METHOD AND APPARATUS FOR THE ON-LINE COATING OF SILICA BASED FIBERS WITH BORON-NITRIDE

Abstract: The on-line drawn fibers are passed through a quartz tube pro­ vided with heating means and exposed to borazine diluted in an inert gas at a rate such that the borazine is pyrolyzed to boron-nitride which deposits as a protective shield on the fiber. The mechanical performances of the coated fibers at high temperature are strongly enhanced.

Description: Field of the invention The present invention relates to the coating of silica optical fibers with protective minerals to improve their physical properties, more especially their resistance to corrosion and fatigue with time under use stress at elevated temperature. The prior art It has been known for long that glass and silica fibers (cal­ led "glass fibers" hereafter for short) need resistance to strong mechanical stress under practical uses, i.e. when manipulated for making fabrics or optical communication lines. Furthermore, the ini­ tial properties of the fibers are often detrimentally affected by external agents such as polluted air, humidity and corrosive atmos­ pheres. These drawbacks mainly result from tiny cracks and crazes in the fiber surface due to mechanical handling during drawing or further manipulations; in the long run, these surface defects ampli­ fy and initiate fissure formation and ultimate rupture or other kind of failure. It has therefore been long sought to remedy the situation by coating the fibers with a protective mineral coating impervious to external agents and providing mechanical reinforcement to the fiber and protection against high temperatures. Many coatings have been tried including silicon oxynitride, silicon nitride, carbon, metals such as Ni and Al, titania and various other metal oxides such as Al₂O₃, ZrO₂, SnO₂, etc.., see for instance "Third International Con­ ference on Integrated Optics and optical Fiber Communication (1981) San-Francisco, Cal.": C.R. KURJIAN, Recent Progress in the strength of Fibers, p. 22; S.M. OH et al, Increased Durability of Optical Fi­ bers through the Use of Compressed Cladding, p. 24; E.G. HANSON et al, Reduction of Static Fatigue in Optical Fibers by Use of a Sili­ con Oxynitride Coating, p. 24. Other references are: P. EHRBURGER et al, Ceramic Bulletin 59 , (1980), 844-847; FR-A-2.320.915 (QUARTZ & SILICE). Recently, the use of BN as a protective cladding on fibers has been proposed in US-A-4,118,211. Several methods for coating boron nitride on glass fibers have been proposed, some of which are listed below: FEURER et al in "Coatings of boron nitride on silica by ther­ mal decomposition of hexachloroborazine" in Proc. Eur. Conf. Chem. Vap. Deposition, 3d 1980 , 49-55 (C.A. 95 , 174092) disclose that hexa­ gonal BN was deposited by VPC at 900°C from B₃N₃Cl₆ to provide 0.5-1 µm films with good adhesion to SiO₂. SANO et al: "Chemical vapor deposition of thin films of boron nitride onto fused silica and sapphire" in Thin Solid Films 1981 , 83(2), 247-51 (C.A. 95: 213137) disclose that BN films were grown on silica at a rate of about 0.5 µm/h using the reaction BCl₃ + NH₃ at 600 - 1100°C. Such rate would be insufficient to provide a pro­ tective coating on optical fibers by an on-line deposition method. SAVELEV et al: "Properties of boron nitride films produced by decomposition of borazine in a high-frequency plasma" in Fiz. Khim. Obrab. Mater 1891, (2), 85-8, report the conversion of borazine to BN in a high-frequency electric discharge with a deposition rate of 3-4 µm/hr. Obviously the techniques disclosed in the prior art are much too slow for the on-line cladding of silica fibers in the drawing of preforms which can easily reach velocities in the range of 10 m/min or more. Indeed, if it is assumed that an on-line BN-coating reac­ tor can have approximately 0.1-0.3 m in length, the holding time at 10 m/min will be about 0.1 to 2 sec. If it is assumed that the thick­ ness of the BN coating to be efficiently protective should be at least 0.01 µm, the deposition rate will have to be in the range of 0.2-0.6 µm/min. Description of the invention The method of the invention, as defined in claim 1, enables to achieve such performances. This method can be implemented practical­ ly by means of the apparatus defined in claim 2. Description of the preferred embodiments The inert gas used as a blanket for drawing the fiber preform and as a dilutant to the borazine can be a noble gas such as helium, neon, krypton or xenon; argon is preferred for reasons of easy avail­ ability. The temperature at which the preform is heated to effect draw­ ing is not critical provided it is sufficient to enable the drawing of about 50 to 200 µm thick fibers at a rate of about 2-20 m/min. The preferred drawing temperatures are in the range of 1900 to 2200°C. The proportion of borazine in the gas mixture that circulates in the quartz coating tube and the flowing rate of said mixture is dependent on the desired coating performances. In other words, the higher the depositing rate sought after, the higher the borazine con­ centration in the gas or the faster the circulation of the gas mix­ ture. However, care must be taken to match the rate of arrival of the borazine reagent with the rate of thermal decomposition of said reagent in contact with the moving fiber in the coating enclosure. Appropriate operative parameters are in the case of fiber drawing speeds of 2-20 m/min: CVD temperature, 900-1150°C; total gas mixture flow, 0.5-5 l/min; borazine concentration in the argon carrier, 0.1-1.5% v/v; optimal borazine concentrations are in the 0.2-0.6% range. In this cases the coating thickness may range from a few nm to a few hundreds nm. Brief description of the drawing The annexed drawing schematically illustrates an installation for coating optical fibers with a boron nitride protective layer. The apparatus The illustrated apparatus comprises a furnace enclosure 1 for the melting and drawing of an optical silica preform 2 under the heat­ ing of a heating element 3 connected to an energy source not repre­ sented. The furnace 1 is connected by an inlet 1ª to an argon sup­ ply 4 via a flow-meter 5. An outlet 1 b of the furnace 1 relieves a possible over pressure due to an excess of blanketing argon. The enclosure 1 is provided at the bottom thereof with a leader tube 6, made of quartz, which enables to direct a fiber 7 drawn from preform 2 to penetrate into another tube 8 coaxial with tube 6. Tube 6 is also provided with gas inlet and outlet openings, respective­ ly 6a and 6b, for connecting to the argon source 4 via a flowmeter 9. The tube 8 is wrapped in a mantle 10 and opens up into a gra­ phite susceptor 11 surrounded by a HF heating coil 12. The spacing between the tube 8 and the mantle 10 is preferably flushed with argon (through openings therein not represented) to prevent contamination by air. Situated substantially upstream of said susceptor, the tube 8 is provided with input openings 8a and 8b and downstream thereof with an outlet 8c. Input 8a is supplied with pure argon from a source 13, via a flowmeter 14 whereas input 8b is supplied with a borazine vapor/Ar mixture generated in a bubbling bottle 15 held at a cons­ tant controlled temperature under the effect of an argon stream cir­ culating via a flowmeter 16. The borazine/argon mixture penetrates into tube 8 well ahead of the position of susceptor 11 relative to the fiber 7 displacement. Therefore the gas mixture is given ample time to homogeneize around said fiber and a homogeneous BN coating will result all around the fiber surface. The decomposition of the B₃N₃H₆ occurs in the graphite susceptor under the heat provided by induction from the HF coil 12. The exhausted gases then leave the tube 8 by the output 8c thereof. In a modification of the above-mentioned equipment, a mixing bottle can be inserted in the path of the borazine/Ar mixture before penetrating into inlet 8b to further improve the mixing of the gas­ es. After exiting from tube 8 by its lower end, the coated fiber 7a is wound on a take-up rotating cylinder 17. The operation of the present installation is self-understandable from the aforementioned description and needs only briefly reviewed. After heating in the oven 1, the glass preform 2 is drawn into a fiber 7 which, after leaving tube 6, enters tube 8 and passes through susceptor 11 heated to the right temperature to effect the pyrolysis of the borazine component of the gas mixture supplied by the bottle 15. Thus, the fiber 7 gets coated with a film of boron nitride and is converted into coated fiber 7a which, after cooling, is finally collected on spool 17. Reduction to practice The following examples illustrate the invention Examples 1 to 12 An apparatus such as that disclosed with reference to the an­ nexed drawing was used. A series of experiments were carried out using silica based optical preforms of diameter 6-10 mm, length about 500 mm and an OH content less than 100 ppb. After drawing, the resulting fibers had a diameter of about 120-170 µm. The operating parameters for examples 1 to 12 are listed in Table I below. The effect of the coating thus achieved was tested in the case of Examples 6 - 12 in connection with the minimum rupture bending ra­ dius. Results are shown below: It appears from the above data that optimal results are obtain­ ed with 0.1-0.6% B₃N₃H₆.

Claims: 1. A method for the on-line coating of silica fibers with a pro­ tective shielding film of boron nitride, characterized in the steps of: (i) drawing a fiber from a silica preform at a temperature of 1900 to 2200°C under a blanket sheath of an inert gas; (ii) on-line passing said hot drawn fiber into a vertically oriented quartz tube provided with means for heating the moving fiber to a temperature of 900-1150°C; (iii) circulating into said quartz tube a gaseous mixture of borazine and an inert gas at a concentration and a rate sufficient to provide by CVD on the moving fiber a boron nitride coating result­ ing from the pyrolysis of said borazine, said coating being at least 0.01 µm thick; (iv) on-line withdrawing the coated fiber from said quartz tube and allowing it to cool for winding on a spool. 2. An apparatus for carrying out the method of claim 1, this apparatus comprising: a) means for heating a glass preform to the melt drawing tempe­ rature under a blanket of inert gas, b) means for drawing a fiber from said heated preform and spool­ ing it on rotating cylinder, c) coating means interposed between the drawing and the spool­ ing means for coating the fiber with a protective film of boron of ni­ tride, said coating means consisting of a vertical quartz tube axial­ ly passed through by said fiber and containing a HF heated graphite susceptor flushed by a gaseous flow of borazine diluted in an inert gas. 3. The method of claim 1, characterized in that the speed of drawing of the fiber is from 2 to 20 m/min and the thickness of the boron-nitride coating is from 10 nm to 1 µm. 4. The method of claim 1, chacacterized by the fact that it pro­ vides to the fibers a protection against loss of strength even after heating for more than 100 hours at 900°C.

Inventors: OBERSON, BERNARD, SCHACHNER, HERBERT, TIPPMANN, HEINZ
Applicants: BATTELLE MEMORIAL INSTITUTE
Application year: 1985
Main classification: C03C25
IPC codes: C03C25, C23C16, G02B6