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2. APPLICATIONS OF FUSED-SILICA FIBERS
Although the first fused-silica fibers were made by Gaudin in 1839  and some tubes and spirals were made and exhibited by Gautier in 1869 , these articles were more curiosities than useful tools. It was not until about 1887 when Boys  drew fibers and used them in his gravimetric work  that their usefulness was exploited. Today silica fibers in the form of paper, matting, or wool are used in sizable quantities in heating and insulating devices [77, 80, 313]. These devices are well suited for high temperature work because of the highsoftening range, low-heat conductivity, and resistance to thermal shock exhibited by the fibers. However it is in the usage of single fibers that the unique properties such as high strength, elasticity, thermal, electrical, and chemical resistance are most fully utilized. It is with these individual fibers and their adaptability to precision measuring instruments that this review is chiefly concerned.
Individual fibers, with or without metallic coatings, have been used as suspensions, as sensing elements and as unit assemblies in many precision instruments. Silica fibers have been used as galvanometer suspensions [1, 65], in gravimetric balances [65, 66], in electroscopes and electrometers [9, 70, 74], in low pressure manometers , in radiameters , in ionization chambers , and in magnetometers . They may possibly be of some use as electrodes in electron tubes. One of the more extensive uses of silica fibers is in the microbalance field. The entire weighing system can be made of fibers, thus avoiding any errors due to differing densities and coefficients of expansion of the members. Silica fibers are particularly desirable for these instruments because of their high strength in tension and in torsion, nearly perfect elasticity, negligible hysteresis. Further, the resistance to chemical attack, low rate of sorption, low permeability, and ease of cleaning allow such balances to be used under many difficult weighing conditions. Only brief mention is
made here of the major developments in fiber microbalances. Reviews of silica fiber microbalance developments [12 to 18, 50] and papers on specific balances should be consulted for complete information about theories and principles of operation and design.
The earliest microbalance using silica fibers was that designed by Salvoni . This type of balance [20 to 24] relies on the rigidity of a cantilever fiber to indicate displacement due to load. Shortly afterward Nernst  introduced a microbalance consisting of a horizontal silica fiber attached to and supporting a fine glass rod with a counterpoised pointer to indicate deflection due to load. This type of balance, which uses a silica fiber as a knife edge, is still in use although in forms slightly modified from the original [26 to 36]. Steele and Grant [37, 38] were the first to construct an arm balance entirely of fused silica. The balance, made from fine fused silica rods with fiber load suspensions and a fused-silica knife edge, utilizes a method of weighing by pressure. Several modifications in the design or in the operation of the SteeleGrant balance have been made for adaptation to particular uses [39 to 42, 46, 48]. Pettersson [43 to 45] introduced suspension of the arm balance by two fine vertical fibers as a replacement for the conventional knife edge; and developed further the theory of such suspension. Neher  designed a microbalance which consisted of a silica-fiber crossarm attached to a horizontal silica tor sion fiber, the twist of which is proportional to the load. This balance can be brought to an equilibrium position by rotating the torsion fiber with a wheel calibrated to indicate the amount of twist in mass units.
From this point microbalances using silica fibers were usually designed by combining certain principles developed in the earlier fiber balances. The balance of Kirk and Craig [23, 47, 49] and modifications by Carmichael , El-Badry and Wilson , and Garner [54, 310] consist of a nominally
3. PRODUCTION AND FABRICATION METHODS
3.1 Fused Silica
One of the greatest drawbacks in the commercial use of fused-silica fibers has been the difficulty and expense of preparing the bulk product from which the fibers are drawn. Prior to World War II no fused silica of good optical quality was produced commercially in the United States . However since that time many of the technologic difficulties have been overcome and fused-silica rods are available in many grades and sizes.
The two forms of fused silica, transparent and nontransparent, are products of quartz rock crystal and of quartz sand, respectively. Sand contains occluded gases and impurities which are difficult to remove entirely in the cleaning process or in the fusion process with the result that the fused product contains volatile and nonvolatile impurities of approximately 0.06 and 0.1 percent, respectively . These impurities are revealed in the masses of tiny bubbles which make the product nontransparent in varying degrees. On the other hand quartz rock crystal can be selected with few if any internal impurities and can be more effectively cleaned so that the occluded gases and other impurities on the surface are removed . The resulting product is relatively free of bubbles and is highly transparent. Quartzites, natural deposits of quartz and vein quartz, which contain more impurities than rock crystal, are poor substitutes for rock crystal; they do however make a better product than quartz sand for the nontransparent form of fused silica . A more recent development is the production of fused silica from "noncrystalline" materials [293, 314].
The production and forming methods for fused silica in no way resemble those for commercial glasses . This is because of the high temperatures needed for complete fusion of the highly viscous melt, complicated by volatilization. Although the melting point of quartz is probably below 1,470°C, the rate of fusion of quartz and the rate of change of the resulting glass into cristobal
ite are about the same below about 1,500°C.
Silica can be fused in arc furnaces, resistor furnaces, graphite molds, and by directed flame. Each of the methods has certain difficulties including those caused by losses from volatilization and by reduction in the presence of carbon and hydrogen. With carbon or graphite electrodes or molds, carbon monoxide is formed and silicon carbide and elemental carbon vapor or silicon metal may be formed . A purer product is obtained if the fusion is done in an oxidizing atmosphere which negates the effects of the reducing tendencies of carbon. There is no tendency for quartz or fused silica to adhere to the carbon if both are pure, for the oxides of carbon prevent close contact between the molten silica and the carbon electrodes or molds [82,88]. However, particles from the electrodes, molds or from the walls of the furnace deposited in the melt cause infusible hard zones which impair the quality of the final product. Further refinements of the fusion techniques may eliminate this type of impurity.
3.2 Fused-Silica Fibers
With the developments in the fusion and forming techniques for fused silica, some of the difficulties of fiber production have been overcome. Early workers with fused-sil
.ca fibers [1 to 3, 92] were hampered by the ack of commercially available stocks of ransparent rods from which fibers could be rawn. By fusing small pieces of fused silca into the form of a rod and working this od until it was fairly smooth, these men obained a rod from which fine sticks and then ibers could be drawn. This was a tedious and time-consuming process and limited the quantity and quality of fibers which could be rawn in a reasonable time. Threlfall  entioned spending 14 days to get two fibers bout 2.5 microns in diameter by about 13 nches long.
The production of silica fibers may be lassified by two major methods. One method ncludes those processes used for limited roduction of single fibers. Namely, blowing f fibers with a flame and the drawing of fiers by hand, and by simple mechanical means hereby the length is limited or the amount rawn is small. The other method includes hose processes in which automatic or semiutomatic machines are used for continuous roduction of single fibers.
⠀ The process of blowing fibers in a flame epends on the action of friction of the ases used. Two slightly different methods re used, one [9, 195] producing a long fine iber, the other [2, 92] a mass of short fiers. In the first process the stock, a fier about 25 microns in diameter, is held ertically in a long flame until a finer fier is blown out. The size of the fiber thus lown depends on the size of the original tock, the temperature and size of the flame nd the time interval between starting the iber and its removal from the flame. The ibers are smooth in appearance but have a slight curl which decreases the strength . In the second method the fiber stock is first drawn apart in the flame and the two pieces stroked back and forth in the flame, which is placed in a horizontal postion. The fibers are blown out horizontally and caught on a cloth or board placed a few feet from the flame. These fibers are deposited in a angled mass and must be eased apart. They re fairly short and may be damaged by supericial scratches from the contacts. Fibers can also be drawn by a hand method [1, 5, 9] in which the stock is melted at a point and
the two ends drawn rapidly apart to produce a rather heavy fiber; or by the gravity method  in which the stock, weighted at one end, is heated at a point until melted sufficiently so that the falling weight draws out a fiber. Boys  and Threlfall  each developed a simple mechanical method for drawing fibers. Boys used a crossbow with a straw arrow which when released drew from the fiber stock a fine fiber as long as 60 feet. Threlfall used a modified catapult the slider of which when released drew short thick fibers.
Machines for continuous drawing of silica fibers generally have three elements: a chuck to hold the silica stock stationary, to rotate it about its axis, or to move it lengthwise into the flame; a holder to maintain the torch at the proper angle and either to hold it steady or to move it along the fiber stock; a rotating reel to draw the fiber from the molten stock at a given rate [11, 102 to 104, 107). These elements can be geared to be regulated independently or as a unit in order to draw a desired size of fiber under readily reproduced conditions. A convenient base for the elements is a lathe or a drill press used either horizontally or vertically. This type of machine has been used to draw fibers from 600 to 0.7 microns from stock of 12 to 0.15 millimeters in diameter. Once the flame is adjusted to give sufficient heat for thorough melting of a particular size of stock then the reel speed and the rate of feed of the stock into the flame can be related to the approximate diameter of the fiber [11, 103]. The ratio of fiber pulling to fiber melting can be regulated within limits above which the fiber breaks off and below which the stock is not sufficiently melted for uniform drawing . Drive slippage and speed recovery accompanied by alternate drawing of cold and hot silica into the fiber results in variations in strength and in the diameter of the fiber .
Many gas mixtures have been used in all of the above methods: oxy-hydrogen, oxy-coal gas, oxy-natural gas, oxy-acetylene. It has not been mentioned whether the gases used affect the fibers, although the heat of the flame must be regulated so that it is neither high enough to volatilize an excessive amount of the fused silica nor low enough to prevent
sufficient melting. With the thicker stock, curly fibers result from uneven heating [1, 2, 94, 195], so curly in some cases as to resemble watch springs . These fibers can be straightened by "soft flaming" but are weakened as a result . A slight bowing due to the effect of gravity on larger fibers has been noticed when they are drawn horizontally; however, the small fibers can be drawn either horizontally or vertically with no noticable effects .
With flame-blown silica fibers about 21 microns in diameter and 15 cm in length, Horton  observed variations in the diameter of about 1 to 4 percent. The fibers had the appearance of a sequence of irregular conic sections within an over-all tapered fiber. Similar irregularities in shape of about six percent in a quarter of an inch were observed in some machine drawn fibers . The long term variations could be associated with variations in the stock and with the pulling phase. For example, rungs or tapes used on sectional reels cause an irregular rate of pulling which results in a slight taper in the fiber between the markers , a variation which is especially noted in fine fibers. The short term variations in the diameter could arise from insufficient melting of the stock, oscillations of the flame, vibrations in the drawing equipment, or even a natural oscillation between the rate of drawing and the rate of melting. Although these diameter variations were not mentioned by many workers, it is possible that they were not sought or that they were so small as to be hardly perceptible with the measuring equipment used.
Although fused silica fibers seem to lose strength with age [188 to 191, 199, 200] the real effects of actual storage conditions on this loss of strength are not known. It would appear that a method of storing fibers might be determined which would help to preserve the strength and to prevent damage by dust, moisture, certain atmospheres, or accidental scratching.
Contamination and scratching of the fibers from dust, salts in the air, and from handling with the fingers tends to weaken the fibers. When the fibers are fused or heated the silicates formed by impurities on the surface can cause brittle joints and weak fi
bers [50, 315, 316]. Finger marks on large. pieces of fused silica appear as devitrification marks on the material after it has been worked in a flame . Opinions regarding methods to counteract this contamination and subsequent weakening differ among workers. Some workers prefer to prevent contamination by working under scrupulously clean conditions, using only those fibers which are freshly drawn, free of dust and other contamination and by never touching that part of the fiber which is used in a piece of apparatus [9, 315]. Others prefer to remove any contamination on the fibers by cleaning with a solution. such as chromic acid, before the joints are fused or the fibers worked [104, 106]. The finished piece is also cleaned to remove any deposits which result from handling during the working of the fibers. The effects of cleaning solutions on the strength of fibers and on the finished piece are not known or have not been detected. The effects are complicated by the fact that while cleaning, rubbing with the fingers, and adsorption of moisture reduce the tensile strength, these processes tend to increase the resistance to scratching or to any further surface injury .
3.3 Silica-Fiber Apparatus
The fineness of the silica fibers used necessitates the use of holders or jigs to prevent the fibers from blowing away while they are being fabricated into the various devices. A number of different types of holders and jigs are used. These aids, used mostly in the fabrication of microbalances and electrometers, are described more thoroughly by the individual experimenters.
The holders used by Neher  and by Carmichael  consist of forks with two prongs whose spacing is either adjustable or is fixed for a particular length of fiber. These forks hold individual fibers only at the ends of the fiber and allow the fiber to be held in any position while joints are fused. The holders can be held in a micromanipulator, or the proper sized holder can be placed in position in a jig designed to insure reproducible construction of a particular device. The used portion of a fiber is