO, the proportion of which are respectively 
70 mol %, 10 mol % and 20 mol %. The resistivity of this ferrite is about 
10 ohm cm and the relative permeability is about 50. 
As other conductive ferrite, is also known a ternary system ferrite, which 
is composed of Fe.sub.2 O.sub.3 in 70-80 mol %, MnO in 10-20 mol % and ZnO 
in 10-20 mol %, respectively. 
The difference of the material composing the the present invention from the 
conventional conductive ferrite is that zinc oxide particles are blended 
with the ferrite particles in the present invention. This fact was proved 
by the experimental data of the ferrite material of a single phase and the 
material of the present embodiment, which were measured by means of a 
X-ray photoelectronic spectroscope. FIG. 6 shows the result of the ferrite 
material of a single phase, and FIG. 7 is of the mixed crystal of the 
present embodiment composed of zinc oxide and ferrite, both of which were 
measured by means of a X-ray electronic spectroscope. As apparent from 
FIG. 7, there are peaks P1, P2, P3 and P4 caused by linkage of Zn-O so 
that the existence of zinc oxide crystal grains are found in the materials 
according to the present embodiment. 
In the present embodiment, ferrite with high permeability is dispersed in 
zinc oxide. For suppressing the high-frequency noise current, the material 
with high permeability is preferred to be disposed in the flowing way of 
the current. The high permeability material is compounded by solid 
solution of other additional material in 50 mol % with ferric oxide in 50 
mol % from a stoichiometrical point of view. However, it is hard to make 
the conductive ferrite to have a high permeability, for ferric oxide 
occupies large ratio as afore-mentioned. 
According to the present invention, zinc oxide with additional high 
permeability material is integrally sintered, resulting in the high 
suppression effect on radio noises. Also the range of frequency that can 
be absorbed by the material can be adjusted by selecting an added magnetic 
material with a proper permeability. 
EMBODIMENT 2 
Next, various kinds of mixture were prepared by changing respectively the 
composition ratio of zinc oxide and ferrite. And the noise suppression 
effect in response to the composition of ferrite were measured so that the 
result is shown in FIG. 8, in which the energy loss is also shown. The 
formation of ferrite was similar to that of the first embodiment. This 
result shows that the addition of ferrite above 55 mol % is an obstacle to 
ignition facilies because it reduces the discharge energy at the ignition 
plugs. Therefore, the composition of ferrite has to be depressed under 55 
mol %. Reversely, the noise-suppression effect is reduced when the ferrite 
is under 20 mol %. 
EMBODIMENT 3 
Another ferrite (Mn-Zn ferrite) was employed as an additional magnetic 
material to zinc oxide, which was a ternary compound comprising ferric 
oxide, manganese oxide and zinc oxide. By the similar process with the 
first embodiment, Mn-Zn ferrite was compounded from the material composed 
of ferric oxide in 50 mol %, manganese oxide in 20 mol % and zinc oxide in 
30 mol %. Relative permeability of abovementioned compounded ferrite is 
3000. FIG. 10 shows a frequency characteristics for the radio noise 
suppression of a distributor, which employs electrodes composed of such 
compounded ferrite as 70 mol % for powdery ferrite and 30 mol % for zinc 
oxide. And it is found that the noise suppression effect of the compound 
according to the present embodiment is rather high at the lower frequency 
side, compared with a compound comprising Zno and Ni-Zn ferrite in the 
first embodiment. 
According to the present invention, a material for high frequency 
suppression can be obtained. The present material is not limited to be 
used for the above mentioned electrodes for a distributor, but also 
employed for electrodes for an ignition plug and for an electric discharge 
machine. 
Obviously, numerous modifications and variations of the present invention 
are possible in light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims, the invention may 
be practiced otherwise than as specifically described herein.