Metallized synthetic cable

Volume 2,number SB

August 1984

MATERIALSLETTERS

METALLIZEDSYNTHETICCABLE* T.E. JONES and T.R. OGDEN Naval Ocean Systems Center, Code 4343, San Diego, CA 92152,

USA

Received7 June 1984

Lightweight synthetic conducting materials such as conducting polymers and graphite intercalation compounds have been under development for several years with the promise of a new class of materials combining the desireable properties of polymers with the useful electronic properties of conventional However, these new materials generally lack one or more of the attributes metals and semiconductors. In this paper, a hybrid needed for practical use and they are very often chemically unstable as well. cable is described consisting of a bundle of synthetic fibers coated with a thin metallic layer. This results in a metallized synthetic cable, where the synthetic fibers act as the strength member Such a lightweight structure can easily be made and only a small amount of metal is actually used. neutrally buoyant by choosing the appropriate dielectric insulation.

1.

INTRODUCTION

The continuing evolution of all aspects of undersea technologies has produced many innovations which help to solve the formidable problems posed by this inhospitable environment. While some noteworthy advances have been made in the optoelectronic field, the more or less conventional electronic sensors and their associated electrical cables still perform most of the undersea data acquisition Ideally, sQch an and transmissron tasks. electrical link should transmit the desired information 'with little or no attenuation, be liqht in weiqht and neutrally buoyant as well as- very st;ong, flexible -and jnexpensive. However, the weight of suspended electrical cables can be immense. Current cable systems based on copper conductors meet many of these criteria and for this reason copper conductors are usual1 the material of choice 'with current techno T ogy. However, copper is less than an ideal material when its weight is (specific gravity is 8.7). considered Furthermore, the general deterioration of copper in an ocean environment when elaborate safeguards aren't provided, and the increasing cost of copper for some expendible systems, make the selection of copper conducocean tors less desireable in many applications. In recent years, neutrally buoyant undersea cables have been developed and they are widely used. Floats are attached at predetermined intervals along the length of the (*) This work was funded by NADC as part the RESOC effort of the UTS block program.

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cable to make it neutrally buoyant and care must be taken when the cable is being deployed or retrieved to guard against damagObviously this arrangement ing the floats. is bulky and requires special handling considerations. there is a need for a Thus, neutrallv buovant undersea electronic data which has sufficient comnuni&ionlink strength and possesses suitable bandwidth and frequency response characteristics while being of compact size. 2.

MATERIALS

Several synthetic conducting materials have been under development for some time. They possess very interesting electronic properties such as high electrical conductivity. These synthetic materials with electrical, thermal, and optical properties similar to elemental metals and alloys, are referred to non-metallic conductors or synthetic i=%als (1,2). Most of these materials are light in weight compared with conventional metals. Although current materials are less conducting than copper, they can have tensile strengths greater than copper while weighing considerably less. New materials are continually being developed and improvements in morphology should in time allow these synthetic materials to provide an alternate material for undersea data links in marine systems when strength and weight are paramount concerns. This type of application imposes several constraints on any candidate material. One such constraint is that the material must be available in stranded or at least filamentary form in a manner simiar to conventional 0 167-557~/84/$03.00OElsevier

(North-Holland

Science Publishers B.V. Physics Publishing Division)

Volume 2. number 5B

MATERIALS

copper wire. The electrical conductivity must be adequate to transmit the desired signal with a minimum of attenuation. The material should be light in weight to achieve neutral buoyancy with little or no added material. In addition, the tensile strength and the general mechanical integrity must be adequate to achieve reliable operation. Finally, the cost should not be prohibitive. Such candidate synthetic conducting materials include graphite fibers, intercalated graphite fibers, conducting polymers, intercalated graphite swaged in metal sheaths. and metallized svnthetic fibers. Pure graphite fibers, althou h rather conducting in comparison with insu 4ators, have a conductivity two orders of magnitude less than copper and calculations have shown that losses would be too great even in lengths of only a few hundred meters. Very high conductivities have been reported in the literature for graphite intercalation compounds (3-6). It should be remembered. however. that these results were achieved on-small, high quality crvstals and not on lona fiber tows where the results would be less dramatic. In addition, most of these graphite intercalation compounds are very reactive compounds and they usually degrade on exposure to air and moisture. Conducting polymer materials have also shown great promise, but the conductivities are not high enough for a cable and the morphology of current conducting polymers is not developed to the extent that long, high tensile strength fibers can be produced. Intercalated graphite swaged in a metal sheath has also been reported to yield high conductivities, but this approach has not advanced to the point where a practical small diameter cable could be constructed (7). Finally, we come to metallized synthetic fibers which is the subject of this Letter and which we would like to discuss in the remaining space.

LETTERS

August 1984

illustrates the type of metallization being discussed. This figure is from an MCI report to the U.S. Navy (8). Note the rather complete metallization on each individual fiber in the lower riaht microaraoh. The ohotos were taken in-cross seciion of a metallized tow of commercial Union Carbide VSB32 araphite fibers. The light colored concentric rings are the metal coatings on the individual graphite fibers which appear as dark cores in the photos. Some properties of these metallized graphite fibers are shown in table 1. Note, the added weight is given in mg/m so that for a one kilometer cable. the added weiqht would only be about 800 grams. The data- in the table is from MCI, Inc. (8). The VSB32 fibers have a density of approximately 2.0 2000 strands/tow and 10 micron diamegm/cc, ter fibers. The Union Carbide Thornel-300 fibers have a density of approximately 1.8 gmlcc, 3000 strands/tow and 7 micron diameter fibers. The tensile strengths of the metallized fibers are slightly less than those of the pristine fibers. Such metallization of synthetic fibers has also been reported elsewhere (9). The type of composite multifibrillar construction described above provides a means of using only a minimum amount of metal. The thin metal coating provides the electrical conduction and the synthetic fiber core provides the strength and structural integrity. Since the synthetic core does not influence the electrical conductivity to any extent, it does not have to be even a poor conductor like graphite. In fact, a material of choice at this time is actually Kevlar which is an insulator. Many cable designs currently utilize Kevlar fibers as the strength members. In the configuration described here they may also provide the conducting medium

Metallization as used in this context refers to the process of coating the individual fibers in a tow of graphite, for example, with a thin layer of metal such as copper, aluminum, nickel, etc. This type of composite conductor then relies on the large tensile strength of the synthetic core as a strength member and the metal coating provides the electrical conduction. The resulting density will be intermediate between the two. This technique is a means of utilizing a very small amount of metal since the metallization is typically 1.0 or 2.0 microns thick. The technique for performing this metallization using electrodeposition on graphite fibers has been developed by MCI, Celanese, and American Cyanamid, at least. MCI has also succeeded in metallizing Kevlar fibers using an initial electroless deposition technique. Figure 1 463

MATERIALSLETTERS

Volume 2,numberSB

August1984

COMPLETE COVERAGE USING OPTIMUM PLATING CONDITIONS

Figure 1. Graphite fibers individuallymetallized with copper. via the metallization process. Furthermore, the technique of weaving long thin strands of Kevlar is rather well developed, whereas graphite is not normally used in this manner and can be difficult to handle. 3.

UNDERSEA

CABLE APPLICATIONS

It should be clear from the previous 464

sec-

tions that the use of synthetic conductors or composite conductors for undersea cables will In order to involve design compromises. evaluate the trade-offs involved, a computer model was developed to predict cable performance-for materials covering a broad range in The difficulty in predicting conductivities. cable performance with lossy conductors is that most handbook methods assume the conductors are highly conducting as appropriate for

Approximations are often made in the copper. formal development of the equations and the validity of directly applying these handbook equations to the case of a graphite cable, for example, should be investigated carefulAlthough it is anticipated that a ly. conductor candidate synthetic successful material would have to be highly conducting, many of the prototype materials currently available have conductivities in the range of 10 to 1000 times less than copper. Althou h such lossy materials would not be like 9y final choices, it was desired to have a working computer model which would be valid in this range of conductivities. This extended range model would then be valuable in the development by predicting actual materials' performance as the materials evolve and improve. It also became apparent that fibrillar multistranded conductors and metallized strands, such as metallized graphite or Kevlar fibers, mi ht play a dominant role in a synthetic cab 3 e. The model was developed to include these cases.

greater than copper and silver to conductivities a thousand times lower and including the metallized fibrillar fibers discussed above. The predictions of the model for standard cables were checked for accuracy. Figure 2 shows how this computer model is used to predict the performance of a particular synthetic cable design. In the figure, the loss in dB is plotted vs. frequency for two cases, both computed for a coaxial line of length 305 meters. The loss in dB is defined as lOLOG(Pi/Po), where Pi is the power input to the cable and PO is the output power. In both cases, the inner conductor consists of 2000 Kevlar fibers each with a mean radius of 5.97 microns. The thickness of the metal layer was 1 micron in both cases. The outer radius of the inert material surrounding the inner conductor was 0.539 nnn. Curve 1 in the figure has a solid copper return with an outer radius of 0.600 mm. For curve 2, a seawater return path is used instead of an outer copper conductor. The figure illustrates the advantage of using a seawater return. In addition to saving weight and volume, the losses are actually less up to almost one MHz. Figure 3 provides a comparison of the losses expected for various metallization thicknesses and contrasts those losses with those expected for a solid copper core conductor. Curve 4 shows that the lowest losses are for the all copper case, of course. Then the results expected for three different thicknesses of metallization are

The computer model was developed by Gordon Martin under contract to NOSC, from first principles, starting with Maxwell's equations of electromaanetism (10). He developed the mathematics of the model using standard references and implemented the results as a Fortran program on the NOSC computer faciliSpecial care was taken to insure the ty. model's validity over a broad range of electrical conductivities from conductivities

2.

[B

AugustI984

MATERIALSLETTERS

Volume 2,number5B

ATTENUATION

_=1000

fl

and load se+ to aptimize losses 01 011 requencies. hrce

: copper return

/I4 I ’

1

2

I

I11111 5

HZ

I 104

I

2

I

I111111

5

I I05

I 2

I’IIIII~ 5

I

P

FREQUENCY

Figure 2. Attenuation vs. frequency for metallized Kevlar coaxial cables. 465

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MATERIALS

LETTERS

August

1984

DB ATTENUATION *O 7

. = loooft ;ource and load 11 each frequency I= thichnCSS coating.

)

Of

optimaed

Copper

5

2

HZ

Figure 3.

Attenuation

vs. frequency

for met allized Kevlar coaxial

shown. Figure 3 illustrates the results computed with-a seawater return and both figures 2 and 3 result from optimizing source and load impedances for minimum attenuation at each frequency. Figure 3 demonstrates that the small additional losses incurred by using the lightweight synthetic cable should be acceptable for most undersea applications requiring relatively short lengths and moderate frequencies. 4.

CONCLUSION

Recent innovations in the metallization of high strength synthetic fibers such as graphite and even Kevlar have yielded potential composite synthetic conductors which combine many of the advantages of the synthetic fibers with I those of the highly conducting This process, whereby the metallic coatings. graphite or Kevlar fibers are coated with a thin layer of a conventional metal, is being rr;sFd by_ at least three companies in the . . . Individual graphite or Kevlar fibers, in a tow of hundreds or thousands of fibers, can be coated with a thin layer of a metal on the order of one micron thick. In the case of graphite, this is accomplished by electrochemical deposition. To metallize insulating Kevlar fibers, an electroless chemical deposition final must the precede electrochemically deposited coating. Calculations show that the most beneficial way to use such composite conductors, for 466

FREOUENCY

cables with a seawater

short undersea cables utilizing

return.

systems, is in multistranded a seawater return.

The losses achievable with such metallized synthetic cables, in diameters of a few millimeters and 300 meter lengths, appear to be acceptable in a seawater return undersea sysIn addition to acceptable losses, a tem. real weight saving should be achieved and-the metallized fibers may also serve as the strength member of the cableK:bu;rprecluding the need for additional support layers. REFERENCES Petroleum Derived 1. John A. Woolam, in: Oeviney and ii;. Carbons, eds. O'Grady, ACS Sympos~;~'Series 21, p. ,

1976. Kenneth J. Wynne and G. Naval Research Reviews, :;:I. 2.

l?ndo/*:*327,

Ubbelohde Proc. 289 (19j2). Fischer

~hys??T~day,

5.

ed.

F.

Bryan Street, p. 38, Spring

Roy.

and Thomas E.

July 1978.

sot.

Thompson,

Lincoln Vogel, in: Molecular Metals, William F. Hatfield, Plenum 1979.

MATERIALS

Volume 2. number SB

6. F. Lincoln Vogel and Claude Zeller, in: Molecular Metals. ed. William E. Hatfield. Plenum Publishing-Co. 1979. Lincoln Vogel, :;9 !i979/*0).

Synthetic

Metals

1,

Technical final report on contract no. NOOOZ4-80-~-5389, prepared by MCI Corp. for the Naval Sea Systems Command, May 20, 1981.

8.

Schmitt, 9. C.P. Beetz, Jr., and R.J. American Cyanamid Co., SAMPE Journal May/June 1983. For completeness, the three companies

LETTERS

August 1984

known to metallized

the authors at this time, who have araohite fibers are the followina: 666 North Hague Avenue, 'Ink 2. Celanese Plastics &lumbzi! Ohio ij204: and Specialties Company, 26 Main Street, 3. American Cyanamid Chatham, N.J. 07928; Chemical Research Division, StamCompany, ford, Connecticut. 10.

G.E. Martin, NOSC Contractor's report 167 October 1982; see also, T.E. %ies and'T.R. Ogden, NOSC Technical Note 1311, September 1983.