U.S. patent application number 11/624632 was filed with the patent office on 2007-06-28 for overvoltage protection materials and process for preparing same.
Invention is credited to Chi-Ming Chan, Ying Kit Cheung, Kai-Mo Ng, Catherine Yuen-Chien Wong.
Application Number | 20070148440 11/624632 |
Document ID | / |
Family ID | 32093174 |
Filed Date | 2007-06-28 |
United States Patent
Application |
20070148440 |
Kind Code |
A1 |
Chan; Chi-Ming ; et
al. |
June 28, 2007 |
Overvoltage protection materials and process for preparing same
Abstract
The invention provides a process for preparing an overvoltage
protection material comprising: (i) preparing a mixture comprising
a polymer binder precursor and a conductive material; and (ii)
heating the mixture to cause reaction of the polymer binder
precursor and generate a polymer matrix having conductive material
dispersed therein, wherein the polymer binder precursor is chosen
such that substantially no solvent is generated during the
reaction.
Inventors: |
Chan; Chi-Ming; (Hong Kong,
HK) ; Ng; Kai-Mo; (Hong Kong, HK) ; Wong;
Catherine Yuen-Chien; (Hong Kong, HK) ; Cheung; Ying
Kit; (Hong Kong, HK) |
Correspondence
Address: |
BERKELEY LAW & TECHNOLOGY GROUP, LLP
1700 NW 167TH PLACE
SUITE 240
BEAVERTON
OR
97006
US
|
Family ID: |
32093174 |
Appl. No.: |
11/624632 |
Filed: |
January 18, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10274904 |
Oct 21, 2002 |
|
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11624632 |
Jan 18, 2007 |
|
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Current U.S.
Class: |
428/329 |
Current CPC
Class: |
H05K 2201/0738 20130101;
Y10T 428/257 20150115; H05K 1/0259 20130101; H05K 1/0257 20130101;
H05K 1/167 20130101; H01B 1/22 20130101; Y10T 428/31663
20150401 |
Class at
Publication: |
428/329 |
International
Class: |
B32B 5/16 20060101
B32B005/16 |
Claims
1-32. (canceled)
33. An overvoltage protection material preparable according to a
process comprising: (i) preparing a mixture comprising at least one
polymer binder precursor and a conductive material; and (ii)
heating the mixture to cause reaction of the polymer binder
precursor and generate a polymer matrix having conductive material
dispersed therein, wherein the polymer binder precursor is chosen
such that substantially no solvent is generated during the
reaction.
34. An overvoltage protection material according to claim 33
wherein the mixture comprising at least one polymer binder
precursor and a conductive material is substantially solvent
free.
35. An overvoltage protection material according to claim 33
wherein the polymer binder is formed via an addition polymerisation
process.
36. An overvoltage protection material according to claim 35
wherein the polymer binder is substantially free of voids.
37. (canceled)
38. An overvoltage protection device preparable according to a
process comprising: (i) preparing a mixture comprising at least one
polymer binder precursor and a conductive material wherein the
polymer binder precursor is chosen such that substantially no
solvent is generated during the reaction; and (ii) heating the
mixture to cause reaction of the polymer binder precursor and
generate a polymer matrix having conductive material dispersed
therein wherein the overvoltage protection device has a trigger
voltage of less than 300 V.
39. An overvoltage protection material comprising a polymer binder
having a conductive material dispersed therein, said overvoltage
protection material being obtainable by an addition polymerisation
reaction of a mixture comprising a polymer binder precursor and a
conductive material.
40. An overvoltage protection material according to claim 39 which
is substantially free of voids.
41. An overvoltage protection material according to claim 39
wherein the addition polymerisation reaction is a bulk free radical
polymerisation reaction.
42. An overvoltage protection material according to claim 39
wherein the addition polymerisation reaction comprises the reaction
of a diisocyanate component and a polyol component.
43. An overvoltage protection material according to claim 42
wherein the polymer binder is a polyurethane.
44. An overvoltage protection material according to claim 39
wherein the addition polymerisation reaction is a hydrosilylation
reaction.
45. An overvoltage protection material according to claim 44
wherein the hydrosilylation reaction comprises the reaction of a
hydrosilane and a vinylsilane.
46. An overvoltage protection material according to claim 44
wherein the polymer binder is a polycarbosilane.
47. An overvoltage protection material comprising a polymer binder
having a conductive material dispersed therein, said material being
substantially free of voids.
48. An overvoltage protection material according to claim 47
wherein the material is obtainable by an addition polymerisation
reaction of a mixture comprising a polymer binder precursor and a
conductive material.
49-54. (canceled)
Description
FIELD OF THE INVENTION
[0001] The present invention relates to materials for protecting
sensitive electronic circuits against transient high electrostatic
pulses. More particularly, it relates to a new solvent-free
methodology for producing these materials, and devices comprises
these materials.
BACKGROUND OF THE INVENTION
[0002] Transient high voltages can induce harmful currents and
voltages in electronic circuits and electric equipment. Normally,
these transient voltages are caused by electrostatic discharge,
lightning or inductive power surges.
[0003] Overvoltage protection materials and devices have been
developed which aim to protect such electronic and electrical
equipment from these transient high voltages. The overvoltage
materials have non-linear electrical resistance characteristics,
specifically having a very high electrical resistance (e.g. >30
MOhm) at a normal operating voltages (e.g. <200 V for normal
electronic instrument), but switching to an essentially conducting
state when subject to a transient high voltage. The voltage at
which the material changes from its high resistance state (or "off
state") to its conducting state (or "on state") is known as the
threshold value, or trigger voltage. It is necessary for the
materials to have a very fast switching time between the high
resistance state and conducting state in order to provide adequate
protection for the electronic or electrical equipment being
protected. An acceptable switching time is usually of the order of
nanoseconds, and is preferably less than one nanosecond. The device
returns back to its normal high resistance state after the
transient high voltage threat has passed.
[0004] The term overvoltage protection material usually refers to a
composite material containing a conductive material within a
polymer matrix system. The overvoltage protection material may
comprise only a two components (the conductive material and the
polymer matrix), or can contain other components such as a
semi-conductive material or a non-conductive material.
[0005] Conventional methods for preparing overvoltage protection
materials include solvent processing or polymer condensation
processes. Usually, the finished polymeric systems should be a
network structure or a thermoset in order to achieve good thermal
properties. Therefore, both of these processing techniques should
be carried out in the presence of a cross-linking component.
(a) Solvent Processing Method
[0006] One method of making thermoset plastics suitable for use as
overvoltage protection materials is a solvent processing method, as
described in U.S. Pat. Nos. 4,977,357 and 5,248,517. The
overvoltage conductive materials are prepared by mixing a dissolved
silicone polymer solution, nickel powder, silicon carbide powder;
inorganic filler and cross-linking agent (e.g. organic peroxide) to
form a paste like composition. This material is then applied to ac
prepared substrate which contains metal electrodes, to form an
uncured device. The device is then cured in an oven at 125.degree.
C. for 4 hours.
[0007] However, this solvent processing technique has a number of
disadvantages. First, the polymer must be dissolved in a suitable
solvent, the choice of which is critical to the success of the
reaction. Not all solvents can be used (e.g. all chlorinate and
some ketone-type solvents are unsuitable), and it can take a great
deal of time to determine which solvents are suitable. Second, a
relatively large amount of solvent must be removed (e.g. sometime
as much as 1:1, solid to solvent weight content) under the high
temperature curing step. Removal of solvent can be damaging to the
device, generating voids within polymer matrix and causing a
resulting degradation in performance of the final overvoltage
protection device. Third, the processing temperature condition
required to evaporate the solvent is relatively high, and the
curing time is relatively long (e.g. approximately 4 hours at
125.degree. C.).
(b) Polycondensation Process Method
[0008] U.S. Pat. No. 5,928,567 discloses a liquid conductive
material designed to protect electrical components from high pulse
static electricity. The process comprises combining a solvent-free
liquid silicone polymer composition (General Electric RTV 11), a
conductive metallic powder, a non-conductive inorganic powder and a
catalytic amount of the curing agent (e.g. dibutyl tin dilaurate)
and mixing these components in a conventional multi-blade mixer.
The paste like material is applied to an appropriate substrate, and
is then cured in a convection oven at 80.degree. C. for 2 hours.
The process is described as solvent free because no conventional
organic solvent is added in the process described.
[0009] However, while no solvent is added to the mixture, the
process itself will generate solvent as a by-product. In the
process specifically described in this patent, the polymerization
process is a common condensation reaction between di- or
tri-hydroxy polydimethylsiloxane and di- or tri-methoxy
polydimethylsiloxane prepolymers. A general reaction scheme is
shown below in Scheme 1. ##STR1##
[0010] From this it can be seen that, although a solvent is not
required as an initial component of the reaction mixture, some
solvent (in this case methanol) will be generated during the course
of the curing reaction. In this instance, one mole of solvent is
generated for every mole of product. During the curing reaction the
solvent present in the mixture will evaporate and in doing so will
generate unwanted voids or bubbles in the polymer matrix. This in
turn diminishes the ability of the device to respond properly
during conditions of transient high voltage.
[0011] From a consideration of the prior art processes it can be
seen that performance of overvoltage protection devices is reduced
due to the presence of solvent either as an added component of the
reaction mixture, or as a by-product formed during the curing
process. It is therefore an object of the invention to provide an
improved and convenient method for preparing overvoltage protection
materials. In particular, it is an object of the invention to
provide a solvent-free, or "clean", process for preparing an
overvoltage protection material suitable for protecting electronic
circuits and electrical devices from transient high voltages, where
no solvent is added to the reaction mixture, and where no
substantially solvent is generated during the course of the
process. It is another object of the invention to provide an
improved overvoltage protection material. It is also an object of
the invention to provide new overvoltage protection devices
comprising this improved overvoltage protection material.
SUMMARY OF THE INVENTION
[0012] The invention provides a process for preparing an
overvoltage protection material comprising: [0013] (i) preparing a
mixture comprising a polymer binder precursor and a conductive
material; and [0014] (ii) heating the mixture to cause reaction of
the polymer binder precursor and generate a polymer matrix having
conductive material dispersed therein, wherein the polymer binder
precursor is chosen such that substantially no solvent is generated
during the reaction.
[0015] The invention also provides a process for preparing an
overvoltage protection device comprising: [0016] (i) depositing a
metallic layer on a substrate, and etching the metallic layer in
order to provide metallic electrodes separated by gaps; [0017] (ii)
preparing a mixture comprising at least one polymer binder
precursor and a conductive material, and applying this mixture
between and in contact with adjacent electrodes; and [0018] (iii)
heating the device to cause reaction of the polymer binder
precursor and generate an overvoltage protection material
comprising a polymer binder having conductive material dispersed
therein; wherein the polymer binder precursor is chosen such that
substantially no solvent is generated during the reaction.
[0019] In another embodiment, the invention provides an overvoltage
protection material preparable according to a process comprising:
[0020] (I) preparing a mixture comprising at least one polymer
binder precursor and a conductive material; and [0021] (ii) heating
the mixture to cause reaction of the polymer binder precursor and
generate a polymer matrix having conductive material dispersed
therein, wherein the polymer binder precursor is chosen such that
substantially no solvent is generated during the reaction.
[0022] In a further embodiment, the invention provides an
overvoltage protection material comprising a polymer binder having
a conductive material dispersed therein, said overvoltage
protection material being obtainable by an addition polymerisation
reaction of a mixture comprising a polymer binder precursor and a
conductive material.
[0023] In each embodiment above, the mixture of polymer binder
precursor and conductive material is preferably substantially
solvent-free. The invention also relates to the polymer binders
described and prepared in the methods above.
[0024] The invention also provides an overvoltage protection
material comprising a polymer binder having a conductive material
dispersed therein, said material being substantially free of
voids.
[0025] Finally, in another embodiment the invention provides a
circuit comprising electronic components, at least a first and
second conducting region, an amount of an overvoltage protection
material being disposed between and in contact with said conducting
regions, the overvoltage protection material being as described
above.
BRIEF DESCRIPTION OF THE FIGURES
[0026] FIG. 1 is a cross-sectional view of an overvoltage
protection device 10.
[0027] FIG. 2 is a plan view of an overvoltage protection device
substrate without an overvoltage protection material.
[0028] FIG. 3 is a plan view of an overvoltage protection device
containing an overvoltage protection material 15.
[0029] FIG. 4 is a diagram of a testing circuit for measuring the
electrical properties of the overvoltage protection materials
according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0030] The present invention describes new processes for producing
overvoltage protection materials, and more specifically describes
some very unique and clean organic synthetic methodologies for
preparing cross-linked polymeric and highly robust networked
systems, and to the formation of conductive composite matrix
materials which are free of unwanted contaminants. The materials
produced according to the invention are useful as overvoltage
protection materials, behaving like electrical insulators under
normal operation, but switching to became conductors under high
transient voltage. The materials also have a high degree of heat
resistance (many thermoplastic polymers become soft or even melt at
temperatures of about 80.degree. C. to about 200.degree. C.).
Furthermore, the simplified processing steps used in the process of
the invention reduce unnecessary contaminants in the final cured
composite materials.
[0031] The process of the invention relates to the synthesis of a
highly cross-linked polymer system which contains a conductive
material, with the resulting composite material being useful as an
overvoltage protection material. During the process, no solvent is
required in order to produce the resulting composite material, nor
is any substantial amount of solvent generated as a by-product of
the reaction. The phrase "substantially no solvent" is intended to
mean that no significant amount of solvent is generated during the
reaction. If any solvent is generated, this is present in a very
small amount, generally less than about 2 wt %, and preferably less
than about 1 wt %, based on the total weight of the composite
material. Obviously it is preferred to prevent any solvent being
generated at all.
[0032] Reducing the amount of solvent generated during the process
has a number of advantages. For example it reduces the likelihood
of unwanted voids or bubbles being produced in the resulting
composite material, such voids or bubbles being caused by fast
evaporation of solvent during the curing process. By reducing the
presence of these irregularities in the composite material, the
electrical properties of overvoltage protection devices made from
this material are improved. When an overvoltage protection material
of the invention is described as being substantially free of voids,
this is intended to mean that voids account for less than 5% of the
volume of the material, preferably less than 1% of the volume of
the material. Clearly, it is advantageous to ensure that the
material is entirely free of voids, however if voids are present,
they are preferably less than about 1 mm in diameter, more
preferably less than about 1 .mu.m in diameter.
Polymeric Binder
[0033] The polymeric binder used in the invention serves two major
roles: first it acts as a media for separation of the conductive
particles, and second it provides enhanced thermal resistance
against shape deformation of the polymer matrix system. The term
"polymer binder" when used herein refers to a cross-linked polymer
structure, also known as a polymer matrix, in which is dispersed
the conductive material. The polymer binder is prepared from a
polymer binder precursor. The term "polymer binder precursor"
represents a precursor composition Which, upon heating, reacts to
form the cross-linked polymer binder, or polymer matrix. The
precursor may comprise a single type of monomer, oligomer or
prepolymer, or may comprise a mixture of different monomers,
oligomers and/or prepolymers. It may also comprise other components
which may be required to form the polymer binder, such as
cross-linking agents and/or catalysts.
[0034] There are numerous techniques and synthetic methodologies
known in the art for producing thermoset polymer systems. However,
it is a requirement of the present invention that the reaction
takes place without a significant amount of solvent being generated
during the reaction. Furthermore, it is preferred that the entire
reaction is carried out under substantially solvent-free conditions
(i.e. with substantially no solvent being present at the beginning
of the reaction, and substantially no solvent being generated
during the reaction). A synthetic route which is therefore suitable
is an addition polymerization reaction. Such a reaction may be
carried out in the absence of solvent (i.e. no solvent is required
to dissolve the reactants prior to curing).
[0035] Addition polymerization reactions differ from those
described in the prior art such as U.S. Pat. No. 5,928,567 because
substantially no solvent is generated as a by-product. As shown in
Scheme 1 above, polycondensation reactions involve the reaction of
two molecules with the elimination of a solvent molecule, thus a
significant amount of solvent is generated as a by-product. In
contrast, the polymer addition reactions of the present invention
involve the reaction of at least two molecules without elimination
of a solvent molecule.
[0036] The polymer binders of the present invention can be prepared
from a variety of different polymerizable functional monomers,
oligomers or prepolymers according to a number of different
methodologies. When used in the present invention, the polymer
binder precursors are reacted to form the polymer binder in the
presence of the conductive material, in order to form a matrix
having conductive material dispersed therein. However, the
following addition polymerisation reactions will be described
generally, without stating the presence of the conductive material
which will, in the processes of the invention, always be
present.
[0037] Reactions which are particularly suitable in the process of
the invention include bulk free radical polymerisation,
polyurethane synthesis, epoxy resin synthesis and
polyhydrosilylation.
Bulk Free-Radical Polymerization:
[0038] Many "bulk" (i.e. solvent-free) free-radical polymerisation
reactions are known and could be used in the present invention.
Some reactions which are particularly useful in the present
invention involve alkyl acrylates or alkyl methacrylates monomers
or liquid formed prepolymers carrying acrylate or methacrylate
functional groups.
[0039] In a typical "bulk" free radical polymerization, monomers or
oligomers of acrylates or methacrylates may be reacted in the
presence of methacrylate- or acrylate-based cross-linking agents
and a free radical type catalyst (e.g. peroxides or AIBN). The
formation of the copolymer can be best represent by the following
reaction scheme 2: ##STR2##
[0040] The acrylate and methacrylate components are preferably
selected from the group consisting of methyl acrylate, ethyl
acrylate, butyl acrylate, 2-ethylhexyl acrylate, methyl
methacrylate, ethyl methacrylate, butyl methacrylate and
2-ethylhexyl methacrylate.
[0041] The crosslinking agent is a compound having more than one
functional group, and is preferably selected from the group
consisting of 1,4-butanediol diacrylate, 1,4-butylene glycol
diacrylate, ethylene diacrylate, 1,6-hexamethylene diacrylate,
1,4-butanediol dimethacrylate-1,4-butylene glycol dimethacrylate,
ethylene dimethacrylate, 1,6-hexamethylene dimethacrylate,
trimethylpropane trimethacrylate, pentacrythediol tetraacrylate and
pentacrythediol tetramethacrylate.
[0042] In general, the reaction can be easily initiated by heating
the content mixture. Normally, the initiating temperatures are all
depends on the type of the radical initiator use which can be
ranging from 40.degree. C. to over 150.degree. C.
Polyurethanes Synthesis,
[0043] Polyurethanes are among the most common polymers in the
global polymer market. Polyurethanes can be synthesised according
to a well established addition process between isocyanates and
polyols. A wide range of isocyanate and polyol compounds are known
in the art, and these compounds may contain aliphatic and/or
aromatic moieties.
[0044] Polyurethane formation usually occurs via a step growth
polymerization process in which the chain length of the polymer
increases as the reaction progresses. The polymer may be a linear,
or slightly branched, thermoplastic material, or may be in the form
of a cross-linked thermoset network. To be suitable for use in the
overvoltage protection devices of the present invention, the
polyurethane should be an electrical insulator.
[0045] The most widely used method of synthesising polyurethanes is
by the reaction of a diisocyanate and a polyol, and this synthetic
reaction may be used in the present invention. Thus, in a preferred
embodiment the polymer binder precursor comprises a diisocyanate
component and a polyol component. These components can react
together to form a polyurethane polymer binder. The reactants are
preferably used in their liquid forms, and may be present as
monomers, oligomers or prepolymers.
[0046] The diisocyanate component is preferably selected from the
group consisting of 1,4-diisocyanatobutane, 1,6-diisocyanatohexane,
1,8-diisocyanatooctane, 1,12-diisocyanatododecane, 2,4-toluene
diisocyanate, isophorone diisocyanate terminated
poly(1,4-butandiol), tolylene 2,4-diisocyanate terminated
poly(1,4-butandiol), and other commercially available diisocyanates
such as those available under the trade marks Desmodur.RTM. L67BA,
ISONATE.RTM. M342 and ISONATE.RTM. 143 L. The diisocyanate
component is preferably present in the reaction mixture in an
amount of from 5 to 50 wt %, based on the total weight of the
reactants.
[0047] The polyol component may be a diol, and is preferably
selected from the group consisting of short chain dihydroxy
alcohols such as 1,2-ethandiol, 1,3-propandiol, 1,2-propandiol,
1,4-butandiol. The polyol component may also be selected from the
group consisting of poly(ethylene glycol) (e.g. having
M.sub.n.about.200-400), hydroxy terminated poly(dimethylsiloxane)
(e.g. having M.sub.n.about.500-10,000) and dihydroxy terminated
poly(dimethylsiloxane-co-diphenylsiloxane), or may be a polyol
which is commercially available under trade marks such as
Terathane.RTM. 650 Polyether glycol, Terathane.RTM. 1000 Polyether
glycol, Terathane.RTM. 2000 Polyether glycol, Desmophen.RTM. 1600U
and Desmophen.RTM. 1900U. The polyol component is preferably
present in the reaction mixture in an amount of from 1 to 50 wt %,
based on the total weight of the reactants.
[0048] In a preferred embodiment, the polymer binder precursor may
additionally comprise a cross-linking agent. The cross-linking
agent is a multi-functional cross-linking compound, and is
preferably selected from compounds having a functionality of more
than two. For example, the cross-linking agent may be selected from
compounds having more than two functional groups such as hydroxy or
isocyanate groups. Particularly preferred cross-linking agents
include monomers, oligomers or prepolymers having more than two
isocyanate groups, and in particular tri- and tetra-functional
compounds such as polymethylene polyphenylisocyanates.
Representative compounds are commercially available from Dow
Chemical under the trade mark PAPI (e.g. PAPI 27 and PAPI
2940).
[0049] Another group of preferred cross-linking agents consists of
compounds containing more than two hydroxy groups, and in
particular tri- and tetra-functional compounds, such as
1,2,3-propantriol and polycaprolactone triol (e.g. having M.sub.w
.about.300), and branched polyalcohol compounds available from
Aldrich under the trade mark Desmophen (e.g. Desmophen 550U,
Desmophen 1910U, Desmophen 1145 and Desmophen 1150).
[0050] These cross-linking agents may be employed in the reaction
scheme in any amount sufficient to provide compositions having the
desired material properties, but are preferably present in an
amount of from 0.1 to 10 wt %.
[0051] Many catalysts can be used to catalyse the reaction of the
isocyanate and polyol components. Preferred catalysts are those
based on metals such as tin and zinc, and in particular tin (II)
octate and zinc (II) octate catalysts. Dibutyltin dilaurate is also
preferred. The amount of catalyst may be determined by those
skilled in the art, but is preferably from about 0.01 to 5 wt %,
based on the total weight of the reactants.
[0052] In a typical cross-linked type polyurethane synthesis, the
procedure normally consists of four components: (1) a monomeric or
oligomeric diisocyanate, (2) a monomeric or oligomeric diol, (3) a
cross-linking agent, and (4) a catalyst. An exemplary synthetic
scheme is shown in Scheme 3 below. ##STR3##
[0053] The R groups in this scheme can be any type of repeating
units, for example a straight-chain or branched alkyl group. They
may be based on a number of molecules such as polyethylene glycol,
polydimethylsiloxane, polytetrahydrofuran (e.g. Terathane.RTM.
Polyether glycol) or polyester. The preferred groups are those
based on low molecular Terathane.RTM. Polyether glycol,
polyethylene glycol and polydimethylsiloxane.
Polyhydrosilylation:
[0054] The addition polymerisation reaction used to prepare the
polymer binder of the invention may alternatively be a
polyhydrosilylation reaction. Hydrosilylation is a well-known
methodology in organic synthesis which takes place under mild
conditions. In a hydrosilylation reaction Si--H and terminal
carbon-carbon double bonds react to produce various alkylsilane
compounds.
[0055] Prepolymers or oligomers which may be used in such a
reaction include materials selected from the group consisting of
1,5-hexandiene, 1,7-octandiene, 1,4-divinylbenzene,
1,3-divinylbenzene, 1,3-divinyl-1,1,3,3-tetramethyldisiloxane,
1,3-divinyl-1,1,3,3-tetraphenyldisiloxane,
1,3-divinyl-1,3-dimethyl-1,3-diphenyldisiloxane,
1,3-divinyl-1,1-dimethyl-3,3-diphenyldisiloxane,
poly(dimethylsiloxane) divinyl terminated,
poly(dimethylsiloxane-co-diphenylsiloxane) divinyl terminated and
poly(dimethylsiloxane-co-methylphenylsiloxane) divinyl terminated
(these compounds providing the C.dbd.C bond necessary for the
hydrosilylation reaction).
[0056] Other preferred prepolymers or oligomers include compounds
selected from the group consisting of
1,4-bis(dimethylsilyl)benzene, 1,3-Bis(dimethylsilyl)benzene,
1,1,3,3-tetramethyldisiloxane, 1,1,3,3-tetraphenyldisiloxane,
1,3-dimethyl-1,3-diphenyldisiloxane,
1,1-dimethyl-3,3-diphenyldisiloxane, poly(dimethylsiloxane)
dihydride terminated, dihydride terminated
poly(dimethylsiioxane-co-diphenylsiloxane) and dihydride terminated
poly(dimethylsiloxane-co-methylphenylsiloxane), (these components
providing the Si--H bond necessary for the hydrosilylation
reaction).
[0057] In one example, the hydrosilylation reaction can involve the
reaction of a hydrosilane and a vinylsilane which can react
together to form a polycarbosilane polymer binder. In a preferred
polymer synthesis, hydrosilylation occurs between
.alpha.,.omega.-divinyl compounds and
.alpha.,.omega.-bis(hydrosilyl) compounds in the presence of
suitable cross-linking agents and catalysts, to afford thermally
stable polycarbosilanes.
[0058] In a general hydrosilylation methodology, vinylsilanes and
hydrosilanes are reacted together, usually in the presence of a
transition metal catalyst (see scheme 4). The reaction involves the
addition of Si--H bonds (e.g. from the hydrosilane component) to
C.dbd.C double bonds (e.g. from the vinylsilane component).
##STR4##
[0059] R and R' can be selected from a number of substituents known
to those in the art, although alkyl groups and phenyl groups are
particularly preferred. In particular, methyl, ethyl, propyl, butyl
or phenyl groups may be used.
[0060] Hydrosilylation methodology can thus be used for the
preparation of the polycarbosiloxanes or polycarbosilanes, which
contain Si--C bonds in the polymer backbone structures. Such
polycarbosiloxanes or polycarbosilanes can be obtained by reaction
of a di-, tri-, tetra-, or poly-methylhydrosilyl-containing
organopolysiloxane, and a di-, tri-, tetra-, or
poly-vinyl-containing organopolysiloxane. The resulting
polycarbosiloxane or polycarbosilane polymeric binders are
preferably thermoset polymers or rubbers, and are electrical
insulators (that is, they have an electrical conductivity of
generally less than about 10.sup.-9 (.OMEGA.cm).sup.-1).
[0061] The methylhydrosilyl-containing organopolysiloxane component
preferably comprises a compound of formula (a), (b) or (c):
##STR5##
[0062] In the formulae (a), each R group is independently selected
from a substituted or unsubstituted monovalent hydrocarbon group.
Each R group preferably contains from 1 to 8 carbon atoms, with
exemplary groups including alkyl groups such as methyl, ethyl,
propyl, butyl, hexyl and octyl, aryl groups such as phenyl, tolyl
and naphthyl, and aralkyl groups such as benzyl and phenylethyl.
The preferred R groups are methyl and phenyl groups. The letter x
is an integer which can range between 0 and 1000, and is preferably
between 0 and 200.
[0063] Reactive monomers like 1,1,3,3-tetramethyldisiloxane,
1,1,3,3-tetraphenyldisiloxane, 1,3-dimethyl-1,3-diphenyldisiloxane,
1,1-dimethyl-3,3-diphenyldisiloxane, poly(di-methylsiloxane)
dihydride terminated, poly(dimethylsiloxane-co-diphenylsiloxane)
dihydride terminated and
poly(dimethylsiloxane-co-methylphenylsiloxane) dihydride terminated
are particularly useful in this present invention. ##STR6##
[0064] In formula (b), R is as defined above in relation to formula
(a), n is an integer of at least 2, m is an integer inclusive of 0,
and the sum of n plus m is between 3 to 8. Reactive molecules like
2,4,6,8-tetramethylcyclotetrasiloxane,
pentamethylcyclopentasiloxane and
2,4,6,8,10,12-hexamethylcyclohexasiloxane are possible compounds of
formula (b). ##STR7##
[0065] In formula (c), R is as defined above, and each R' group is
independently a hydrogen atom, or a methyl or trimethylsilyl group.
The proportion of x (i.e. the wt % of the methylhydrosiloxane
monomer unit) in the molecule ranges from 10 to 95 wt %, preferably
from 10 to 50 wt %.
[0066] The vinyl-containing organopolysiloxane preferably has a
formula according to either of the formulae (d), (e) and (f) below:
##STR8##
[0067] Formula (d) represents a vinyl-terminated siloxane component
wherein each R group is independently selected from a substituted
or unsubstituted monovalent hydrocarbon group. The R group
preferably contains from 1 to 8 carbon atoms, with preferred
examples including alkyl groups such as methyl, ethyl, propyl,
butyl, hexyl and octyl, and aryl groups such as phenyl, tolyl and
naphthyl. The letter x is 0 or an integer ranging from 1 to 1000,
and is preferably between 0 and 200, and the proportion of the
vinyl group (in wt % based on the total weight of the
vinyl-containing organopolysiloxane component) ranges from 0.1 to
30 wt %. ##STR9##
[0068] Formula (e) represents a vinyl-containing
cycloorganosiloxane component, wherein n is an integer of at least
2, m is an 0 or an integer between 1 and 6, and the sum of n plus m
is between 4 and 8. ##STR10##
[0069] In formula (f), each R group is as defined above in relation
to formula (d), and each R' group is independently selected from
allyl, vinyl, methyl, and trimethylsilyl groups. The value of y is
chosen such that the proportion of the vinyl-containing
organosiloxane monomer unit (i.e. the wt % of this component
relative to the total weight of the compound) ranges from 1 to 50
wt %. The preferred range is from 1 to 10 wt %.
[0070] The linear silicon-containing polymers (e.g. siloxanes or
silanes) described above possess a highly robust chemical structure
with exceptional thermal resistance properties. The thermal
resistance can be further improved by incorporating some
cross-linking agent in the polymer binder precursor. Potential
cross-linking agents include tri-, tetra- or poly-methylhydrosilyl
(or -vinyl) containing organosiloxane cross-linking components,
which are preferably employed in the presence of a platinum
catalyst system. These provide additional heat resistance
performance, resulting in compositions with excellent thermal
stability performance. A typical reaction scheme is shown below in
Scheme 5: ##STR11##
[0071] In this scheme P can be any suitable linking group. For
example, it may be chosen such that the methylhydrosilyl component
is the same as the product formed in scheme 4 above. P may be
repeating unit based on any suitable group known to those in the
art, for example it may consist of a polyethylene glycol-,
polydimethylsiloxane- or polyterahydrofuran-based group.
[0072] The cross-linking component (alternatively called a
multifunctional monomer) is a molecule which contains three or more
reactive functional units. For example, these functional units may
be carbon-carbon single bonds or C.dbd.C double bonds (e.g. a vinyl
group), or hydrogen-silicon or carbon-silicon bonds (e.g. a
methylhydrosilyl group). Each of these reactive functional units
(e.g. C.dbd.C, or H--Si) can independently act as an active site
and subsequently participate in the growth of the polymer chain,
while the other reactive functional units can facilitate the
formation of the branched polymer which grows and eventually
connects to another chain in order to create a cross-linked polymer
structure. Thus, the multifunctional components mentioned above are
capable of producing highly cross-linked structures. Preferred
cross-linking components are those having three or more reactive
functional units which are either vinyl groups or methyhydrosilyl
groups.
[0073] Thus, in a preferred embodiment, the present invention
provides a cross-linked polycarbosiloxane polymer binder
composition comprising the reaction product of: [0074] (1) a di-,
tri-, tetra-, or poly-methylhydrosilyl-containing
organopolysiloxane; [0075] (2) a di-, tri- tetra- or
poly-vinyl-containing organopolysiloxane; [0076] (3) a tri-,
tetra-, or poly-vinyl-containing, or a tri-, tetra-, or
poly-methylhydrosilyl-containing organosiloxane cross-linking
component.
[0077] The cross-linking component is preferably present in liquid
form, and is preferably in an amount of from 0.1 to 10 wt %, based
on the total weight of the total composition.
[0078] Preferably the polymer binder composition further comprises
an organometallic catalyst. Suitable catalysts include platinum
catalysts, which may be selected from chloroplatinic acid or
platinum vinylsiloxane complexes. Platinum complexes are
particularly advantageous in that they lower the reaction
temperatures required for thermal curing. The catalyst is
preferably present in an amount of from 0.01 to 5 wt %, based on
the total weight of the polymer.
Other Components
[0079] In addition to the polymer binder component, overvoltage
protection materials of the present invention comprise a conductive
material, which is preferably present in the form of conductive
particles dispersed throughout the polymer matrix. The overvoltage
protection materials may also include other components, for example
semi-conductive materials and/or non-conductive materials.
[0080] The conductive polymeric material composite may be formed as
a multifunctional polymeric composite matrix system, having three
or more components. One particularly preferred system can be
represented by the formula: where A is a polymeric binder (e.g. an
insulating polymer matrix comprising silicon rubber or other type
of cross-linked polymer), B is a conductive material (e.g.
aluminium, nickel or iron particles), C is a non-conductive
material (e.g. inorganic particles that control the spacing between
the conductive particles), and D is a semi-conductive material
(e.g. inorganic particles which modulate the electrical properties
of the system and increase the heat dispersion effect). The nature
of components A, B, C and D, and the relevant proportions of these
components (represented by x, y, z and n) can be altered in order
to achieve the desired overvoltage protection material.
[0081] The preferred values of x, y, z and n are those which result
in a polymer matrix where A is present in an amount of from 10 to
60 wt %, B is present in an amount of from 10 to 70 wt %, C is
present in an amount of from 1 to 40 wt %, and D is present in an
amount of from 1 to 40 wt %.
(i) Conductive Materials
[0082] The ability of the overvoltage protection materials to
protect devices from transient high voltage surges is dependent
upon a number of factors, including the electrical properties of
the polymeric binder, and the size of the gap between the
electrodes of the resulting overvoltage protection devices. The
nature of the conductive material also influences the properties of
the overvoltage protection materials. For example, the electrical
conductivity of the conductive material, the size of the particles
of this material, and the loading of conductive material within the
polymer binder (and hence the interparticle separation of the
conductive material) all influence the overvoltage protection
capabilities.
[0083] The conductive materials used in the present invention have
bulk conductivities greater than 1000 (.omega.cm).sup.-1, and are
preferably present in the form of conductive particles. These
conductive particles generally have an average particle size (APS)
of generally less than 20 microns, such as 0.1 to 20 microns, and
more preferably less than 10 microns. A particularly preferred
average particle size is between 1 and 10 microns. Conductive
particles having average particle sizes between 1 and 30 microns
are easily obtainable from commercial suppliers.
[0084] A wide range of suitable conductive particles is available.
Examples include metallic particles such as aluminium, copper,
gold, iron, silver, beryllium, bismuth, cobalt, magnesium,
molybdenum, palladium, tantalum, tungsten and tin particles;
particles of metal alloys such as stainless steel, bronze and
brass; carbide powders such as carbides of titanium, boron,
tungsten and tantalum; carbon-based powders such as carbon black
and graphite; metal nitrides and metal borides. Recently,
intrinsically conducting polymers such as polyaniline and
polypyrrole have become available, and these would also be of use
in the invention. Preferred conductive particles include nickel,
aluminium, copper, carbon black, graphite, gold, iron, stainless
steel, silver, tin and metal alloys, with nickel and aluminium
being most preferred.
[0085] The conductive material may be present in any amount
necessary to achieve the desired overvoltage protection
capabilities. Generally it will be present in an amount of from 10
to 70 wt %, based on the total weight of the composition, more
preferably from 20 to 60 wt %, most preferably 30 to 55%.
(ii) Semi-Conductive Materials
[0086] The semi-conductive materials used in the present invention
generally have bulk conductivities less than 100
(.omega.cm).sup.-1. They are employed to modulate the electrical
property of the overvoltage protection materials, and to increase
the heat dispersion effect.
[0087] The materials are preferably particulate inorganic
materials, and the average particle size of the semi-conductive
materials is usually less than 10 microns, for example from 0.1 to
10 microns, and more preferably the average particle size is less
than 2 microns, such as 0.1 to 2 microns.
[0088] Suitable semi-conductive materials include aluminium
nitride, barium titanate, boron nitride, silicon nitride, titanium
dioxide, silicon carbide and zinc oxide, with silicon carbide being
preferred. When present in the composition, the semi-conductive
materials are preferably present in an amount of from 1 to 40 wt %,
based on the total weight of the composition, more preferably from
10 to 30 wt %.
(iii) Non-Conductive Materials
[0089] The non-conductive materials used in the present invention
generally have bulk conductivities less than 10.sup.-6
(.OMEGA.cm).sup.-1. They are employed primarily to control the
spacing between particles of the conductive material.
[0090] The non-conductive materials are present in the form of
particles, with the average particle size generally being less than
5 microns, for example 0.005 to 10 microns, and more preferably
being within the range 0.01 to 1 micron. These materials are
preferable organic materials, and are suitably selected from
non-conducting materials (or inorganic fillers) including coated-
or uncoated- aluminium oxide, barium oxide, silica, barium
carbonate, calcium carbonate, magnesium carbonate, calcium sulphate
and magnesium sulphate. Particularly preferred are silica,
aluminium oxide and calcium carbonate.
[0091] When present in the composition, the non-conductive
materials preferably comprise from 1 to 40 wt %, based on the total
weight of the composition, more preferably from 5 to 20 wt %.
Processes for Preparing Overvoltage Protection Materials:
[0092] The overvoltage protection material of the invention may be
produced according to the general processes described earlier. Due
to the choice of the polymer binder precursor, substantially no
solvent is generated during the curing step to form the polymer
binder, and this has a number of advantages. For example, by
reducing the amount of solvent generated, there is less solvent to
evaporate during curing, hence less energy is required for the
curing step. Also, reducing the amount of solvent generated results
in a material having fewer voids in its structure. The overvoltage
protection materials of the present invention preferably have
substantially no voids.
[0093] The overvoltage protection materials may be prepared by a
standard mixing technique of the components using conventional
apparatus. A high speed (e.g. >2000 rpm) continuous stirring
tank reactor is particularly suitable.
[0094] According to a preferred embodiment of the invention, there
is provided a solvent free process of forming an overvoltage
protection material, wherein a highly cross-linked polymer matrix
is prepared having a conductive material dispersed throughout. The
polymer matrix is generated from a polymer binder precursor which
reacts via an addition polymerisation reaction, as described
earlier. Other components, such as semi-conductive and/or
non-conductive materials may also be included within resulting
overvoltage protection material. This is a `one-pot` synthetic
technique, in which all the materials are mixed or blended together
in a mixing vessel, and this reaction mixture is then heated in
order to form the polymer binder (also called the polymer matrix)
having a conductive material dispersed therein. The one-pot
methodology has the advantage that it reduces the number of
processing steps, and hence reduces costs. Suitable synthetic
routes are shown in the Examples below.
Device Fabrication
[0095] The overvoltage protection devices of the invention may be
prepared according to a process comprising depositing a metallic
layer on a substrate, and etching the metallic layer in order to
provide metallic electrodes separated by gaps; preparing a
solvent-free mixture comprising at least one polymer binder
precursor and a conductive material, and applying this mixture
between and in contact with adjacent electrodes; and heating the
device to cause reaction of the polymer binder precursor and
generate an overvoltage protection material comprising a polymer
binder having conductive material dispersed therein; wherein the
polymer binder precursor is chosen such that substantially no
solvent is generated during the reaction. The polymer binder
precursor and conductive material are as discussed above.
[0096] The polymer binder precursor preferably reacts via an
additional polymerisation reaction, and the polymer binder formed
during this process is substantially free of voids. The particular
components (e.g. polymer binder precursor and conductive material)
used to make the device can be optimised according to the
application for which the resulting device will be used. However,
it is preferred to choose the components such that the overvoltage
protection device has a trigger voltage of less than 300V. The
devices preferably have a very fast switching time, for example
less than 1 nanosecond.
[0097] The substrate and metallic layer will be chosen depending on
the specific application of the device. For example, the device may
be used in a semiconductor device, and the substrate may then be
selected from materials known in the art to be suitable for this
application. The metallic layer is preferably between about 1 to 20
microns thick, more preferably from about 5 to about 15 microns
thick. The metallic materials suitable for use as the electrodes
are known to those skilled in the art, and include nickel, silver
and copper, with nickel and copper being preferred. The deposition
step can be any conventional method for depositing a thin layer of
metal onto a substrate. Possible methods include sputtering,
nickel-electrodless plating, electroplating, thermal evaporation
and metal-epoxy lamination.
[0098] A suitable fabrication procedure is as follows. A thin
metallic layer is coated onto a substrate surface by any suitable
method, such as sputtering, thermal evaporation or electroplating.
Alternatively, a commercially available copper laminated board,
where a layer of copper is already present on a substrate, may be
used. Exemplary substrates include, but are not limited to,
laminated epoxy fibre glass (e.g. FR4), glass and ceramics.
[0099] The device pattern is formed on the surface of the device by
a number of suitable procedures, for example by lithographic
patterning or wet etching. The resulting gaps between the
electrodes are generally of the order of microns, for example
between 10 and 1000 microns. A preferred electrode separation is
between 10 and 200 microns, more preferably between 30 and 150
microns.
[0100] An exemplary device, prior to addition of the overvoltage
protection material, is shown in FIG. 2, which depicts two
electrodes (12, 13) on a substrate (14). The electrodes are
separated by micro-gap (11).
[0101] Once the basic device structure has been fabricated, the
solvent-free mixture, comprising the polymer binder precursor and
the conductive material, can be applied onto the above mentioned
device by a number of dispensing methods such as hand brushing,
screen printing and others casting procedure. Hand brushing is used
in the following examples, although screen printing is preferred
for large-scale production. After the material has been applied to
the device, the polymer binder must be cured in order to form the
final overvoltage protection device. Suitable curing conditions
will be known to those skilled in the art, but temperatures of
between 40 and 150.degree. C., more preferably between 25 and
100.degree. C., have been found to be particularly suitable for
making devices according to the invention.
[0102] The finished device (10), after addition of the overvoltage
protection material (15), is shown in FIG. 1 (cross-sectional view)
and FIG. 3 (plan view).
[0103] The overvoltage protection device may be fabricated as part
of a larger circuit. For example, the invention provides a circuit
comprising electronic components, at least a first and second
conducting region, an amount of an overvoltage protection material
being disposed between and in contact with said conducting
regions.
[0104] The following Examples are intended to illustrate the
present invention, and are not intended to limit the invention in
any way.
EXAMPLES
[0105] A number of formulations were been prepared by a mixing
technique which is described in the examples below. After curing of
the overvoltage protection material, devices comprising these
materials were tested. In particular, the devices were tested to
determine: (1) electrical resistance; (2) trigger voltage (using
the Human Body Model (HBM), a standard methodology for generating a
transient voltage, in which the voltage is generated by an ionised
Charge Plate); (3) leakage current (recorded by Current-Voltage
(I-V) method) measured by scanning a voltage between 0 to 24 V
(d.c.) through the overvoltage protection device and then taking
the current reading at 12V; and (4) capacitance (measured at 1 MHz
frequency). The devices were tested using an electrical circuit as
shown in FIG. 4, in order to determine the trigger voltage. FIG. 4
depicts a pulse generator (16), an overvoltage protection device
according to the invention (17), a resistor (18), a ground (19) and
a high speed oscilloscope (20).
Example 1
[0106] A conductive polymeric composite material comprising
polyurethane as a polymeric binder was prepared according to the
following formulation: TABLE-US-00001 Poly(1,4-butanediol),
isophorone diisocyanate terminated 24.5 wt % Terathane 650,
polyester glycol 5 wt % Polycaprolactone triol, Mw .about.300 0.5
wt % Dibutytin dilaurate 0.6 wt % Nickel powder, APS 2.2-3.0 micron
45 wt % Calcium carbonate powder, APS 40 nm 5 wt % Silicon carbide
(12 S), APS 0.7 micron 20 wt %
[0107] In a typical preparation procedure using a polyurethane
polymeric binder, poly(1,4-butanediol), isophorone diisocyanate
terminated, Terathane 650 polyester glycol, polycaprolactone triol,
nickel powder, calcium carbonate powder and silicon carbide powder
were mixed in a round bottomed flask and stirred with mechanical
stirrer for 10-20 minutes until the materials were well mixed using
a high speed (e.g. >2000 rpm) continuous stirring tank reactor,
and a viscous material was obtained. Then, a catalytic amount of
dibutyltin dilaurate (e.g. 0.6 wt % based on the polymer content)
was added into the well mixed viscous mixture, and the composition
was stirred at room temperature for a further 10-15 minutes.
[0108] The resulting viscous material was cast onto the gap between
electrodes on a device as shown in FIG. 1 by a normal hand casting
method. The device was then placed in a conventional hot-air oven
at 80.degree. C. to cure the polymer. After about 30 minutes curing
the overvoltage protection device was removed and its electrical
properties measured as described above. The results are shown below
in Table 1.
Example 2
[0109] A conductive polymeric composite material was prepared
according to the following formulation: TABLE-US-00002
Poly(dimethylsiloxane-co-methylhydrosiloxane) 38.25 wt %
Polydimethylsiloxane, vinyldimethyl terminated 4.5 wt %
1,3,5,7-Tetramethylcyclotetrasiloxane 2.25 wt % Platinum
vinylsiloxane catalyst (Pt 0.1 wt %) 1.3 wt % Aluminium powder, APS
6.0 micron 40 wt % Calcium carbonate powder, APS 40 nm 7.5 wt %
Alumina, AKP20, APS 0.3-0.6 micron 7.5 wt %
[0110] In a typical preparation procedure,
poly(dimethylsiloxane-co-methylhydrosiloxane), polydimethylsiloxane
(vinyidimethyl terminated), 1,3,5,7-tetramethylcyclotetra-siloxane,
aluminium powder, calcium carbonate powder and alumina powder were
mixed in a round bottom flask and stirred with mechanical stirrer
for 10-20 minutes at 60.degree. C. until the materials were well
mixed. Then, a catalytic amount of platinum vinylsiloxane
containing 0.1 wt % platinum (e.g. .about.1 wt % based on polymer
content) was added into the well mixed viscous mixture. The mixture
was stirred at room temperature for a further 10-15 minutes.
[0111] The resulting viscous material was cast onto the gap between
electrodes on a device as shown in FIG. 1 by a normal hand casting
method. The device was then placed in a conventional hot-air oven
at 80.degree. C. to cure the polymer. After about one hour of
curing the overvoltage protection device was removed and its
electrical properties measured as described above. The results are
shown below in Table 1, from which it can be seen that the
overvoltage protection device had essentially the same
characteristics as the overvoltage protection device in Example
1.
Example 3
[0112] A conductive polymeric composite material was prepared
according to the following formulation: TABLE-US-00003
Poly(dimethylsiloxane-co-methylhydrosiloxane) 38.25 wt %
Polydimethylsiloxane, vinyldimethyl terminate 4.5 wt %
1,3,5,7-Tetramethylcyclotetrasiloxane 2.25 wt % Platinum
vinylsiloxane catalyst (Pt 0.1 wt %) 1.3 wt % Nickel powder, APS
2.2-3.0 micron 33 wt % Calcium carbonate powder, APS 40 nm 11 wt %
Alumina, AKP20, APS 0.3-0.6 micron 11 wt %
[0113] Exactly the same procedure was used as described above in
Example 2, but using the formulation above, where nickel powder
with APS 2.2-3.0 micron was used instead of aluminium powder. After
curing, the resulting overvoltage protection device had essentially
the same characteristics as the protection device in Example 1 (see
Table 1).
Example 4
[0114] A conductive polymeric composite material was prepared
according to the following formulation: TABLE-US-00004
Poly(dimethylsiloxane-co-methylhydrosiloxane) 30 wt %
Polydimethylsiloxane, vinyldimethyl terminate 10 wt % Platinum
vinylsiloxane catalyst (Pt 0.1 wt %) 1.3 wt % Aluminium powder, APS
6.0 micron 50 wt % Calcium carbonate powder, APS 40 nm 5 wt %
Silicon carbide (7 S), APS 5 wt %
[0115] Again, the procedure described in Example 2 was followed,
but in this example, the formulation contained an additional
component of silicon carbide powder. The electrical properties of
the resulting device are shown in Table 1. The overvoltage
protection material composite was a little softer than the
composites of Examples 2 and 3 because an additional crossing
linking agent was omitted from the formulation
Example 5
[0116] A conductive polymeric composite material was prepared
according to the following formulation: TABLE-US-00005
Poly(dimethylsiloxane-co-methylhydrosiloxane) 27.8 wt %
Polydimethylsiloxane, vinyldimethyl terminate 9.2 wt % Platinum
vinylsiloxane catalyst (Pt 0.1 wt %) 1.3 wt % Aluminium powder, APS
6.0 micron 50 wt % Calcium carbonate powder, APS 40 nm 6.5 wt %
Silicon carbide (7 S), APS 6.5 wt %
[0117] The procedure of Example 2 was again followed, but with a
different formulation. After curing, the electrical properties of
the resulting overvoltage protection device were measured, and the
results are shown below in Table 1.
Example 6
[0118] A conductive polymeric composite material was prepared
according to the following formulation: TABLE-US-00006
Poly(dimethylsiloxane-co-methylhydrosiloxane) 26.25 wt %
Polydimethylsiloxane, vinyldimethyl terminate 8.75 wt % Platinum
vinylsiloxane catalyst (Pt 0.1 wt %) 1.3 wt % Aluminium powder, APS
6.0 micron 50 wt % Calcium carbonate powder, APS 40 nm 7.5 wt %
Silicon carbide (7 S), APS 7.5 wt %
[0119] The procedure of Example 2 was again followed, but with a
different formulation. After curing, the electrical properties of
the resulting overvoltage protection device were measured, and the
results are shown below in Table 1.
Example 7
[0120] A conductive polymeric composite material was prepared
according to the following formulation: TABLE-US-00007
Poly(dimethylsiloxane-co-methylhydrosiloxane) 26.25 wt %
Polydimethylsiloxane, vinyldimethyl terminate 8.75 wt % Platinum
vinylsiloxane catalyst (Pt 0.1 wt %) 1.3 wt % Aluminium powder, APS
6.0 micron 50 wt % Calcium carbonate powder, APS 40 nm 10 wt %
Silicon carbide (7 S), APS 5 wt %
[0121] The procedure of Example 2 was again followed, but with a
different formulation. After curing, the electrical properties of
the resulting overvoltage protection device were measured, and the
results are shown below in Table 1.
Example 8
[0122] TABLE-US-00008 A conductive polymeric composite material was
prepared 26.25 wt % Polydimethylsiloxane, vinyldimethyl terminate
8.75 wt % Platinum vinylsiloxane catalyst (Pt 0.1 wt %) 1.3 wt %
Aluminium powder, APS 6.0 micron 50 wt % Calcium carbonate powder,
APS 40 nm 11.28 wt % Silicon carbide (7 S), APS 3.72 wt %
[0123] The procedure of Example 2 was again followed, but with a
different formulation. After curing, the electrical properties of
the resulting overvoltage protection device were measured, and the
results are shown below in Table 1. TABLE-US-00009 TABLE 1
Resistance Leakage Gap Trigger off state current Capacitance
between Voltage (G.OMEGA.) Response (nA) (pF) electrodes Example
No. (V) (@ 6 V) Time (ns) (@ 12 VDU) (@ 1 MHz) (.mu.m) 1 214 6
<1 <1 0.40 35 2 192 142 <1 <1 0.20 140 3 259 160 <1
<1 0.14 100 4 240 3 <1 .about.0.001 0.55 60 5 213 1 <1
.about.0.014 0.50 35 6 200 1 <1 .about.0.025 0.43 35 7 186 121
<1 .about.0.033 0.44 35 8 207 138 <1 .about.0.010 0.47 35
[0124] The preceding examples describe the starting mixtures,
preparation methods and process conditions used, and the
characteristics of the resulting conductive polymeric composite
materials, as normal fabrication procedures according to the
present invention. These examples are illustrative of but a few of
the many possible permulations of chemicals and process parameters
that could be used to prepare conductive polymeric composite
materials of the invention.
[0125] The foregoing is offered primarily for the purposes of
illustration. It will be readily apparent to those skilled in the
art that numerous variations, modifications and substitutions may
be made in the materials, procedural steps and conditions described
herein without departing from the spirit and scope of the
invention.
* * * * *