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BACKGROUND AND OBJECTS OF THE INVENTION
This invention relates to a process for preparing electrically conductive
fibrous material from a thermally stabilized acrylic fibrous material, and
to the fibrous material produced thereby. The invention further relates to
an electrically conductive composite comprising electrically conductive
thermally stabilized acrylic fibrous material surrounded with a continuous
polymeric or resinous matrix and to a process for preparing the same. The
invention is useful for EMI (electromagnetic interference) shielding, and
electrostatic discharge as well as in forming electrically conductive
resins and paints.
It is known in the art to treat polyacrylonitrile fibers with cupric
sulfate, hydroxylamine, and thiosulfate to produce electrically conductive
fibers having adsorbed thereto copper sulfide in the forms of digenite,
chalcocite, and covellite, alone or in conjunction with sulfides of noble
metals, in a total amount of up to 30 percent in terms of elemental copper
based on the weight of the starting fiber. (See Tomibe et al, European
Pat. No. 0 086 072 and U.S. Pat. No. 4,336,028.) However, these fibers
possess various deficiencies: the polyacrylonitrile fibers are relatively
heat unstable and tend to lose their integrity in various applications;
for example, if the fibers are contacted with molten resinous material,
the fibers disintegrate. Further, the copper sulfide content is only
partially in the form of covellite, the most conductive of the forms of
copper sulfide, thus rendering the fibers inadequately conductive for many
applications. Additionally, high levels of copper sulfide incorporation
(e.g., greater than about 30 weight percent) are not possible according to
the processes of the prior art.
It is also known to produce copper sulfide-coated electrically conductive
fibers from other synthetic or natural polymers. (See Tomibe et al, U.S.
Pat. Nos. 4,364,739, 4,378,226, and 4,410,593.) However, each of these
fibers possesses the same deficiencies as the above-described fibers.
It is also known to produce elemental copper-plated
acrylate/styrene/acrylonitrile articles or articles of other polymers by
depositing a copper compound and subsequently reducing with a borohydride.
(See U.S. Pat. Nos. 4,234,628 and 4,246,320 to DuRose and Coll-Palagos et
al, respectively.) However, many of the above-noted deficiencies are
inherent in these articles.
Further, it is known in the art to produce composite articles by loading
organic fibrous material and/or inorganic fillers into a resinous matrix.
For example, U.S. Pat. No. 2,956,039 to Novak et al discloses metal-plated
fibers (e.g., of wool, polyethylene terephthalate, or nylon) or metal
particles in admixture with an epoxy resin to produce an electrically
insulating composition. U.S. Pat. No. 3,658,750 to Tsukui et al discloses
an electrically insulating composition comprising a thermosetting resin
and 40 to 80 volume percent of a powdered filler which may be cuprous
sulfide or cupric sulfide. U.S. Pat. No. 4,155,896 to Rennier et al
discloses a composition comprising copper plated steel or glass fibers
dispersed in an organic coating. U.S. Pat. No. 3,658,748 to Andersen et al
discloses a composite comprising reinforcing fibers (e.g., of
polyacrylonitrile) embedded in a thermosettable resin. However, each of
these compositions possesses various deficiencies, including insufficient
conductivity for certain applications and difficulty of processing the
composite due to poor thermal stability of the filler material.
It is therefore an object of the present invention to provide a process for
preparing improved electrically conductive fibrous materials, particularly
highly conductive materials.
It is a still further object of the present invention to provide a process
for preparing an improved electrically conductive fibrous material which
is flexible and ductile.
It is a further object of the present invention to provide an improved
electrically conductive fibrous material having covellite copper sulfide
in association therewith, wherein the copper sulfide is substantially
entirely in the form of covellite copper sulfide.
It is a still further object of the invention to provide a process for
preparing a composite article which incorporates an improved electrically
conductive fibrous material which is heat stable and which may be
processed in a molten polymeric matrix without destruction of the fibrous
material.
It is a still further object of the invention to provide an electrically
conductive monolithic composite incorporating an improved electrically
conductive fibrous material.
It is a still further object of the invention to provide an electrically
conductive polymer composition incorporating an improved electrically
conductive fibrous material.
It is a still further object of the invention to produce fibrous material
which is suitable for use in electrostatic discharge and EMI shielding
applications and other applications where electrically conductive
composites are desired.
These and other objects, as well as the scope, nature, and utilization of
the claimed invention will be apparent to those skilled in the art by the
following detailed description and appended claims.
SUMMARY OF THE INVENTION
According to the present invention, an electrically conductive fibrous
material is prepared from a thermally stabilized acrylic fibrous material
by
(a) supplying a source of cuprous ions to the thermally stabilized acrylic
fibrous material to produce a cuprous ion-impregnated thermally stabilized
acrylic fibrous material;
(b) contacting the resulting cuprous ion-impregnated thermally stabilized
acrylic fibrous material with a sulfiding agent capable of sulfiding the
cuprous ions to form covellite copper sulfide in association with the
thermally stabilized acrylic fibrous material; and, optionally,
(c) washing the resulting thermally stabilized acrylic fibrous material
containing associated covellite copper sulfide to remove residual
reactants adhering to the same.
In a preferred embodiment, an electrically conductive fibrous material is
prepared from a thermally stabilized acrylic fibrous material by
(a) cuprous ion-impregnating the thermally stabilized acrylic fibrous
material with an aqueous solution of between about 0.25 and about 10
weight percent of copper ions, added as cupric sulfate, and between about
0.5 and 10 weight percent of an hyroxylamine reducing agent while at a
temperature of between about 80.degree. and about 105.degree. C. for
between about 1 and about 2 hours;
(b) subjecting the resulting cuprous ion-impregnated fibrous material to a
sulfiding treatment in a solution comprising a thiosulfate sulfiding agent
in a concentration of approximately 5 to 15 percent by weight while at a
temperature of between about 90.degree. and about 105.degree. C. for an
additional perid of time between about 1 and about 2 hours to produce an
electrically conductive fibrous material having covellite copper sulfide
in association therewith; and
(c) washing the resulting thermally stabilized acrylic fibrous material to
substantially remove residual reactants adhering to the same.
In another aspect of the invention, an electrically conductive fibrous
material is provided which comprises thermally stabilized acrylic fibrous
material in association with approximately 5 to 60 percent, and preferably
35 to 60 percent, by weight of covellite copper sulfide, based upon the
total weight of the product.
In another aspect of the invention, an electrically conductive composite
article is prepared by a process comprising the steps of:
(a) cuprous ion-impregnating a thermally stabilized acrylic fibrous
material with a solution of a cupric salt and a reducing agent capable of
reducing cupric ions to cuprous ions;
(b) subjecting the resulting cuprous ion-impregnated thermally stabilized
fibrous material to a sulfiding treatment in a solution comprising a
sulfiding agent capable of sulfiding the cuprous ions to covellite copper
sulfide in association with said fibrous material to produce electrically
conductive thermally stabilized acrylic fibrous material;
(c) washing the resulting electrically conductive thermally stabilized
acrylic fibrous material to substantially remove residual reactants
adhering to same; and
(d) incorporating the resulting electrically conductive fibrous material
within a substantially continuous polymeric matrix to produce a monolithic
electrically conductive composite article.
In still another aspect, a monolithic electrically conductive composite
article is provided which comprises electrically conductive thermally
stabilized acrylic fibrous material in association with approximately 5 to
60 percent by weight of covellite copper sulfide based upon the total
weight of the conductive fiber product, incorporated within a
substantially continuous polymeric matrix.
In yet another aspect, a polymer composition suitable for use in
electrically conductive end uses is provided, comprising electrically
conductive thermally stabilized acrylic fibrous material in association
with approximately 5 to 60 weight percent of covellite copper sulfide and
a polymeric carrier.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a magnified (880X) photograph of the electrically conductive
thermally stabilized acrylic fibrous material produced in accordance with
the procedure of Example I.
FIG. 2 is a magnified (9200X) photograph of the surface of the electrically
conductive thermally stabilized acrylic fibrous material produced in
accordance with the procedure of Example I.
FIG. 3 is a magnified (10,000X) photograph showing a cross-section of the
fibrous material depicted in FIGS. 1 and 2.
FIG. 4 is an X-ray diffraction pattern of the electrically conductive
thermally stabilized acrylic fibrous material produced in accordance with
the procedure of Example I, showing the covellite copper sulfide phase in
a Debye-Scherrer pattern.
FIG. 5 is a graph of the resistance variation with temperature of the
electrically conductive thermally stabilized acrylic fibrous material
produced in accordance with the procedure of Example I.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The fibrous material which is rendered electrically conductive in
accordance with the present invention is a thermally stabilized acrylic
fibrous material which can be produced by methods previously known in the
art.
The acrylic fibrous material prior to thermal stabilization may be formed
by conventional solution spinning techniques (i.e., may be dry spun or wet
spun), or high pressure melt spinning, and commonly is drawn to increase
its orientation. As is known in the art, dry spinning commonly is
conducted by dissolving the polymer in an appropriate solvent, such as
N,N-dimethylformamide or N,N-dimethylacetamide, and passing the solution
through an opening of predetermined shape into an evaporative atmosphere
(e.g., nitrogen) in which much of the solvent is evaporated. Wet spinning
commonly is conducted by passing a solution of the polymer through an
opening of predetermined shape into an aqueous coagulation bath. High
pressure melt spinning is conducted by applying high steam pressure to
the polymer, which has been heated to near the melting point, thus forcing
an extrudate through an opening of predetermined shape.
The acrylic polymer prior to thermal stabilization is formed primarily of
recurring acrylonitrile units. For instance, the acrylic polymer may be an
acrylonitrile homopolymer or acrylonitrile copolymer containing at least
85 mole percent acrylonitrile units (e.g. at least 95 mole percent
acrylonitrile units) and up to about 15 mole percent of one or more
monovinyl units copolymerized therewith (e.g., up to at least 5 mole
percent of one or more monovinyl units). Representative monovinyl units
may be derived from styrene, methyl acrylate, methyl methacrylate, vinyl
acetate, vinyl chloride, vinylidene chloride, vinyl pyridine, and the
like. A preferred acrylic polymer prior to stabilization is an
acrylonitrile copolymer containing approximately 98 mole percent
acrylonitrile units copolymerized with approximately 2 mole percent of
recurring methyl acrylate units.
The acrylic fibrous material prior to thermal stabilization may optionally
be drawn in accordance with conventional techniques in order to improve
its orientation. For instance, the starting material may be drawn by
stretching while in contact with a hot shoe at a temperature of about
140.degree. to 160.degree. C. Additional representative drawing techniques
are disclosed in U.S. Pat. Nos. 2,455,173; 2,948,581; and 3,122,412, which
are herein incorporated by reference. It is recommended that the acrylic
fibrous materials prior to thermal stabilization be drawn to a single
filament tenacity of at least about 2.5 grams per denier. If desired,
however, the starting material may be more highly oriented, e.g., drawn up
to a single filament tenacity of about 7.5 to 8 grams per denier, or more.
The acrylic fibrous material prior to thermal stabilization may be provided
in a variety of physical configurations. For instance, the acrylic fibrous
material prior to thermal stabilization may be in the form of a staple
yarn, continuous filament yarn, multifilamentary tow, tape, strand, cable,
fibrils, fibrids, paper, woven fabric, nonwoven fabric, etc. Continuous
filament yarns may be provided with a twist of about 0.1 to 5 tpi, and
preferably about 0.3 to 1.0 tpi, in orer to improve handing
characteristics. Alternatively, one may select bundles of acrylic fibrous
material which possess substantially no twist.
The thermal stabilization reaction commonly is conducted by heating the
acrylic fibrous material in an oxygen-containing atmosphere at a
temperature within the range of approximately 200.degree. to 350.degree.
C. to render the same non-burning when subjected to an ordinary match
flame. Such thermal stabilization reaction may be conducted in accordance
with techniques known in the art. For instance, the multiple stage thermal
stabilization process of U.S. Pat. No. 3,539,295, which is herein
incorporated by reference, may be employed. The oxygen-containing
atmosphere preferably contains about 1 to 40 percent by weight of
molecular oxygen, and in a particularly preferred embodiment is air. The
fibrous material preferably is maintained under longitudinal tension at a
substantially constant length during the thermal stabilization reaction.
Residence times for the thermal stabilization reaction at a temperature
within the range of approximately 200.degree. to 350.degree. C. are
commonly about 1 to 5 hours, or more, and are influenced by the denier of
the fibrous materials as will be apparent to those skilled in the art.
Batch or continuous processing techniques may be employed.
At the conclusion of the thermal stabilization reaction the fibrous
material is black in appearance and commonly contains a bound oxygen
content of at least 6 percent by weight (e.g., 7 to 12 percent by weight)
as determined by the Unterzaucher analysis. While not wishing to be bound
by theory, it is believed that the thermal stabilization reaction involves
(1) an oxidative cross-linking reaction of adjoining molecules as well as
(2) a cyclization reaction of pendant nitrile groups to a condensed
dihydropyridine structure.
Alternatively, the thermal stabilization reaction may be assisted by the
use of various processing techniques which tend to shorten the time
required to accomplish the desired thermal stabilization. For example,
thermal stabilization techniques employing high energy sources such as
lasers can be used. Representative processes which can be used to form the
thermally stabilized acrylic fibrous material on an accelerated basis are
disclosed in U.S. Pat. Nos. 3,416,874; 3,592,595; 3,647,770; 3,650,668;
3,656,882; 3,656,883; 3,708,326; 3,729,549; 3,767,773; 3,813,219;
3,814,577; 3,820,951; 3,850,876; 3,917,776; 3,923,950; 3,961,888;
4,002,426; 4,004,053; 4,295,844; 4,364,916; 4,370,141; etc. The
disclosures of these patents are herein incorporated by reference.
It has been found that better adhesion of the copper sulfide as formed is
obtained when the thermally stabilized acrylic fibrous material is washed
with a solvent to remove impurities, preferably at an elevated
temperature, e.g. from about 30.degree. C. to the boiling point of the
solvent. The solvent for such washing can be an aliphatic alcohol having
from 1 to about 3 carbon atoms, a halocarbon having from 1 to about 3
carbon atoms, or a halogenated hydrocarbon having from 1 to about 3 carbon
atoms. In a preferred embodiment, the fibrous material is washed in
methanol under reflux conditions.
The thermally stabilized fibrous material which is to be made electrically
conductive in accordance with the present invention is cuprous
ion-impregnated by contact with a source of cuprous ions in a solution.
Cuprous ions have been found capable of dispersing into the fibrous
material more readily and more completely than cupric ions or elemental
copper. Firstly, elemental copper cannot be incorporated into the fibrous
material except by physical entrapment or plating. By analytical methods
(X-Ray Absorption Near Edge Spectra) capable of distinguishing between
cupric and cuprous ions it has been determined that the copper species in
the treated fibers is substantially cuprous. While not wishing to be bound
by theory, it appears that the cuprous ions are preferentially complexed
by the pre-oxidized acrylic material, since hydroxylamine is a moderate
reducing agent and reduces only about 1 percent of the cupric ions in
solution at any given time, but the final proportion of the cuprous ions
in the fibrous material is much higher than would be predicted by their
concentration in the treatment solution.
The solvent for the cuprous ion solution may be water, or nonaqueous media
such as acetonitrile, propylene carbonate or butyrolactone. In a presently
preferred embodiment, an aqueous solution is employed.
Inasmuch as most commercially available cuprous compounds (e.g., cuprous
chloride, cuprous oxide, cuprous cyanide, cuprous iodide and the like) are
insoluble in water, the cuprous ions are preferably supplied by in situ
reduction of cupric ions. In a preferred embodiment, cupric ions are
supplied in a reducing agent-containing aqueous solution in the form of a
water-soluble cupric salt such as cupric sulfate, cupric chloride, cupric
nitrate, cupric acetate, cupric formate, cupric bromide, cupric
perchlorate, complex salts of copper and the like, and mixture thereof,
such that reduction of cupric ions to cuprous ions occurs in solution. In
a most preferred embodiment, the source of cupric ions is cupric sulfate
in an aqueous solution.
The cupric salt is supplied in a solution at a concentration sufficient to
produce a cupric ion concentration of approximately 0.1 to 15 percent by
weight, based on total weight of the solution. In a preferred embodiment,
the cupric salt is supplied at a concentration sufficient to produce a
cupric ion concentration of approximately 0.25 to 10 percent by weight
based on total weight of the solution. In a most preferred embodiment for
good conductivity and physical properties, the solution comprises cupric
ions in a concentration of approximately 2 percent by weight. The
conductivity of the fibrous material treated generally varies with the
concentration of the cupric ion in solution and available for reduction,
but at the higher concentrations of cupric ion, the advantage of higher
conductivity may be offset by mechanical deterioration of the fibers due
to overimpregnation.
A reducing agent is supplied with the cupric ion source to reduce cupric
ions to cuprous ions in solution, preferably in an equivalent
concentration. Preferably, the reducing agent is hydroxylamine, or an
hydroxylamine addition salt, e.g., hydroxylamine sulfate, hydroxylamine
hydrochloride, hydroxylamine nitrate, hydroxylamine acetate, hydroxylamine
formate, hydroxylamine bromide, and the like, and mixtures thereof, with
the most preferred reducing agent presently being hydroxylamine sulfate.
However, other salts such as sodium hypophosphite, sodium bisulfite,
sodium dithionite, sodium formaldehyde sulfoxylate and zinc formaldehyde
sulfoxylate can also be used. The latter two salts are available
commercially from Virginia Chemicals Co. under the trademarks Discolite
and Parolite, respectively. Copper metal can also be used as the reducing
agent, in forms such as powder, turnings, wire or other finely divided
materials.
The soluble reducing agent (i.e., other than copper metal) is supplied in
an amount which is soluble in the cupric ion-containing solution and which
is sufficient to at least partially reduce the cupric ions present to the
cuprous oxidation state. The concentration for the reducing agent in the
solution will generally range from approximately 0.1 to 20 percent by
weight of active ingredient (e.g., hydroxylamine) based on the total
solution weight. In a preferred embodiment, the reducing agent is present
in the solution as between about 0.5 and about 10 percent by weight of the
solution based on the total solution weight. In a most preferred
embodiment, the reducing agent comprises about 5 percent by weight of the
solution. When copper metal is used as the reducing agent, it need only be
present in a quantity at least sufficient to substantially completely
reduce the cupric ions present to the cuprous oxidation state, and is
preferably present in a slight excess.
The pH of the solution may be controlled at approximately 1 to 5 by the
addition of sulfuric acid, hydrochloric acid, nitric acid, acetic acid or
other acids, and sodium hydroxide, potassium hydroxide or other bases to
the solution. Control of the pH can be achieved by buffering agents such
as potassium hydrogen phthalate, citrate, tartrate, and the like.
The temperature of the resulting cuprous ion-containing solution is
preferably elevated (e.g., above about 60.degree. C.). In a preferred
embodiment, the temperature of the aqueous solution during the cuprous
ion-impregnating step is between about 80.degree. and about 105.degree. C.
at atmospheric pressure. In a most preferred embodiment, the temperature
of the aqueous solution is about 100.degree. C. Higher temperatures, e.g.,
in the range of from about 100.degree. to about 150.degree. C., can be
used in high pressure equipment such as pressure dyeing equipment, and in
steam-heated ovens. Long filaments, tow or roving can also be treated
continuously in a steam oven. Elevated temperatures are expected to
shorten the duration of treatment.
Contact time between the thermally stabilized acrylic fibrous material and
the cuprous ion-containing solution in the cuprous ion-impregnating step
may be between about 5 minutes and about 10 hours in duration. In a
preferred embodiment, the contact time is between about 15 minutes and
about 2 hours in duration. During such contact, the thermally stabilized
acrylic fibrous material is preferably maintained at a constant length.
The required contact between the thermally stabilized acrylic fibrous
material and the cuprous ion-containing solution can be accomplished by a
variety of techniques including immersion, spraying, drip feeding,
padding, etc. In small quantities, loose hanks of filaments or tow can be
immersed in the solution, while in larger quantities, it is convenient to
wind the filaments loosely on a bobbin which can be immersed and gently
rotated in a tank of the solution. In a preferred embodiment for
production, a continuous length of the fibrous material can be passed in
the direction of its length through a bath containing the cuprous
ion-containing solution which is continuously or intermittently
replenished, or passed through a zone where the solution is applied by
spraying, padding or drip feeding.
Following a cuprous ion impregnating step of appropriate duration, the
thermally stabilized acrylic fibrous material comprises cuprous ions
dispersed substantially uniformly throughout the fibrous material. This
fact is evidenced by elemental mapping using the characteristic X-ray
emission in an electron microscope. However, the uniform penetration and
distribution of cuprous ions throughout the fibrous material is not
essential, as the desired conductivity may in some cases be achieved by
cuprous ion impregnation which is limited to surface areas. If a
relatively low concentration of the cuprous ions in the fibrous material
is desired, e.g., for production of low conductivity fibers, the material
may optionally be washed prior to contact with the sulfiding agent.
Following the cuprous ion-impregnating step, the cuprous ion-impregnating
thermally stabilized acrylic fibrous material is contacted with a
sulfiding agent which is capable of sulfiding cuprous ions to form
electrically conductive copper sulfide in association with the thermally
stabilized acrylic fibrous material. Suitable sulfiding agents include
sodium thiosulfate, potassium thiosulfate, lithium thiosulfate, rubidium
thiosulfate, cesium thiosulfate, sodium sulfide, sulfur dioxide, sodium
hydrogen sulfite, sodium pyrosulfite, sulfurous acid, dithionous acid,
sodium dithionite, thiourea dioxide, hydrogen sulfide, sodium formaldehyde
sulfoxylate, and zinc formaldehyde sulfoxylate and the like, or mixtures
thereof. Some of these agents, such as, e.g., sodium hydrogen sulfite,
sodium dithionite, sodium formaldehyde sulfoxylate, and zinc formaldehyde
sulfoxylate can serve as combination reducing and sulfiding agents. The
preferred sulfiding agents are the alkali metal thiosulfates. The most
preferred sulfiding agent at present is sodium thiosulfate.
The sulfiding agent is preferably contacted with the cuprous
ion-impregnated thermally stabilized acrylic fibrous material by addition
of the sulfiding agent directly to the cuprous ion-containing solution.
The contact occurs for an additional time period of between about 15
minutes and about 10 hours. In a preferred embodiment, the additional
contact time is between about 1 and about 2 hours in duration. During such
contact, the thermally stabilized acrylic fibrous material is preferably
maintained at a constant length. Again, the required contact between the
cuprous ion-impregnated fibrous material and the sulfiding
agent-containing solution may be accomplished by a variety of techniques
including immersion, spraying, drip feeding, padding, etc. In a preferred
embodiment, a continuous length of the fibrous material is again passed in
the direction of its length through a bath containing the sulfiding
agent-containing solution which is continuously or intermittently
replenished. In an embodiment, a solution of a copper thiosulfate complex
chilled to a temperature where it is homogeneous (e.g. 0.degree.-5.degree.
C.) is applied to the fibrous material, then precipitates copper sulfide
when the material is warmed to at least about room temperature.
The sulfiding agent comprises between about 0.1 and about 30 percent by
weight of the solution which is contacted with the cuprous ion-impregnated
fibrous material, based on total solution weight. Preferably, the solution
comprises between about 5 and about 15 percent by weight of the sulfiding
agent. Most preferably, the solution comprises about 10 percent by weight
of the sulfiding agent, based on total solution weight.
Preferably, the aqueous solution comprising the sulfiding agent is again
maintained at an elevated temperature, e.g., between about 90 and about
105.degree. C. at atmospheric pressure. Most preferably, the aqueous
solution is maintained at about 100.degree. C. Higher temperatures,
preferably at superatmospheric pressure, can be used to accelerate the
treatment. At present, the highest conductivities are obtained in an
embodiment in which the cuprous solution is cooled, e.g., to a temperature
of about 80.degree. C., a sulfiding agent such as a thiosulfate is added,
and the temperature of the solution is then raised, e.g., to the range of
about 100.degree.-103.degree. C.
Following the sulfiding treatment, the resulting fibrous material is
preferably washed to remove residual reactants adhering thereto, and
dried. Washing may be achieved by rubbing or agitating in a tank or under
running water, spraying with a jet of water, and the like. Drying may be
accomplished by hot air, superheated steam or vacuum drying.
Following the sulfiding treatment, substantially all of the copper ions are
sulfided. In a preferred embodiment, at least about 80 percent, and
preferably between about 90 and about 98 percent of the sulfided copper
(i.e., copper sulfide) is in the covellite form, with the remainder
generally being in the form of digenite, having the empirical formula
Cu.sub.9 S.sub.5. In a most preferred embodiment, the copper sulfide is
substantially entirely (e.g., at least 97 percent) in the covellite form.
Preferably, the resulting copper sulfide consists essentially of covellite
copper sulfide.
By the term "covellite" is meant copper sulfide of a stoichiometric formula
CuS, with a crystallographic structure identical to that of the copper
sulfide mineral covellite of the same stoichiometry. The crystal structure
is described by R. W. G. Wyckoff in CRYSTAL STRUCTURES, 2d Ed., Vol. I, R.
E. Krieger Publ. Co. (1982), at page 145, which is herein incorporated by
reference. Contrary to expectation, the copper is not in the cupric
(divalent) state and all the copper and sulfur atoms are not equivalent.
The structure is hexagonal with an elongated six molecule cell; a.sub.o
=3.796 .ANG. and c=16.36 .ANG.. Of the six sulfur atoms per unit cell,
four are associated to two S.sub.2 groups (S-S: 2.05 .ANG.); two of the
six copper atoms have triangular coordination (CuS: 2.19 .ANG.) and the
other four have tetrahedral coordination (Cu-S: 2.31 .ANG.). All the
copper is reduced to Cu.sup.+ and CuS is diamagnetic. The monosulfide is a
metallic conductor at room temperature and is superconducting below
1.62.degree. K.
It is highly desirable that the copper sulfide is in the covellite form, as
covellite is the most highly electrically conductive known form of copper
sulfide. The chemical structure of the copper sulfide is verified by X-ray
diffraction techniques.
FIG. 4, an X-ray diffraction pattern of the electrically conductive
thermally stabilized acrylic fibrous material produced in accordance with
the procedure of Example I, shows the covellite copper phase in a
Debye-Scherrer pattern. The pattern was identified as that of covellite by
a computer search of JCPDS files, correlating with JCPDS card 6-464. (The
JCPDS card for digenite is card 23-962.) The proportion of covellite
produced can be affected by the duration of the sulfiding treatment; for
example, after the fiber has soaked in cuprous ion solution for 1 hour,
mixtures of covellite and digenite can be observed after sulfiding for one
half or 1 hour, but only covellite is observed after 2 hours of sulfiding.
At this point, every line in the x-ray diffraction pattern can be
attributed to the covellite phase, with no lines characteristic of the
digenite phase being discernible. The digenite phase, if present at all,
is believed to constitute less than about 3 percent of the crystalline
phases. While not wishing to be bound by theory, observations of trials
thus far are consistent with a mechanism wherein both covellite and
digenite phases are formed initially, with generation of the digenite
continuing, but then disproportionating to form a covellite phase.
With respect to the physical configuration of the copper sulfide relative
to the fibrous material, durin | | |
