U.S. patent number 3,669,907 [Application Number 04/687,229] was granted by the patent office on 1972-06-13 for semiconductive elements.
This patent grant is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Tadao Kohashi, Kazunobu Tanaka.
United States Patent |
3,669,907 |
Kohashi , et al. |
June 13, 1972 |
**Please see images for:
( Certificate of Correction ) ** |
SEMICONDUCTIVE ELEMENTS
Abstract
Semiconductive elements having a specific resistivity of the
semiconductor range (that is, 10.sup.5 to 10.sup.8 ohm-cm) which
have an ohmic resistance even in an considerably high electric
field and are highly durable in adverse physical and atmospheric
conditions, which are manufactured by heating a mixture of powder
of vitreous binding material and powder of semiconductive metal
oxide dispersed in the former powder. Further, the dielectric
strength and the uniformity of resistivity of said semiconductive
elements can be improved by adding, to the mixture, powder of an
oxide whose conductivity is much higher than that of said
semiconductive metal oxide.
Inventors: |
Kohashi; Tadao (Yokohama,
JA), Tanaka; Kazunobu (Kawasaki-shi, JA) |
Assignee: |
Matsushita Electric Industrial Co.,
Ltd. (Osaka, JA)
|
Family
ID: |
13725477 |
Appl.
No.: |
04/687,229 |
Filed: |
December 1, 1967 |
Foreign Application Priority Data
|
|
|
|
|
Dec 7, 1966 [JA] |
|
|
41/80696 |
|
Current U.S.
Class: |
252/520.1;
338/308; 501/18; 501/79; 252/520.2; 252/520.4; 252/521.3; 501/17;
501/76 |
Current CPC
Class: |
H01C
17/06533 (20130101); H01B 1/00 (20130101) |
Current International
Class: |
H01B
1/00 (20060101); H01C 17/06 (20060101); H01C
17/065 (20060101); H01b 001/06 (); C03c
017/00 () |
Field of
Search: |
;252/518,519,520,521
;117/229 ;106/39,48,49,46A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Drummond; Douglas J.
Claims
What we claim is:
1. A method for manufacturing a semiconductive element having
particles of pulverized semiconductive metal oxide dispersed in a
vitreous binding material comprising the steps of mixing a powder
of glass with a powder of at least one semiconductive metal oxide
selected from the group consisting of SnO.sub.2, TiO.sub.2,
WO.sub.3, Sb.sub.2 O.sub.5, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3,
V.sub.2 O.sub.5 and Bi.sub.2 O.sub.3, whose grain size is larger
than the grain size of said glass powder, the amount of said metal
oxide being 10 to 35 percent in volume, until said particles of
said metal oxide powder are well dispersed in said glass powder,
and heating the mixture.
2. A method for manufacturing a semiconductive element having
particles of pulverized semiconductive metal oxide dispersed in a
vitreous binding material, comprising the steps of mixing
pulverized glass, whose average grain size is smaller than 1
micron, with a powder of at least one semiconductive metal oxide
selected from the group consisting of SnO.sub.2, TiO.sub.2,
WO.sub.3, Sb.sub.2 O.sub.5, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3,
V.sub.2 O.sub.5 and Bi.sub.2 O.sub.3, whose average grain size is
smaller than 5 microns but larger than the grain size of said
glass, the amount of said metal oxide being 10 to 35 percent in
volume; stirring the mixture with a volatile liquid additive until
the mixture acquires a sol-like state; applying said mixture in the
form of a layer on the surface of a heat-resistive substrate whose
softening point is higher than the softening point of said glass;
and heating said layer of mixture at a temperature higher than the
softening point of said glass but lower than the softening point of
said heat-resistive substrate, to thereby adhere said layer to said
substrate.
3. A method for manufacturing a semiconductive element having
particles of pulverized semiconductive metal oxide and of
pulverized oxide whose specific resistivity is higher than that of
a semiconductive metal oxide dispersed in a vitreous binding
material comprising the steps of mixing powder of glass with powder
of at least one semiconductive metal oxide selected from the group
consisting of SnO.sub.2, TiO.sub.2, WO.sub.3, Sb.sub.2 O.sub.5,
Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, V.sub.2 O.sub.5 and Bi.sub.2
O.sub.3 and powder of an oxide whose specific resistivity is higher
than that of said semiconductive metal oxide, the grain size of
said metal oxide powder and said oxide powder being larger than
that of said glass powder, and particles of said metal oxide and
said oxide being well dispersed in said glass powder, and heating
the mixture.
4. A method for manufacturing a semiconductive element having
particles of pulverized semiconductive metal oxide and of
pulverized oxide whose specific resistivity is higher than that of
a semiconductive metal oxide dispersed in a vitreous binding
material comprising the steps of mixing pulverized glass, whose
average grain size is smaller than 1 micron, with powder of at
least one semiconductive metal oxide selected from the group
consisting of SnO.sub.2, TiO.sub.2, WO.sub.3, Sb.sub.2 O.sub.5,
Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, V.sub.2 O.sub.5 and Bi.sub.2
O.sub.3 and pulverized oxide, whose specific resistivity is higher
than that of said metal oxide, the average grain size of said metal
oxide and said oxide being smaller than 5 microns but larger than
that of said glass, and the amount of said metal oxide being 10 to
35 percent in volume; stirring the mixture with a volatile liquid
additive until said mixture acquires a sol-like state; applying
said mixture in the form of a layer on the surface of a
heat-resistive substrate whose softening point is higher but lower
than the softening point of said heat-resistive substrate to
thereby adhere said layer to said substrate.
Description
This invention relates to semiconductive elements, particularly to
semiconductive elements which provide electric resistors with
excellent properties.
One object of this invention is to provide semiconductive elements
which are made from the mixture of a vitreous material and a
semiconductive material or materials and which have an ohmic
resistance of a specific resistivity of the order of 10.sup.5 to
10.sup.8 ohm-cm in the semiconductive region.
Another object of this invention is to provide a useful method for
manufacturing the above-mentioned semiconductors.
The conventional conductive glasses which are made from the mixture
of a vitreous material and powder of a metal, are intended to
provide glasses which are very near, or as near as possible, to a
conductor in resistivity, or semiconductive glasses with a
directional resistivity whose specific resistivity is of the order
of 10.sup.5 to 10.sup.8 ohm-cm and is in the range of a
semiconductor but unaffected by an electric field. In the
manufacture of the conventional semiconductive glasses, troublesome
precaution must be taken about the processing atmosphere in order
to prevent the powder of metal from being oxidized or degenerated
because of the high temperature during the heating operation.
Further, metals, generally having a high maleability, are very
difficult to be pulverized beyond a certain limit with the present
techniques of crushing or granulation. Moreover, with the
conventional conductive glasses, control of the resistivity is very
difficult because the resistivity of the metal is substantially
low.
Another formation of the semiconductive elements is organic resins
or plastics containing a conductive powder, the former serving as
binder for the latter particles. Such a formation, however,
presents an extremely non-linear voltage-current characteristics
owing to the incomplete contact between the conductive particles
and the binder, and to the surface state barrier of the conductive
particles. Thus, the intended ohmic resistivity is not obtained by
this kind of formation. Further, the temperature rise caused by an
excessive current due to the non-linearity of the resistance,
deteriorates the binding resin which is of low heat-resistivity,
and shortens the life of the element.
According to this invention, new semiconductive elements which are
highly resistive to the environmental conditions and have an ohmic
resistivity in the semiconductive range (10.sup.5 to 10.sup.8
ohm-cm), are obtained by using a pulverized metal oxide as
semiconductive material which is stable in an atmosphere of
considerably high temperature and can be pulverized comparatively
easily.
Now, this invention will be explained in detail with reference to
the attached drawings, in which;
FIG. 1 is a sectional view of a semiconductive element embodying
this invention;
FIG. 2 is a diagram showing the current vs voltage characteristics
of the semiconductive element shown in FIG. 1;
FIG. 3 is a sectional view of another embodiment of this
invention;
FIG. 4 is a plan view of another embodiment of this invention;
FIG. 5 is a sectional view of still another embodiment of this
invention;
FIG. 6 is a diagram showing the current vs voltage characteristics
of the element shown in FIG. 5;
FIG. 7 is a sectional view of another embodiment of this invention;
and,
FIG. 8 is a plan view of another embodiment of this invention.
Referring to FIG. 1, pulverized tin oxide (SnO.sub.2) 103 is
contained, as a semiconductive material, in vitreous binding
material 102. This semiconductive layer is formed by applying the
mixture of the semiconductive material 103 and the powder of the
vitreous material on the surface of iron plate 104, and then dried
by heating. The iron plate 104 of glazing quality serves as an
electrode and at the same time as a heat-resistive substrate.
Electrode 101 consisting of an electro-conductive paint is applied
on the above-mentioned semiconductive layer.
Manufacturing steps for the above-mentioned embodiment of this
invention will be described hereunder, by way of example. The
composition of the mixture which consists of pulverized frit
(hereafter referred to as A) as the binding material and pulverized
tin oxide (hereafter referred to as B) as the semiconductive
material, is shown in the following table.
Material Percent volume
__________________________________________________________________________
Pulverized frit:A 85% Pulverized SnO.sub.2 :B 15%
__________________________________________________________________________
the percentage of the metal oxide may be varied in the range of 10
to 35 percent depending on the desired resultant resistivity. The
average grain size of A is preferably about 1 micron and that of B
about 5 microns, at most. A larger grain size will cause less
dispersion of the particles and a more difficult forming of the
layer, resulting in a porous layer. On the other hand, a smaller
grain size is desirable, presenting better dispersion, stable
electric resistivity and easy forming. However, it should be noted
that the conditions that the grain size of A is smaller than that
of B should be maintained. If the grain size of A is larger than
that of B, the particles of the frit will be covered with the
particles of the metal oxide and will be prevented from contacting
each other. Accordingly, the layer will not be formed when
heated.
Next, the mixed powder of A and B thus measured is thoroughly
stirred in a suitable mixing or stirring machine. In this example,
the mixture of A and B added with a volatile liquid such as
diacetone-alcohol or octyl-alcohol of equivalent weight was stirred
in a ball-mill for 12 hours. A proper period for stirring is half a
day to 1 day. A shorter period will cause insufficient dispersion
of the particles, while a longer period will result in a less
difference of grain size between A and B which is not enough to
ensure the condition that the grain size of A is larger than that
of B. Thus mixed and dispersed powder is substantially in a
sol-like state, the liquid serving as dispersion medium. The
mixture is applied on the surface of the heat-resistive substrate
by the silk screen method which is a preferable method but to which
this invention is not limited. The object of heating is to fuse the
vitreous binder without solving the metal oxide particle into the
vitreous material, and to cause the binding material to adhere to
the substrate, forming a layer with a smooth surface. Thus, at
least the surface portion of the metal oxide particles makes solid
solution with the vitreous binder, thereby lowering the barrier in
the surface of the metal oxide particles. This improves the
electrical contact between the binding material and the
semiconductive material, rendering an ohmic resistivity to the
layer. A heating temperature as low as possible is preferable.
Accordingly, a frit of lower viscosity in a low temperature, that
is, of lower softening point should be selected. The following
Table 1 shows the composition of the vitreous material used in this
embodiment.
TABLE 1
Ingredients %(weight) Ingredients %(weight) SiO.sub.2 20.01
Na.sub.2 O 10.84 B.sub.2 O.sub.3 28.58 K.sub.2 O 4.05 ZnO 18.33
TiO.sub.2 2.31 BaO 14.34 Al.sub.2 O.sub.3 0.41 CaO 0.74 Fe.sub.2
O.sub.3 0.009 MgO 0.016 PbO 0.012
the softening point of this frit was about 600.degree. to
640.degree. C. An iron plate of approximately the same heat
expansion coefficient as the vitreous binder was used as the
substrate. Heating was performed in an electric oven without any
difficulty. That is, the heating can be carried on in the
atmospheric environment without necessity of any special
arrangement, because tin oxide (SnC.sub.2) used as the
semiconductive material is very stable at a temperature around
640.degree. C in the air and because the substrate used has
substantially the same heat expansion coefficient as that of the
vitreous material. The sol-like mixture of A and B is applied on
the substrate and heated up to 650` C in about 20 minutes after
being placed in an electric oven, during which the liquid component
is evaporated leaving the mixture of powder on the substrate. The
mixture on the substrate is maintained at that temperature
(650.degree. C) for about 5 minutes, followed by a cooling period
of about 20 minutes, after which the whole process is completed. If
a silk screen of 130 mesh is used for sieving the powder of about 1
micron in grain size, a layer of about 20 microns in thickness can
be formed with one cycle of the heating. An additional layer can be
made on the already formed layer, if required for some purposes. As
the viscosity of a glass is considerably high, it will not occur
that the first layer of the glass flows or changes in thickness at
the temperature of 650.degree. C during the short time of 5
minutes. FIG. 2 shows a current vs voltage characteristics of the
semiconductive layer thus formed, the DC voltage being applied
across the electrodes 101 and 104. It will be seen from the
excellent straightness of the characteristics that the layer has
ohmic resistivity. This layer has a considerable large current
bearing capacity, no doubt being heat-resistive.
FIG. 3 shows another embodiment of this invention in which titanium
oxide (TiO.sub.2) is used in place of the tin oxide of the previous
embodiment. In FIG. 3, binder 301 is a vitreous material having a
softening point of 550.degree. to 580.degree. C and contains
pulverized TiO.sub.2 dispersed therein. Transparent electrodes 303
and 304 are formed separately on the glass substrate 305 whose
softening point is 680.degree. to 700.degree. C. A semiconductive
layer consisting of binding material 301 and semiconductive
particles 302 is formed over the electrodes and the substrate
including the space between the two electrodes. The embodiment
shown in FIG. 1 is intended to be a resistor across the thickness
of the layer, whereas this embodiment shown in FIG. 3 is intended
as a lateral pass for the electric current. Another feature of this
embodiment is that the vitreous layer is formed on the surface of a
glass or vitreous substrate. The method for forming the element and
the requirements for the grain size are the same as in the previous
embodiment.
As to the substrate, the material is limited to iron plate,
ceramics and other similar materials in the previous embodiment,
whereas glass is also usable in this embodiment. There are two
conditions for a glass to be usable as substrate. One is that the
softening point of the binder be lower than that of the substrate.
This is an essential condition, because the forming will be
impossible if the substrate softens earlier than the binder during
the heating. The other condition is that the volume expansion
coefficient of the substrate be approximately the same as that of
the vitreous binder. If the expansion coefficients of both
materials are significantly different, the substrate will crack
from the strain caused during cooling, or at least the electrodes
303 and 304 will lose the conductivity owing to the strain. Table 2
shows the volume expansion coefficients and the softening points of
the binder and the substrate used in this embodiment.
TABLE 2
Expansion Coeff. Softening point
__________________________________________________________________________
Binding material 270 .times. 10.sup.-.sup.7 appr. 580.degree.C
Plate glass (substrate) (270-300) .times. 10.sup.-.sup.7
650-700.degre .C
__________________________________________________________________________
in this example, the heating operation was performed at 630.degree.
C. The transparent electrodes (Nesa electrodes) 303 and 304 do not
lose the conductivity through the heating.
FIG. 4 shows another embodiment of this invention, in which the
element is formed in a shape suitable for a potentiometer.
Transparent electrodes (Nesa electrodes) 401 and 402 are formed on
the heat-resistive substrate (ceramics or glass) 403. The
semiconductive layer 404 which comprises a binding material
containing pulverized metal oxide is formed on the substrate and
the electrodes.
In these embodiments, the semiconductive material is not limited to
tin oxide (SnO.sub.2) or titanium oxide (TiO.sub.2) as in the
previous embodiments, but can be one of other semiconductive metal
oxides such as WO.sub.3, Sb.sub.2 O.sub.5, Cr.sub.2 O.sub.3,
Fe.sub.2 O.sub.3, V.sub.2 O.sub.5 and Bi.sub.2 O.sub.3, or a
mixture of any two or more selected from these oxides.
It will be understood that this invention is not limited to the
above-described embodiment, but can be embodied in variously
modified formations according to the respective purposes.
As described above, this invention provides semiconductive elements
wherein particles of semiconductive metal oxide are contained in a
vitreous binding material. Owing to the fact that metal oxide is
used for the component which imparts the conductivity to the
element, the element has a high resistivity against severe
environmental conditions, being stable even at a considerably high
atmospheric temperature and accordingly can be manufactured without
necessitating complicated and expensive equipment and operations.
Thus, according to this invention, semiconductive elements are
provided which have an ohmic resistance of a specific resistivity
of the semiconductive range (that is, 10.sup.5 to 10.sup.8 ohm-cm),
the resistivity being controlled by the relative volume of the
metal oxide contained in the binding material.
In short, the semiconductive elements of this invention can be
easily manufactured by a process comprising the following steps,
that is; a step in which pulverized glass whose average grain size
is smaller than 1 micron is mixed with pulverized semiconductive
metal oxide whose average grain size is smaller than 5 microns but
larger than that of said glass, the amount of the oxide being 10 to
35 percent in volume, and stirred with a volatile liquid additive
until the mixture aquires a sol-like state; a step in which said
mixture is applied in the form of a layer on the surface of a
heat-resistive substrate whose melting point or softening point is
higher than the softening point of said glass; and a step in which
said layer of mixture is heated at a temperature higher than the
softening point of said glass but lower than the melting point or
softening point of said heat-resistive substrate. According to the
method of this invention, the heating can be carried on in an
atmospheric environment without necessity of any special
consideration about the ambient conditions. Further, a smooth and
strongly bound layer is obtained as the result of the fact that the
grain size of the glass powder is selected to be smaller than that
of the metal oxide powder. Moreover, the fact that the grain size
of the glass powder and the metal oxide powder are smaller than 1
micron and 5 microns respectively, eliminates the possibility of
producing a porous layer and ensures the satisfactory dispersion of
the oxide particles into the binding glass, thus making possible
the manufacture of resistors with uniform and reliable properties,
and at the same time rendering the formation of the layer very
easy. Thus, semiconductive elements having a specific resistivity
of the semiconductor range (that is, 10.sup.5 to 10.sup.8 ohm-cm)
can be produced in a very simple manner.
The semiconductive elements of this invention are stable even at a
considerably high temperature and highly resistive against moisture
and other ambient conditions.
In the following paragraphs, another aspect of this invention will
be described with other embodiments.
In this aspect of this invention, a pulverized insulating material
or materials are mixed with the above-described metal oxide powder
in order to aid the dispersion of said metal oxide particles.
According to this embodiment, extremely durable semi-conductive
elements with uniform properties are obtained which have an ohmic
resistance in the semiconductor range (10.sup.5 to 10.sup.8 ohm-cm)
and have an improved dielectric strength. The insulating material
to be mixed is only required to be thermally stable, since this
material is used to prevent the incomplete dispersion or
maldistribution of the oxide particles and in no way contributes to
the conductivity of the element. Here again, the grain size of the
glass powder should be selected to be smaller than those of the
metal oxide powder and the powder of the insulating material.
Mixing, application and heating of the powder are substantially the
same as described in the previous paragraphs with other embodiments
of this invention.
An example of this embodiment will be described hereunder with
reference to the attached drawings. In FIG. 5, vitreous binding
material 102 contains pulverized tin oxide (SnO.sub.2) 103 as
semiconductive material and pulverized BaTiO.sub.3 105 which is
mixed in order to facilitate the dispersion or distribution of the
semiconductive material 103 and to improve the dielectric strength
of the element. The mixture consisting of powders of 102, 103, and
105 is applied, in the form of a layer, on the surface of iron
plate 103, and then dried by heating. The iron plate 103 of glazing
quality serves as an electrode and at the same time as a
heat-resistive substrate. Electrode 101 consisting of an
electro-conductive paint is applied on the above semiconductive
layer. The composition of the mixture which consists of pulverized
frit (hereafter referred to as A), the pulverized tin oxide
(hereafter referred to as B) and the pulverized BaTiO.sub.3
(hereafter referred to as C), is shown in the following table.
Material Percent volume
__________________________________________________________________________
Pulverized frit:A 65% Pulverized SnO.sub.2 :B 15% Pulverized
BaTiO.sub.3 :C 20%
__________________________________________________________________________
the percentage of SnO.sub.2 may be varied in the range of 10 to 35
percent depending on the desired resultant resistivity, with a
corresponding decrease in the percentage of frit. The same
precaution as described in connection with the previous embodiments
is required as to the grain size of B and C in relation to that of
A. The procedures of mixing and stirring of the mixture, the
ingredients of the frit and the heating operation in this example
are substantially the same as in the previous embodiments. FIG. 6
shows a current vs voltage characteristics of the thus formed
semiconductive layer, the DC voltage being applied across the
electrodes 101 and 105. The straightness of the characteristics
shows that the layer has an ohmic resistance. This layer also has a
considerably large current bearing capacity.
FIG. 7 shows another embodiment which is substantially the same as
the embodiment shown in FIG. 3, except that pulverized Al.sub.2
O.sub.3 is added in order to facilitate the dispersion of TiO.sub.2
particles. Namely, binder 301 is a vitreous material having a
softening point of 550.degree. to 580.degree. C and contains
pulverized TiO.sub.2 as the semiconductive material 302 and
pulverized Al.sub.2 O.sub.3 as the dispersing medium 303.
Electrodes of SnO.sub.2 304 and 303 are formed separately on the
surface of glass substrate 305 whose softening point is 680.degree.
to 700.degree. C. The semiconductive layer made from the powders
301, 302 and 306 also fills the space between the two electrodes.
Other structural features of this embodiment are substantially the
same as those of the embodiment described with reference to FIG.
3.
FIG. 8 shows another embodiment of this invention. This embodiment
is substantially the same as the embodiment shown in FIG. 4, except
that the dispersing medium is added in the mixture. In FIG. 8,
transparent electrodes (Nesa electrodes) 401 and 402 are formed on
the heat-resistive substrate (ceramics or glass) 403. Then, the
semiconductive layer 404 is formed on the substrate and the
electrodes.
In these embodiments too, the semiconductive material is not
limited to tin oxide (SnO.sub.2) or titanium oxide (TiO.sub.2) as
mentioned above, but can be any one or any mixture of other
semiconductive metal oxides such as WO.sub.3, Sb.sub.2 O.sub.5,
Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, V.sub.2 O.sub.5 and Bi.sub.2
O.sub.3. Also, the insulating or dispersing medium is not limited
to BaTiO.sub.3 or Al.sub.2 O.sub.3, but can be any other material
(for example, SiO.sub.2) which has a much higher electric
resistance than the above-mentioned metal oxide and a higher
melting point than the vitreous binding material.
As described above, according to the second aspect of this
invention are provided semiconductive elements wherein particles of
a semiconductive metal oxide and particles of an oxide whose
specific resistivity is higher than that of said metal oxide are
contained in a vitreous binding material. Such elements have an
improved dielectric strength and uniformity of resistivity owing to
the sufficient dispersion of the semiconductive particles, besides
the features described in connection with the previous embodiment
according to the first aspect of this invention.
Such semiconductive elements can be easily manufactured by a
process comprising the following steps, that is; a step in which
pulverized glass whose average grain size is smaller than 1 micron
is mixed with pulverized semiconductive metal oxide and pulverized
oxide whose specific resistivity is higher than that of said metal
oxide, the average grain size of said metal oxide and said oxide
being smaller than 5 microns but larger than that of said glass,
and the amount of said metal oxide being 10 to 35 percent in
volume, and the mixture being stirred with a volatile liquid
additive until said mixture acquires a sol-like state, a step in
which said mixture is applied in the form of a layer on the surface
of a heat-resistive substrate whose melting point or softening
point is higher than the softening point of said glass; and a step
in which said layer of mixture is heated at a temperature higher
than the softening point of said glass but lower than the melting
point or softening point of said heat-resistive substrate.
Advantages obtained from such method have been described in
connection with the first aspect of this invention, and therefore
are not repeated here.
* * * * *