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New Forms of Carbon.

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pyramidal inversion in the sulfonium ylide (43),
A H * = 23.3 k c a l / m o l e [ ~ ~is~ lsomewhat
,
lower than
that for sulfonium salts ( A H * = 25-29 kcal/
‘C2H5
431
mole) [9*,991, and much lower than barriers found in
sulfoxides ( A H * = 35-42 kcal/mole)
Although
substantial (2p-3d), bonding is postulated t o occur
in sulfoxides f1521, and, by extension, in sulfilimines, it
[1511 D . Darwish and R . I
. Tomilson, J. Amer. chem. SOC.90,
5938 (1968).
is not clear at present whether the pyramidal stability
of these compounds should be attributed to the effect
of t h e x bonding, rather than to theeffect of theelectronegativity of the oxygen and nitrogen substituent or
to lone pair-lone pair interactions.
We thank the Air Force Ojfice of Scientific Research
for support of this work (AF-49(638)-1625 and AFAFOSR-I 188-B), the National Research Council of
Canada for a Fellowship to one of us (A.R.), and
Professor Morton Raban ,for stimulating discussions.
Received: February 17, 1970
[A 763 IE]
German version: Angew. Chem. 82, 453 (1970)
[1521 a) P . Haake, W . B. Miller, and D . A. Tyssee, J. Amer.
chem. SOC.86, 3577 (1964); b) G . L . Bendazioli, F, Bernard;,
P . Pnlrnieri, and C . Znuli, J . chem. SOC.(A) 1968, 2186.
New Forms of Carbon
By Otto Vohler, Peter-Ludwig Reiser, Renato Martina, a n d Dieter Overhoff [*I
A number of novel carbon materials whose unique properties fit them for many uses have
recently been developed. Pyrolytic graphites are excellent conductors of heat and electricity parallel to the surface, whereas they are semiconductors perpendicular to the
surface. A similar anisorropy is found in graphite foils, which are impermeable, but also
very flexible. Classlike carbon, which is also impermeable, is, however, completely isotropic. Carbon foams and felts are extremely light and exhibit very good thermal insulation up to high temperatures. In addition to very high strength, carbon Jibers have values
of Young’s modulus greater than that of any other fibers or wires. Carbon fiberiresin
composites are therefore more rigid than any other known materials; their specific
Young’s modulus is five times that of steel.
1. Introduction
Up to the end of the 19th century, graphitic carbon
was known practically only in the form of mined
natural graphite. Though synthetic carbons had been
prepared by heating mixtures of anthracite, coke,
charcoal from wood and sugar, and retort carbons
with carbonaceous binders such as sugar syrup, tar,
and pitch, and the partial conversion of these synthetic
carbons into synthetic graphite had repeatedly been
observed, e.g. in an electric arc, the economic production of synthetic or artificial graphite was made possible
only by the graphitizing process patented by Acheson [I]
in 1896.
I n this process, the material to be graphitized forms
the thermally and electrically insulated heating core
[*] Dr. 0. Vohler, Dr. P.-L. Reiser, Dr. Renato Martina, and
Dr. D. Overhoff
SIGRI Elektrographit GmbH
8901 Meitingen (Germany)
[l] E. G . Acheson, US-Pat. 568323 (1895).
414
of an electric resistance furnace, as shown diagrammatically in Figure 1. The charge is heated to about
3000°C by direct passage of current, via a resistor
pack of granular coke. The graphitization that occurs
a t this temperature leads to extensive ordering within
the crystallites of the coke grains and of the binder
coke [ZJ.
b ;
,I
e
I
\
9
Fig. 1. Acheson graphitizing furnace, schematic.
a) Current connection, b) current supply electrodes, c) port, d) resistor
pack of granular coke, e) artificial carbon to be graphitized, f) insulating
layer, g) furnace bed.
[2] E. Donges and 0. Yohler in K. Winnacker and L . Kiichler:
Chemische Technologie. Vol. I, 3rd. Edit., Hanser, Miinchen
1969.
Angew. Chem. internat. Edit.
1 Vol. 9 (1970) 1 NO. 6
As a result of its unique combination of chemicaI, electrical,
mechanical, and high-temperature propertiesr31, artificial
graphite has since become widely used in research and industry, e.g. as electrodes in arc furnaces for the production of
electric steel, in carbide, reduction, and resistance furnaces,
as anode material in aqueous and fused salt electrolysis, as
casting molds and lining material, in the production of chemical equipment, as fuel element and moderator material in
nuclear reactors, in rockets, and for small components,
particularly in electrical engineering (cf. [ZJ).
I n recent years, often as a result of extreme requirements for spacecraft, a series of novel carbons have
been developed, not by additional treatment or transformation of classical artificial graphite, but by fundamentally different processes; examples are pyrolytic
graphite by pyrolysis of gaseous carbon compounds,
flexible graphite by explosive decomposition and subsequent compaction of graphite intercalation compounds, glasslike carbon and carbon foams by controlled carbonization of synthetic resins, and carbon
fiber materials by crosslinkage and carbonization of
suitable organic fibers.
These new carbon materials have unique properties,
as a result of which they have found many uses. Some
examples of extreme fields of application are electrical
heating and thermal insulation, gaskets and porous
filters, flexible parts and composites that are more
rigid than any other material known.
2. Graphite Single Crystals
Some of these new carbons posses the typical graphite
structure only in small regions, and their X-ray reflections are often considerably broadened because of the
small crystallite size; in some cases, in fact, only twodimensional order can be detected. It is nevertheless
convenient to derive their properties from the crystal
structure of graphite.
Graphite crystallizes mainly in a hexagonal layer
lattice of space group D h (Fig. 2). Each carbon atom
within the layers is bonded to three neighboring atoms
by very strong, covalent sp* bonds with a C-C distance of 1.415 A. On the other hand, the TC bonding
between the layers is very weak; the distance between
two neighboring a,b planes is 3.3539 A.
Because of this pronounced layer lattice, the properties of the graphite single crystal are strongly dependent on direction (Table 1); examples are the Young's
modulus, the specific electrical resistance and the thermal conductivity, which is directly related to the
nature and strength of the chemical bonding. Parallel
to the a,b planes, the conductivity of the graphite single
crystal is comparable with that of good metallic conductors such as aluminum or copper; perpendicular
to the layers, on the other hand, it is a semiconductor.
This anisotropy is particularly marked in the thermal
expansion 131.
Table 1.
Properties of graphite single crystals.
Density (glcn13)
Young's modulus (kglcrnz)
Electrical resistance (ohm min2lm)
Thermal conductivity
(kcal m-1 h-' deg-1)
Coefficient of thernial expansion
(20-200 "C) (dee-1)
This strong dependence on direction is found only for
particularly well-formed natural graphite flakes, and
is much less pronounced in the properties of the porous
artifical graphite objects made by the classical methods
for the production of synthetic carbon from solid
fillers and pitch-like binders. In these artifacts, the
anisotropy of the single crystal is greatly weakened by
the various orientations of the crystallites, by microcracks and pores within the grains, and b y the porous
structure of the shaped article. However, the anisotropy i s very pronounced in some new types of carbon,
such as pyrolytic graphite, graphite foils, and the hightensile or high-modulus carbon fibers.
3. Pyrolytic Carbon and'Graphite
The formation of carbon by pyrolytic decomposition
of gaseous or vaporized carbon compounds is not new;
retort carbons were obtained in this way even before
Acheson described his graphitizing process. It was
only in the last decade, however, that suitable equipment has been developed for the production of pyrocarbon as coating and in the form of a compact, selfsupporting body under controlled conditions, and
hence with properties that could be varied as desired.
Pyrocarbon coatings and compacts are produced from
gaseous or vaporized carbon compounds, which are
decomposed pyrolytically on hot graphite supports
(800-2800 "C)[4-61. Examples of suitable carbon
compounds are methane, ethane, propane, acetylene,
benzene, and carbon tetrachloride.
[3] W. N . Reynolds: Physical Properties of Graphite. Elsevier,
Amsterdam 1968.
14) R . J . Diefendorf; J. Chim. physique 57, 815 (1960).
[ 5 ] F. Trombel and .
I
.
Rappeneau: Les Pyrocarbones in Les
Carbones. Vol. 2, p. 779; Masson, Paris 1965.
161 B. Lersmacher, H . Lydtin, and W. F. Knippenberg, Chemie-
a
Fig. 2.
Crystal lattice of graphite (hexagonal modification).
Angew. Chem. internat. Edit. / Vol. 9 (1970) / No. 6
1ng:Techn.
39, 833 (1967).
415
3.1. Pyrocarbon Coatings
The structure and properties of the pyrocarbon layers
obtained depend mainly on the temperature, pressure,
residence time, and nature of the carbonaceous gas.
The variability of pyrographite is extremely wide, and
ranges from almost isotropic, via granular, to the
highly anisotropic laminar structures.
of articles coated in this way has been attacked, the
protective action of the pyrocarbon in the pores
persists.
These pyrocarbon structures have been examined in
particular detail in connection with deposition in the
fluidized bed reactor 171. This deposition has gained
considerable technological importance for the coating
of fuel particles for nuclear reactors.
Multiple layers are used having various mechanical
properties and irradiation behavior and exhibiting
good fission product retention.
Another important application is the production of
pyrocarbon coatings on artificial graphite bodies {see
Fig. 3), the deposition then no longer taking place in a
fluidized bed, but on fixed graphite objects. The pyrocarbon coating gives the artificial graphite a very
smooth surface; it becomes substantially impermeable
and its resistance t o oxidation and erosion is greatly
enhanced.
Fig. 4. Artificial graphite with pyrolytic c a r b o n coating within t h e
pores (magnification 1000 u).
Graphites coated with pyrocarbon are used in particular
where severe erosion and oxidation are likely to occur, e.g.
as sleeves in COz-cooled nuclear reactors. They are also used
for special purposes, such as vaporization dishes in the vapordeposition of metals or as graphite molds for the sealing of
terminals in tube bases. Owing to their high purity, graphites
coated with pyrocarbons are being increasingly used in semiconductor technology as mold materials and as plates for
epitaxy. Since blood has only a small tendency to clot in the
presence of pyrocarbon layers, this could offer a new application in medicine, e.g. for artificial heart valves [91.
3.2. Compact Pyrolytic Graphite
Fig. 3. Artificial graphite with pyrolytic c a r b o n coating on t h e surface
(magnification 1000 x).
If the coating is applied only to the outer surface of
the shaped body, it provides only incomplete protection against slow selective oxidation, such as occurs
e.g. in graphite components in nuclear reactors. If the
protective surface layer is damaged, the oxidizing gas
can pass through the pores of the graphite body and so
destroy the material. To prevent this, it is necessary to
deposit the pyrocarbon within the pores (see Fig. 4).
This is possible at slow deposition rates (low temperature, low partial pressure) [81. Even after the surface
416
Compact pyrolytic graphite bodies are produced a t a
constant temperature and a constant pressure a t the
lowest possible deposition rate. Crystalline order is
obtained only within the layer planes; the material is
turbostratic. Growth and ordering processes that lead
also to extensive crystalline order in the c direction
can be induced by additional heat treatment at temperatures up to 3000 "C.
The growth rate, density, and microstructure depend
mainly on the deposition temperature [lo]. Whereas
pyrolytic carbons having a density of only 1.35 g/cm3
are obtained at 16OO0C, the bulk density of pyrographite deposited at about 2100 "C is between 2.20
and 2.23 g/cm3.
[7] J . C. Bokros and R . J . Price, Carbon 3, 503 (1966).
[8] 0. Vohler, P.-L. Reiser, and E . Sperk, Carbon 6, 397 (1968).
[9] J. C. Bokros, 9 t h Carbon Conference, Paper CP-inv.,
Boston 1969.
[lo] J . C. Bokros and R . J. Price, Carbon 6, 213 (1968).
Angew. Chem. internat. Edit.
/ Vol. 9
(1970)
NO. 6
Because of the good alignment of the layer planes,
pyrolytic graphite bodies, like single crystals, are
strongly anisotropic. Thus the ratio of anisotropy for
the specific electrical resistance (perpendicular/parallel)
is 1400; the parallel/perpendicular ratio for the thermal conductivity is about 440. As in the single crystal,
the thermal expansion is negative parallel to the surface and strongly positive in the perpendicular direction (cf. Table 2).
Table 2 .
Properties of pyrolytic graphite.
1
Bulk density (g/cm3)
Young's modulus (kglcm2)
Flexural strength (kglcmz)
Compressive strength (kg/cm*)
Electrical resistance (ohm mmzjrn)
Thermal conductivity
(kcal m-1 h-1 deg-1)
Coefficient of thermai expansion
(20-200 "C)(deg-1)
parallel to
surface
1
perpendicular to
surface
2.5 x 105
1 x 103
1 x 103
5
350
-
-0.8 x 10-6
+2 5 x 10-6
4~ 103
7 x 103
0.8
A crystallographically almost ideal graphite has been
obtained by uniaxial hot compression of pyrolytic
graphite followed by annealing at a very high temperature and low pressure [11J. The ratio of anisotropy
for the electrical resistance of this material a t 300 "K
was 4000, and increased to 90000 on cooling to 4.2 OK.
insulation perpendicular to the layer planes and by an
increase in the strength of the pyrographite, which is
high even in the unalloyed state.
4. Carbon Foils a n d Membranes
It is well known that foreign atoms can be inserted
between the layers of well-ordered graphite crystals to
give intercalation compounds, such as potassium
graphite, graphite hydrogen sulfate, and graphite
oxide [13-161. In graphite oxide, oxygen and hydroxyl
groups are located between the carbon layers. Luzi
found as early as about 1890 that graphite oxide can
be thermally expanded to more than 100 times its
original volume. A more recent process for the economical production of flexible graphite foils is based
on the compression of this expanded material [173.
The starting material is generally coarsely crystalline,
very pure natural graphite. The graphite is oxidized
with fuming nitric acid or with a sulfuric acid/nitric
acid mixture, and then hydrolyzed. If the graphite
oxide, after drying, is heated in a fraction of a second
to about 1000 "C, it decomposes explosively. The gases
liberated expand the graphite lattice normal t o the
Owing to the high degree of alignment and the resulting strong anisotropy, pyrolytic graphite bodies can
be produced only in limited thickness. The maximum
thickness attainable for slab materials at present is
about 10 mm. In the case of tubes it is proportional to
the radius of curvature, i.e. maximum wall thickness/
radius of curvature = 0.05. Bodies with sharp edges or
thick walls can be produced by further machining or
by combining thinner pyrographite plates.
The marked anisotropy of pyrolytic graphite and its increased
resistance t o erosion and oxidation, together with the high
emission coefficient and high sublimation point of carbon,
offer great advantages e.g. when used as rocket nozzles, as reentry cones of space vehicles, for the nose cones and wing
leading edges of space gliders, o r as protective shields. Owing
to the excellent conductivity parallel to the a,b planes of the
graphite crystal, the heat is well distributed over the surface,
and both the radiation and the ablation take place over the
entire surface. At the same time, underlying structural
material is thermally protected by the insulating effect in the
c direction. Compact pyrolytic graphite plates are often used
when a very uniform distribution of heat over a surface is
desired. Pyrolytic graphite is also used t o advantage in electronic tubes 1121.
Pyrographite alloys, e.g. with B, Si, Hf, and Co, are
obtained when a vaporized organic compound of the
alloying component is added to the gaseous carbon
compound. A particularly well-known example is the
pyrographite-boron alloy, which is characterized by
higher elasticity, lower oxidizability, and greater
[ l l ] I . L. Spean, A. R. Ubbelohde, and D . A. Young, Second
Conference on Industrial Carbon and Graphite, London 1965,
p. 123, Society of Chemical Industry, London 1966.
[12] J . M . Osepchuk and J . E . Simpson, 9th Carbon Conference
Paper CA-18, Boston 1969.
Angew. Chem. internat. Edit. f Vol.'9 (1970) J No. 6
Fig. 5. Electron-scan micrograph of a carbon foil (magnification
125 x). The micrograph was prepared in the Labor fur Raster-Elektronen-Mikroskopie Dr. H . Klingele, Munchen.
[13] W. Luzi, Naturwissenschaften 64, 224 (1892).
[14] U . Hofmann and A. Frenzel, Ber. dtsch. chem. Ges. 63,
1251 (1930).
[15] U . Hofmann and E. Konig, Z . anorg. allg. Chem. 234, 311
(1937).
1161 H. Thiele, Z . anorg. alig. Chem. 206, 407 (1932).
1171 J . H . Shane, R . J. J . Russel, and R. A. Bochman, French
Pat. 1395964 (1963), High Temperature Materials.
417
layer planes by a factor of more than 100. The separated carbon layers of the graphite lattice are then firmly
bonded together by mechanical pressure; the resulting
products are gas-tight, flexible, strongly anisotropic
carbon foils having a density of about 1.1 g/crn3. An
electron scan micrograph (Fig. 5 ) shows the pronouncd
layer structure of this material.
These foils are produced without binders; they consist
of pure carbon, and can therefore be used up to temperatures above 3000 "C in inert and reducing atmospheres or under vacuum. The strong dependence of the
properties on direction is again remarkable. The ratio
of anisotropy for the specific electrical resistance is
even higher than in pyrolytic graphite (cf. Table 3).
The electrical conductivity parallel to the surface is
comparable with that of well-ordered artificial graphite. Special emphasis must be placed on the impermeability of these foils to gases and liquids up to very
high temperatures. Graphite foils are also much more
resistant to oxidation than artificial graphites. Their
surface is very smooth and resistant to abrasion.
Table 3.
a lower pressure and without a binder. These foams
may then be compressed further during use.
Self-adhesive or plastic-coated graphite foils are already on the market.
Carbon membranes only a few pm thick have now
also been developed. Though thin enough to be translucent, these membranes are nevertheless impermeable
to gases and liquids.
The membranes are again prepared from graphite oxide,
which is made into a thixotropic gel in aqueous suspension. This gel is deposited on well-polished surfaces. Very careful drying gives a membrane ofgraphite
oxide, which is reduced in a stream of hydrogen above
500 "C 1181. The resulting carbon membrane, which
has a highly imperfect graphite crystal lattice, is graphitized above 2500 OC; during this process, the density
increases to 1.8-2.2 g/cm3. Carbon membranes of this
nature are used e.g. as dialysis membranes for the
desalination of sea water.
5. Glasslike Carbon
Properties of carbon foils.
parallel t o
surface
Bulk density (g/cm3)
Electrical resistance (ohm mmz/m)
Thermal conductivity
(kcal m-1 h-1 deg-1)
Permeability (air) (cm2/s)
perpendicular t o
surface
0.9 1.1
x 104
8
150
4
-
< 10-6
3
The flexible graphite is produced in the form of foils in
thickness of 0.1 to 0.3 mm. These films are used directly or made into laminates several mm thick, the individual layers of which may be bonded together by carbonizable resin binders. Dense molded bodies can
also be produced without binders. They are then
equivalent to the foils in their resistance to corrosion
and heat and in their thermal and electrical properties.
Owing to their greater thickness, however, they no
longer posses high flexibility.
Graphite foils and laminates have proved excellently suited
for use in several fields:
a) as corrosion-resistant seals (gaskets and packings) for
very low to very high temperatures: high thermal conductivity and hence good heat dissipation, self-lubricating, impermeable with and without cooling, long life, no maintenance,
corrosion-resistant;
b) for the lining of press molds and casting molds: smooth
surface, low friction, dense, abrasion-resistant;
c) as radiation shields in high-temperature furnaces: high
reflection factor and good thermal conductivity parallel to
the surface, combined with poor thermal conductivity normal
to the surface;
d) in electrochemistry and electrical engineering, e.g. as a
surface heater: high anisotropy of electrical and thermal
conductivity;
e) as linings for chemical equipment: very flexible, corrosionresistant, impermeable to gases and liquids;
f) as films for bursting disks: impermeable to gases and liquids, corrosion-resistant, heat-resistant.
Bulk density (g/cm3)
Pore volume (vol.-%)
Young's modulus (kglcmz)
Flexural strength (kglcmz)
Electrical resistance (ohm mmz/m)
Thermal conductivity
(kcal m-1 h-1 deg-1)
Coefficient of thermal expansion
(deg-9
Permeability (air) (crn2is)
1.45- 1.50
<l
2.6 x 105
700
50
3
3 x 10-6
10-11
Expanded graphite can also be made into porous
foams (density 0.05-0.1 g/cm3) by compression under
418
Angew. Chem. internat. Edit.
1 Vol. 9 (1970)/ No. 6
Glasslike carbon is much stronger and stiffer than
conventional carbons and graphites (Table 4). Its
electrical and thermal conductivity are comparable
with those of hard carbons.
Owing to the small number of accessible pores and its
high impermeability, glasslike carbon is very corrosionresistant. However, it is attacked by alkali metals with
formation of intercalation compounds, as well as by
carbide-forming metals 1191.
To prepare glasslike carbon, partially precondensed
phenolic or furan resins are mixed with catalysts,
poured into molds with very smooth surfaces, and
cured. The material is then removed from the molds
and carbonized in accordance with a strictly defined
heating schedule. The resulting shrinkage is pronounced, but quite definite and isotropic, so that moldings can be produced with a dimensional accuracy
of *0.5%. Owing to its hardness, glasslike carbon
can only be worked with diamond tools or by an
ultrasonic technique. It is therefore preferable to work
the cured resin before carbonizing, taking the amount
of further shrinkage into account [21J.
b) A foam made from a synthetic resin that itself gives
a relatively high coke yield, e.g. a phenolic resin
foam [26,271.
The carbon foams obtained by carbonization are extremely light, having densities of only about 0.05 g/
cm3. The percentage decreases in volume and in weight
are the same, so that the carbon foams have the same
bulk density as the synthetic resin foam used. Carbon
foam is composed of very thin carbon cell walls (cf.
Fig. 6); it can be used up to temperatures higher than
Glasslike carbon is produced as plates, tubes, crucibles,
boats, etc. with wall thicknesses of about 3 mm. It has found
a number of applications, particularly in laboratory work. The
similarity of its coefficient of expansion to that of many
borosilicate glasses makes it possible to bond the two materials permanently. Since glasslike carbon is not wetted by most
molten metals, it can be used in metallurgy for melting
crucibles, and also for pipes for passing corrosive gases into
molten metals and as vaporization dishes for the evaporation
of metals. Glasslike carbon has a high resistance t o erosion,
and is therefore also being tested as a rocket nozzle lining 1221.
6. Highly Porous Carbon, Carbon F o a m
Carbons in which up to 75 % Of their volume is OCCUpied by open pores of very uniform structure can be
produced from microcrystalline cellulose without
binders 123,241.
These homoporous carbons are used mainly as electrode materials for the direct conversion of chemical
into electrical energy in electrochemical fuel cells.
Owing to the uniformity of their pores, such electrodes
can accept very high loads.
Carbons with much greater pore volumes (carbon
foams) are obtained by carbonizing foamed synthetic
resins. The starting materials are open-pored rigid
synthetic resin foams. There are two basic types:
a) A skeleton of a foam that gives only a low carbon
yield on pyrolysis e.g. a foam based on polyurethane,
which has been impregnated with a carbon-supplying
synthetic resin, e.g. a phenolic resin c7-51.
[21] French Pat. 1475 809 (1966), Carbone Lorraine.
[22] Chem. Engng. 76, No. 27, p. 38 (1969).
1231 0. Vohler and R. Martina, Allg. prakt. Chemie 17, 500
(1966).
1241 0. Vohler, R. Martina, M . Schmid, and F. Konigsheim,
German Pat. 1254520 (1964), SIGRI.
[ 2 5 ] French Pat. 1388818 (1963), Carbone Lorraine.
Angew. Chem. internat. Edit. / Yol. 9 (1970)/ No. 6
Fig. 6. Electron-scan micrograph of carbon foam (magnificatton
250 x). The micrograph was prepared in the Labor fur Raster-Elektronen.Mikroskop,e D ~ H. , ,y,jngPle, ~
"
~
~
h
~
~
,
3000 "C in a n inert gas or under vacuum. Its thermal
insulating qualities are outstanding (cf. Table 5); the
thermal resistance of foamed synthetic resins is only
slightly better (thermal conductivity about 0.03 kcal
m-1 h-1 "C-1). However, synthetic resin foams can
only be used below their decomposition temperatures,
i.e. below 180 "C.
Table 5 .
Properties of carbon foam.
Bulk density (gicm3)
total (vol.-%)
Pore volume
open (vol: %)
Young's modulus (kgicmz)
Compressive strength (kglcmz)
Electrical resistance (ohm mmzim)
Thermal conductivity
(kcal m-1 h-1 deg-1)
Coefficient of thermal expansion
(20-200 " C )(deg-1)
(20-1000 "C) (deg-1)
Permeability (cmzis)
{
I
0.055-0.062
91
95
1200
4.8
5500
0.04
2.1 x 10-6
2.5 x 10-6
=I02
[26] W . D . Ford, US-Pat. 3121050 (1960), Union Carbide
Corporation.
1271 B. R . Afkins, Brit. Pat. 1016449 (1963), Nobrac Carbon.
419
The thermal resistance of carbon foam is superior to
that of refractory bricks by a factor of thirty, whereas
its density is lower by a factor of about twenty. For the
same weight of material, therefore, the thermal resistance is six hundred times as high.
[C ell-OH
1
The compressive strength of the highly porous carbon
foam is relatively low. It can be increased merely by
variation of the starting material or of the foaming;
however, the thermal conductivity increases at the
same time. The compressive strength can be increased
to 1000-2500 kg/cmz by impregnation e.g. with pyrolytic carbon; this is accompanied by an increase in the
thermal conductivity by a factor of about ten (approx.
0.35 kcal m-1 h-1 "C-1) [281.
The most important application of carbon foam is insulation,
in particular up to high and very high temperatures. It is also
used as a filter for corrosive agents, for frits for the uniform
distribution of gases in reaction mixtures, and as a support
material for catalysts. Fire-resistant sandwich materials
represent another application.
1- HzO
H
0
H
0-
OH OH
"
1
step I1
15O-.24O0C
- H2O
L
J"
I'1
step 111 a
240+400"C
7. Carbon Fiber Materials
6 H bH
Edison (1880) was the first t o carbonize cellulose
filaments for the newly developed carbon filament
lamp. The mechanical strength of these first carbon
filaments was very low, and they were not very flexible.
It was not until 1959 that flexible carbon fibers and
fabrics, which were then available only on a laboratory
scale, were first described in the technical literature 129,301. The starting material was again cellulose,
now in the form of rayon.
In 1961, Shindo investigated the thermal degradation
of polyacrylonitrile fibers and found that this material
(particularly after preoxidation) leads to extremely
strong carbon fibers with good carbon yields 1311.
These investigations formed the main basis of the development of high-tensile and high-modulus carbon
fibers a t Farnborough 1323.
The thermal degradation of other starting materials,
e.g. animal fibers [331, polyvinyl acetate 1341, polyvinyl
alcohol 1351, pyrolysis products of polyvinyl chloride 1361, copolymers of vinyl chloride, vinyl esters, and
vinyl alcohol [371, furfuryl alcohol condensates, and
more recently phenol-formaldehyde condensates, has
also been studied.
[28] B. Lersrnacher, DGLR Yearly Conference Bremen 1969,
Paper No. 57.
[29] Metal Progr. 75, 115 (1959).
[30] J . A . Mock, Mat. Design Engng. 49, 149 (1959).
1311 A. ShindG: Studies on Graphite Fiber. Rep. Government
Ind. Res. Inst., Osaka, No. 317 (1961).
[321 W . Watt, L. N . Phillips, and W . Johnson, Engineer 815,221
(1966).
[33] E. Sperk, 0. Vohler, F. Jeitner, and V. Gierth, German
Pat. 1255629 (1963), SIGRI.
[34] A. Shindo: Highly Crystallite-oriented Carbon Fibers
from Polymeric Fibers. ACS Polymer Preprints Vol. 9, No. 2,
1327 (1968).
[351 A . Shindo, Symp. on Carbon, Tokyo 1964.
[36] S . Otani, Carbon 3, 31 (1965).
[37] Brit. Pat. 1177739 (1967), Wacker.
420
I
step 111 b
240-4OO0C
I
1
Levoglucosan
1
- n20
I
step 111 c
- CO2, CO, H 2 0
24 O-40O0C
Tar
1
C - containing
intermediate
(4 C-Residue)
\
400-700° C
graphite-like layer planes
Scheme 1.
Thermal degradation of cellulose 138, 391.
Step I: desorption of physically adsorbed H20. Step 11: removal of
chemically bound H20. Step 111: depolymerization and chain cleavage
with dehydration, decarbonylation, and decarboxylation. Step IV:
dehydrogenation, demethanation.
In general, materials are suitable for the production
of carbon fibers only if they satisfy the following basic
requirements:
1. The fibrous form should be retained on pyrolysis,
i.e. the material must not melt when heated, or it must
Angew. Chem. internat. Edit.
Vol. 9 (1970)
1 No. 6
be capable of being pyrolytically decomposed below
its melting point.
2. The carbon skeleton should be capable of being
readily changed into a two-dimensional graphite
structure.
3. The pyrolysis should take place without appreciable evaporation of carbon-containing volatile components.
These requirements are satisfied by cellulose and crosslinkable polymers such as polyacrylonitrile. According
to Tang and Bacon, cellulose is decomposed as shown
in Scheme 1 138,391.
The dehydration is assisted by treatment with phosphates, borates, vanadates, etc. Each cellulose unit
ultimately contributes a n average of four C atoms,
which aromatize to form graphite layers.
When polyacrylonitrile is used as the starting material,
it is treated with oxygen at 200 to 300 "C before thermal degradation. This leads to crosslinking and cyclization, with the result that a structure consisting of
condensed dihydropyridine rings is formed. Above
350 "C, the nitrogen is removed as ammonia or hydrogen cyanide. Above 600 "C, further crosslinking takes
place, with dehydrogenation, to give large graphitelike layer molecules. Reactions verified by I R spectroscopy are shown in Scheme 2.
7.1. Carbon Felts, Wools, and Fabrics
When textiles made of organic fibers are subjected to
suitable heat treatment, the organic substances are
ultimately converted entirely into carbon, while the
textile form is retained. Carbon felts, wools, and fabrics can be made in this way[41]. Carbon fabrics are
also made in special cases by weaving continous
carbon fibers (see Section 7.2).
The starting materials are generally textiles made from
cellulose fibers, though materials made from completely synthetic fibers, such as polyacrylonitrile, are used
in some cases. However, only needled felts, which are
relatively spongy, can be produced from thesz fibers.
Suitable starting materials for the production of denser
carbon felts are pressed felts of animal fibers, particularly sheeps' wool. As can be seen in Figure 7, the
surface structure of the wool fibers with flat scale-like
cells is retained even after carbonization.
The heat treatment of the textiles generally takes place
in three steps:
First step (up t o about 300 "C): precarbonizing.
Second step (up to about 1000 "C): carbonizing.
Third step (up to above 2500 "C): graphitizing.
In the first step the fiber material is decomposed into
products that are already black. Crosslinking agents
(up to 5 fused rings stable)
dehydrogenation
P
mornstization
large graphite-like layer molecules
m
*
CN CN CN CN CN
2.
-
- HCN
CN CN CN CN
- HCN
Scheme 2.
further condensed
ring s y s t e m s
C N CN C N C N C N
aromati-
zation
l a r g e graphite-like
layer molecules
Thermal decomposition of polyacrylonitrile 1401.
Because of their fibrous structure, the carbon fiber
materials combine the characteristics of carbon (low
vapor pressure up t o very high temperatures, high
sublimation point, chemical stability) with typical
textile properties such as flexibility, elasticity, and
lightness.
must be used in some cases to prevent softening or
even melting before chemical degradation. In the case
of polyacrylonitrile, for instance, atmospheric oxygen
is used, while a special method has been developed for
animal fibers 1331. The thermal decomposition may also
be carried out in high-boiling liquids[42,431 or in at[41] 0. Vohler and E. Sperk, Ber. dtsch. keram. Ges. 43, 199
[381 M . M . Tang and R . Bacon, Carbon 2, 211 (1964).
1391 R . Bacon and M . M . Tang, Carbon 2, 221 (1964).
[401 E. Buhr: Hochtemperaturbestandige Kunststoffe. Hanser,
Miinchen 1969.
Angew. Chem. internat. Edit. 1 Vol. 9 (1970)
1 No. 6
(I 966).
1421 G . R . Hogg j r . and J . L. Allison, DAS 1234608 (1963),
Great Lakes Carbon.
1431 M. T . Cory, DOS 1469250 (1963), Basic Carbon.
421
Fig. 8. Insulating power of carbon felt (melting of aluminum o n a
hand insulated with carbon felt).
only a fraction of that of carbon powder, suitably insulated
furnaces can be heated and cooled more rapidly. Felt shields
are also reasonably strong, and their strength increases with
rising temperature. Since they can be cut with scissors or
knives and stitched together with carbon yarn, insulators
of any desired shape can be produced quickly and easily.
Furthermore the material is so uniform that no hot spots
occur, and uniform temperatures are obtained inside the
furnace.
Their chemical corrosion resistance also offers other advantages, e.g. for use as a filter material for corrosive substances (gases, solutions, molten metals), as a support
material for catalysts, as corrosion-resistant linings in chemical equipment, and (also in metalized formC461) as “fiberporous” electrodes in accumulators and fuel cells.
The electrical properties of the carbon and graphite felts
provide unique and new methods of resistance heating (more
flexible than the finest metal conductors, very good heat dissipation by black body radiation, high power densities, low
heat capacity, fast heating and cooling).
Owing to their low neutron absorption cross section, carbon
felts and wools are also used in nuclear engineering 1471.
Carbon and graphite wools prepared from artificial cotton or
similar starting materials are particularly suitable for use as
packing materials for high-temperature insulation. Carbon
Fig. 7. Electron-scan micrograph of carbon felt based o n sheeps’ wool
(magnification 1250 x). The micrograph was prepared in the Labor fur
Raster-Elektronen-Mikroskopie Dr. H . Klingele, Miinchen.
mospheres containing halogen or nitric oxide 144,451.
In the second step the precarbonized material is
degraded to elementary carbon in the absence of air.
These carbon felts, wools, and fabrics can be used in
this form for most purposes. For special applications
the materials are heated in a third step to above
2500 “C (graphitization). The resulting products are
known as graphite felts, wools, and fabrics, though
there is crystallographically no pronounced graphitic
structure.
Owing to the wide range of starting materials, it is
possible to obtain products with various combinations
of properties, as is shown by Table 6 for carbon and
graphite felts.
Table 6 .
Properties of carbon and graphite felts
SIGRATHERM FA 5 G
Carbon
graphite
SIGRATHERM F 1/3
carbon
graphite
I
I
Weight per unit area (g/m2)
Bulk density (g/cm3)
Thermal conductivity (20 “C
(kcal m-1 h-1 deg-1)
Square resistance (ohm)
Tensile strength (kgicrnz)
Breaking strain (%)
1
200
0.05
0.05
180
0.05
3.5
0.5
25
1.5
0.3
20
0.10
1
330
0.20
0.08
2
4
15
The bulk density of these felts ranges from 0.05 to
0.20 g/cm3. The felts with the higher bulk densities,
while having practically the same thermal insulating
powers, are more stiff, and can therefore be used e.g.
as self-supporting insulating shields. The insulating
power of these carbon felts is illustrated in Figure 8.
Carbon and graphite felts are thus used mainly for thermal
insulation up to very high temperatures both in resistance
and in induction furnaces. They are cheaper and more effective than metal shields. Their insulating properties are
superior to those of carbon black. Since the bulk density is
1441 C. L. Gutzeii, D A S 1272801 (1965), Hitco.
1451 R . 0. Moyer j r . , D . R . Ecker, and W. J . Spry j r . , US-Pat.
3333926 (1963), Union Carbide Corp.
422
280
0.17
0.15
0.2
I .5
5
I
SIGRATHERM FD 400
carbon
graphite
I
I
350
0.08
0.05
320
0.08
0.30
0.5
0.1
1
0.5
5
10
and graphite fabrics are used basically for the same purposes
as the felts, but not as insulating materials.
All carbon fiber materials have the disadvantage of
restricted application in air above 30O0C, owing to
their ease of oxidation. Their high-temperature properties are fully utilizable up to temperatures above
3000°C only in inert or reducing atmospheres. The
resistance to corrosion and erosion can however be
increased by pyrolytic graphite, carbide, or silicide
coatings.
1461 P . Fuber, D A S 1299056 (1966), Rheinisch-Westfalische
Elektrizitatswerke.
[47] K . Schoen, D A S 1242766 (1965), AVR GmbH.
Angew. Chem. internat. Edit.
Vol. 9 (1970)
1 NO. 6
7.2. Carbon Fibers
Carbon felt, wadding, wool, efc. generally d o not
possess high mechanical strength; the graphite single
crystal, however, owing to its strong covalent bonds
in the a,b direction, has an extremely high Young’s
modulus (about 1 x l o 7 kg/cni2) and a tensile strength
of about 1 x 106 kg/cm2.
In the production of continuous carbon fibers, the a,b
planes can be extensively oriented parallel to the fiber
axis if e.g. carbonized rayon filaments are stretched at
about 250OoC[4*l. Stretching by a factor of 50%
increases the Young’s modulus e.g. from 1 x 106 to
4 x 1 0 6 kg/cm2. The tensile strength simultaneously
increases from 100 t o 300 kg/mm2.
With polyacrylonitrile fibers as the starting materials,
stretching is carried out between 200 and 300 “C ‘321.
Carbon fibers from polyacrylonitrile attain their maximum strength after heat treatment at 1500°C (cf.
Table 7: high-tensile). As the treatment temperature is
further raised, the Young’s modulus rapidly increases;
the high-modulus fibers of Table 7 were heated at
2500 “C. Additional high-temperature stretching above
2500°C results in a further increase in the Young’s
modulus and tensile strength.
under its own weight. The high-tensile carbon fibers
have a tension length of 170 km. As can be seen in
Table 7, metal fibers have tension lengths of only a
fraction of this value.
The decisive advantage of carbon fibers, however, lies
in their extremely high stiffness, which is expressed by
Young’s modulus. Young’s modulus of highly
stretched carbon fibers is inferior only to that of
graphite whiskers. The specific Young’s modulus of
high-modulus carbon fiber is eight times that of a
metal wire.
The carbon fibers are produced in ropes containing
2000 to 1000 individual filaments. In the individual
filaments, which have a diameter of about 8 pm, the
two-dimensionally ordered graphite layer planes are
aligned parallel to the fiber axis. As in textile fibers,
large crystalline zones are combined to form supermolecular structures; under the electron microscope
they exhibit a fibrillar structure in the form of stripshaped zones arranged substantially parallel to the
fiber axis.
Carbon fibers are used mainly for the production of
high-tensile and high-modulus composite materials,
which are discussed in Section 7.4.
7.3. Carbon Whiskers
Table 7. Mechanical properties o f carbon fibers, carbon fiber reinforced plastics, and other materials
Tensile
strength
(kglcm2:
Young’s
modulus
( 1 0 3 kg/cm*)
Tension
length
(km)
Specific
Young’s
modulus
(km) 1 4
21 000
4 200
2 300
3 800
1900
700
2 100
1 200
700
110
I70
115
240
110
53
44
6
22000
13 000
14500
7 600
2 800
2650
2600
2600
Fiber5 and wires
Graphite (high modulus)
Graphite (high tensile)
Boron (on tungsten)
Asbestos
Glass
Steel
Titanium
Aluminum
30000
30000
60000
28000
42000
20000
1700
Young’s moduli and tensile strengths even higher than
those of the carbon fibers described in the last section
can be attained in carbon whiskers. These are hairlike crystals of carbon, a few centimeters long, whose
diameter (about 0.5-5 p n ) is appreciably smaller
than that of carbon fibers. Since the strength of fibers
in general increases rapidly with decreasing diameter,
these carbon whiskers have the highest strength
reached so far. i.e. a tensile strength of up to 2000 kg/
mm2 and Young’s modulus of up t o 7 x 106kg/cmz.
Graphite whiskers were first obtained in the mid1950’s by Bacon and Bowman [491. They were prepared
from graphite vapor by a high-energy electrical discharge. The methods used nowadays, however, make
use of gaseous carbon compounds. Under very strictly
limited deposition conditions, the pyrocarbon (cf.
Section 3) formed on pyrolysis can be made to grow
in the form of whiskers. This leads first to very thin
primary whiskers having a diameter of about 0.01
pm [501, which then thicken as a result of secondary
deposition of pyroIytic carbon. The main difficulties
in the production of whiskers lie in the suppression of
this secondary growth. Several methods have been
used for this purpose, such as the addition of catalysts
to the pyrolyzing gas[51,521. The deposition of the
I
Other matcrials
Graphite fiber/
resin composite
( V f = 0.5) [bl
Glass fiber/
resin composite
(Vf = 0.5) [b]
Steel
8 400
2 100
I
13600
54
9000
500
50
2 800
14 000
2100
18
2 650
[a] Specific Young’s modulus 2 ‘oung’s modulus/density.
[bl Vf = bulk factor (based on the volume fraction)
The tensile strength of high-tensile carbon fibers is
exceeded only by those of asbestos fibers, steel wires,
and of graphite whiskers.
The great advantage of carbon fibers over metals
becomes all the more impressive when one considers
the specific strengths and specific Young’s moduli, i.e.
those based on unit weight. The specific tensile
strength corresponds to the tension length, i.e. the
minimum length of a fiber or wire for which it breaks
[48] G. E . Cranch and J . S . Shinko, DOS 1469492 (1963),
Union Carbide Corp.
Angew. Chem. internat. Edit.
VoI. 9 (1970)J No. 6
[491 R . Bacon and J. C. Bowman, Bull. Amer. physic. SOC. Ser.
11, 2, 131 (1957).
[50] See ref. 151, p. 832.
[51] H . F. Karrffman, D . J . Griffiths, and J . S . Mackay, USPat. 2796331 (1954), Pittsburgh Coke and Chemical Comp.
[521 R . J . Diefendorf, US-Pat. 3107180(1961),General Electric
Comp.
423
whiskers in steep temperature and concentration gradients has proved very favorable 1531.
All these methods are being used only on a laboratory
scale at present. Owing to their outstanding strength
properties, carbon whiskers will undoubtedly be used
in future for materials subject to extreme loads, though
they will always be much more expensive than carbon
fibers.
7.4. Carbon Fiber Reinforced MateriaIs
All the common metals have roughly equal stiffness
for equal weights; they have roughly equal specific
Young’s moduli. This is an inherent property of metals,
and cannot be fundamentally altered by alloying or
the like.
t h e filament winding method. The matrix materials are
generally epoxy resins, though polyester and phenolic
resins are also used. Figure 9 shows an electron-scan
micrograph of the point of fracture of a carbon fiber!
synthetic resin composite. The good adhesion ofthe syntheticresin matrix to thesurface of the fibers is obvious.
As can be seen from Table 7, a composite of this type
with a bulk factor of 0.5, i.e. consisting of 50 vo1.-%
of fibers and 50 v01.- % of synthetic resin, is as stiff as
a steel body having the same dimensions; the specific
Young’s modulus, on the other hand, is five times that
of steel. This means that a component made of carbon
fiber reinforced plastic suffers only one fifth of the
deformation under load exhibited by a comparable
steel component.
Because of the low defect concentration, wires, fibers,
and above all whiskers always have higher tensile
strength and Young’s modulus than compact materials. In the development of new materials with extremely
high specific stiffness, therefore, one starts with thin
fibers or wires, which are combined with matrix substances to produce composite materials. The strength
of these composites is roughly proportional to the
volume fraction of the fibers that are arranged parallel
to the direction of the stress.
Graphite whiskers are ten times as strong as carbon
fibers. However, it is very difficult t o embed them in a
matrix material. Moreover, they do not differ SO much
from carbon fibers in their Young’s modulus.
On the other hand, continuous carbon fibers, like
glass fibers, can be easily embedded in a synthetic resin
matrix in volume fractions of more than SO%, e.g. by
Fig. 10. Rigidity of materials, from left to right: aluminum, carbon
fiber reinforced plastic, steel, glass fiber reinforced plastic.
The high rigidity of the carbon fiber reinforced plastics
is illustrated by Figure 10. Strips of aluminum, glass
fiber reinforced plastic, steel, and carbon fiber reinforced plastic having the same dimensions were loaded
with equal weights. Bending is clearly least for the
carbon fiber reinforced plastic and the steel. However,
the steel strip is five times as heavy as the carbon fiber
product.
Carbon fiber systems are unaffected by moisture. Their
good electrical and thermal conductivity is also particularly advantageous.
These systems will be used in particular where high strength
and high rigidity are required in conjunction with low weight,
i.e. in rapidly rotating components and in aircraft and automobile construction. Examples are turbine blades in the
Rolls-Royce RB 211 engine, heavy-duty jet housings, edges
of aircraft wings, and floors of aircraft (in combination with
honeycomb structures). There are also possible applications
outside of aviation and space technology, e.g. for large centrifuges, spinning and weaving machines, sports cars, mountaineering equipment, and sailboat masts.
Fig. 9. Electron-scan micrograph of the fracture point of a carbon
fiber-synthetic resin composite (magnification 1250 x). The micrograph
was prepared in the Labor fur Raster-Elektronen-MikroskopieDr. H .
Klingele, Munchen.
I531 E. Firzer and H . G. Schi’esinger, DAS Y 269933 (1966),
SIGRI.
424
The application temperature of carbon fiber reinforced plastics is limited by the thermal stability of the
plastic. Important advantages will result from the use
of synthetic resins with high thermal stabilities, such
as polyimides and polyimidazoles. The highest temAngew. Chem. internat. Edit.
/ Vol. 9 (1970) 1 No. 6
peratures at which the composites may be used will be
further raised by the use of glasses or metals as matrix
materials 154,551. Carbon fiber reinforced carbons are
stable up to 3000 “C in inert o r reducing atmospheres.
[54] D . H . Bowen, R . A . J . Sambell, K . A . D. Lambe, and N. J.
Mattingley, DOS 1925009 (1968), U.K.A.E.A.
I551 A . A . Baker and R. J. Bache, Brit. Pat. 1177301 (1968)
Rolls Royce.
However, their production is still in the experimental
stage.
The further development of these new fiber-reinforced
high-temperature materials will open up wide fields of
application for carbon fibers.
Received: March 1 1 , 1970
[A 754 IE]
German version: Angew. Chem. 82, 400 (1970)
Translated by Express Translation in Service, London
Electroplating of Plastics in Theory and Practice
By Kurt Heymann, Wolfgang Riedel, and Gunter Woldt [*I
Electroplated plastics combine many advantages of plastics and of metals; they have the
low weight and ease of shaping ofplastics, together with the luster, hardness, and electrical
conductivity of metals. An important part of any process for the electroplating of plastics
is a pretreatment to ensure good adhesion of the metal film. Etch activation in the straightthrough method requires only six steps for pretreatment.
1. Introduction
The rapid growth of the consumption of plastics in
the last 20 years is primarily due to the ease with which
they can be shaped to obtain useful objects. This was
greatly appreciated by industrial users, who required
versatility of types and shapes with no great increase
in production costs, such as arises e.g. in the processing
of metals.
For this reason the users accepted disadvantages in
color matching, the attraction of dust by static electricity, and the relatively soft surface of injection molded
thermoplastics.
These “made-to-measure materials” offer considerable scope for the work of the chemist. The polymers
can be suited to a wide range of uses by modification
of the nature of the monomers, the polymerization
conditions, and pigmentation, and because of the
possibility of reinforcement with glass fibers. Nevertheless, plastics have to compete with the traditional
metal.
touch. Electroplated plastic feels warm and is much
lighter. Because of its lightness, it is easier to transport
and handle, and this is a further argument in favor of
metalized plastics.
Plastics can be shaped more easily, more cheaply, and
in greater variety than metals. To obtain shiny surfaces, the mold into which the plastic is injected is
polished instead of each individual article. Without
knowing the market, one might think that the electroplating of plastics is a passing fashion. However, this
practice has many technical advantages, apart from
its decorative value.
Many plastics are damaged by ultraviolet radiation.
This damage is prevented by a film of metal. The
flammability of plastics is also reduced by metalization.
Use is often made in the electrical industry of chassis
having complicated shapes. To produce a metal
chassis of this kind, several individual parts have to
be fitted together and welded. It has been found in
It seemed natural, therefore, to seek a good combination of plastic and metal, i.e. a genuine composite
of plastic and metal.
An important factor here is undeniably the wish to
give a plastic surface a metallic luster for decorative
reasons, since large sections of the market like the
glitter of metals. It is in fact impossible to tell by appearance whether a chromium-plated object is made
of metal or plastic; the difference can be felt only by
[ * ] Dr. K . Heymann, Dip1.-Phys. W. Riedel, and Dr. G. Woldt
Schering AG, Galvanotechnik
1 Berlin 65, Postfach 59 (Germany)
Angew. Chem. internal. Edit.
1 Vof. 9 (1970) 1 No. 6
Fig. 1.
Chassis of ABS plastic (Bosch Elektronik GmbH. Berlin).
425
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