SINGLE CRYSTAL WHISKER ELECTRIC LIGHT FILAMENT

Background of the Invention

1. Field of the Invention

The invention described and claimed herein is generally related to electric light filaments, and more particularly to materials used as such filaments.

2. Description of the related art.

Previously known electric light filaments are typically made of materials which are either polycrystalline in nature or which are amorphous, or noncrystalline, in nature. Such materials suffer from the disadvantage that they become brittle with time at elevated temperatures.

Polycrystalline materials, which include the majority of commercially available metallic filaments, are characterized by the presence of crystal grain boundarias, dislocations, voids and various other microstructural imperfections. These microstructural imperfections lead to grain growth and recrystallization, particularly at elevated temperatures, which in turn lead to increased brittleness and diminished strength.

Metallic filaments also suffer from a disadvantage that is a consequence of the relatively low electrical resistance that characterizes metallic filaments. The low electrical resistance requires that the filaments be made quite long.

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which in turn requires the filament to be tightly coiled in order to fit it into a light bulb of suitable size. Coiling of the filament effectively reduces the radiating surface area because the coiled filament partially occludes itself, thereby diminishing the radiative efficiency of the filament.

Another disadvantage of metallic filaments is that metals in general, and particularly tungsten, have a relatively high resistivity/temperature coefficient. From room temperatures to approximately 1200 °C the resistance of metal filaments increases as much a six-fold, resulting in high electrical power consumption at operating temperatures.

Amorphous metals used as filaments undergo various degrees of crystallization at elevated temperatures, resulting in the development of grain boundaries that decrease the strength and toughness of these materials also. Additionally amorphous materials are often of lower strengths initially relative to crystalline materials.

Accordingly, it an object and purpose of the present invention to provide an electric light filament which is of improved strength, durability and resilience, particularly at elevated temperatures.

It is also an object and purpose of the present invention to provide an electric light filament which does not undergo progressive crystallization or recrystallization at incandescent temperatures.

Summary of the Invention

The present invention provides an electric light filament comprising a single crystal fiber known as a "whisker." The whisker is preferably a high emissivity ceramic whisker. In the preferred embodiment the whisker consists essentially of monocrystalline fiber of silicon carbide, most preferably beta silicon carbide. Such filaments are characterized by high mechanical strength and durability at the elevated temperatures required to achieve incandescence. Also, such crystals are characterized by their high surface area to volume ratios, as a result of their typically small cross- sectional diameters, which are on the order of about 5 microns. They are also characterized by their high electrical resistances and high emissivities relative to metals. Additionally, the resistance of silicon carbide does not increase with temperature as much as tungsten, such that power consumption of a silicon carbide whisker is lower at incandescent operating temperatures. In accordance with another aspect of the invention, the silicon carbide whisker is doped with nitrogen to increase the conductivity of the whisker to a level appropriate for use of the whisker as an electric light filament.

These and other aspects of the present invention will be more apparent upon consideration of the following detailed description of the preferred embodiments of the invention.

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Description of the Preferred Enbodimentε

Whiskers are minute, high purity, single crystal fibers. More than a hundred materials, including metals, oxides, carbides, halides, nitrides, and carbonaceous materials have been prepared as whiskers. As a consequence of their high chemical purity and monocrystalline structures, whiskers are characterized by very high mechanical tensile strengths, which in the case of some materials approach the theoretical maximum strength of the material based on actual interatomic bonding forces. Because of their high tensile strengths, whiskers have been primarily of interest as agents used to reinforce ceramic, metallic and even polymeric matrices.

In addition to the high mechanical strength that results from the highly ordered crystalline structure of whiskers, other significant and, to some extent, unexpected changes are obtained in the optical, magnetic, dielectric and electrical conductivity of materials that are formed as whiskers.

Ceramic whiskers are unique in that they can be strained elastically as much as three percent without permanent deformation, compared with about 0.1 percent for bulk ceramic materials. In addition, whiskers exhibit considerably less strength deterioration with increasing temperatures than the best conventional- high-strength metal alloys. Further, no appreciable fatigue effects have been observed in whiskers. They can be handled roughly, milled or chopped, elevated to

high temperatures, and otherwise worked without any appreciable loss of strength.

Whiskers can be produced in a range of fiber sizes and fiber forms. A number of processes are known for producing whiskers in various forms, including forms known by terras such as grown wool, felted paper and loose fibers.

Some ceramic materials are semiconductors, and are known to be very resistant to current flow. However, it is known that by doping silicon carbide with nitrogen, which becomes located interstitially within the silicon carbide crystal structure, the electrical conductivity of the silicon carbide can be increased to a level that permits its use as an incandescent electric light filament. In this regard, the emissivity of silicon carbide is also particularly conducive to this use, lying in the range of 0.9, which is considerably higher than the emissivity of approximately 0.4 that characterizes most metallic filaments.

These characteristics are all conducive to the new use of whiskers, provided by the present invention, as electric light filaments.

A demonstration of the present invention was conducted using a number of single crystal whiskers of beta silicon carbide (SiC). The whiskers were doped green with nitrogen.

The whiskers ranged from three (3) millimeters to thirty (30) millimeters in length and were approximately five (5) microns

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in diameter. The whiskers were mounted between two wire binding posts which were spaced approximately three millimeters apart. A direct voltage was applied to the whiskers across the binding posts, and the temperatures achieved in the whiskers were measured ^' with an optical pyrometer. When 30 volts (d.c.) was applied to the whiskers, the whiskers glowed in the ^' high red heat (800-1,000 °C) region. Higher voltages in air caused the whiskers to burn out due to oxidation. In partial vacuum temperatures were of 1100 °C to 1440 °C were achieved in the whiskers before burnout.

The silicon carbide filaments were compared with conventional tungsten filaments. For both types of filaments the properties of electrical resistance, filament length, filament diameter and filament weight were measured. From th measured voltages and current readings, power requirements at various filament temperatures were calculated and are set forth below. Qualitative analyses of light output in lumens were done by comparing a silicon carbide filament to that of candle, and a candle to a 4 watt clear glass tungsten filamen light bulb. This was done using the dual screen method.

The temperatures, voltages and currents were measured simultaneously on each filament as the voltage was increased. Temperatures were measured using an optical pyrometer. Voltage and current readings were performed on a digital multimeter. The voltages were regulated using a variable

transformer. The masses, lengths, and cross-sectional areas were also measured and/or calculated. For comparison, similar measurements were made on clear glass conventional 25-watt and 4-watt tungsten filament light bulbs. Data obtained from these tests, which compare the silicon carbide filament to the tungsten filament, are given in Tables I through V below.

Table I

Comparative Physical Property Data

Beta SiC Coiled Ratio

Whisker Tungsten SiC/W

Mass (mg) .002 4.5 1/2500

Length (mm) 30 32 1/1

300 (uncoiled) 10/1

Diameter (microns) 5 250 1/50

Resistance (Ohms) 1800-3100 74 (25 watts) 25/1 560 (4 watts) 5/1

Effective Radiating 2.86 .077 36/1 Surface Area to Volume Ratio

Emissivity at 1200 °C .90 40 2.3/1

Resistance Change,

Room Temp, to 1200 °C 2x 6x 1/3

TABLE II Tungsten 4 Watt Bulb

Voltage Current Power Resistance Temperature

(volts) ( a) (watt) (ohms) (°C)

.77 1.36 .001 560

6.0 4.55 .028 1300

10.2 7.36 .075 1390

15.0 9.48 .142 1580 first light

19.6 11.11 .218 1760 800

23.0 12.79 .294 1800 860

28.3 14.32 .405 1980 920

32.7 15.76 .515 2070 980

37.0 17.12 .633 2160 1030

41.6 18.52 .770 2250 1080

46.1 19.72 .909 2340 1120

50.8 20.92 1.06 2430 1180

55.4 22.17 1.23 2500 1235

59.8 23.07 1.38 2590 1290

64.1 24.33 1.56 2630 1340

68.4 25.35 1.73 2700 1390

72.7 26.44 1.92 2750 1390

77.3 27.48 2.12 2810 1420

81.8 28.41 2.32 2880 1420

86.0 29.32 2.52 2930 1430

90.2 30.00 2.71 3000 1480

94.6 31.05 2.94 3050 1480

98.9 31.99 .3.16 3090 1510

103.0 32.76 3.374 3140' 1530

107.0 33.67 3.603 3180 1530

111.4 34.30 3.821 3190 1550

115.7 35.94 4.158 3220 1560

119.7 35.89 4.296 3340 1575

123.6 - 36.72 4.539 3370 1585

TABLE III Tungsten 25 Watt Bulb

Voltage Current Power Resistance Temperature (volts) (ma) (watt) (ohms) (°C)

.77 10.4 .008 74 _—

- 10.2 74.9 .76 136 _.__.

_ 19.6 90.8 1.8 216 ___. 28.3 102.2 2.9 277 37.0 110.5 4.1 335 930

389 1070 46.1 118.6 5.5 55.4 126.9 7.0 437 1212 64.1 134.3 8.6 478 1330 72.7 141.0 10.3 516 ^* 1410 81.8 148.0 12.1 553 1450 90.2 156.6 14.1 576 1550 98.9 162.8 16.1 607 1630 107.0 169.4 18.1 632 1720 115.7 175.9 20.4 658 1830 123.6 181.1 22.4 682 1900

TABLE IV

Silicon Carbide Whlisker Filament ^•

Temperature

Voltage Current Power Resistance (volts) (ma) (watt) (ohms) (°C)

.775 .43 1802 __— 10.6 5.89 __<-_ 1800 ._ _* _.._, 19.6 10.00 __,__. 1960 28.29 13.3 2127 __.•__ 36.99 15.43 2400 46.09 17.07 .78 2700 800 50.77 17.50 .89 2900 850 55.44 18.00 1.00 3080 950 59.79 18.45 1.10 3240 1060 64.13 18.80 1.21 3410 1170 68.42 19.10 1.31 3582 1260

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TABLE V

Silicon Carbide Whisker Filament

Voltage Current Power Resistance Temperature

(volts) ( a) (watt) (ohms) (°C)

.77 .25 .0002 3080 ___

6.0 1.71 .010 3500

10.2 2.50 .025 4080 _—

15.0 3.28 .049 4570 760

19.6 3.96 .078 4950 850

23.0 4.49 .103 5120 900

28.3 4.92 .139 5750 1050

32.7 5.16 .169 6330 1140

37.0 5.45 .191 6790 1250

41.6 5.40 .227 7700 1340

46.1 5.60 .258 8230 1400

As summarized in Table I, the resistance of the tungsten filaments increases six- old over the temperature -re-nge from room temperature to 1200 °C, ^" wherease the silicon carbide filaments increase in resistance only two-fold. Also, the emissivity of the silicon carbide whisker filaments at 1200 °C is on the order of 0.9, whereas the emissivity of the tungsten filament is on the order of 0.4.

One surprising discovery is that silicon carbide whiskers are considerably more efficient as electric light filaments than conventional tungsten filaments. Comparisons with

« conventional tungsten filaments have indicated that, to achieve a particular incandescent temperature, silicon carbide filaments require significantly less electrical power than a comparble tungsten filament. This is thought to

be a consequence of a higher surface area to volume ratio in the silicon carbide whiskers than in tungsten filaments, and possible also due to a higher emissivity in silicon carbide whiskers than in tungsten filaments.

These advantages are considered to be a consequence of a higher resistance and a higher surface area to volume ratio i the silicon carbide whiskers than in tungsten filaments, as well as a higher emissivity in silicon carbide than in tungsten filaments.

From a review of the foregoing data, it is evident that because of the physical structure of the silicon carbide whisker and its significantly different physical, mechanical and electrical properties, single crystal whisker filaments have many superior performance properties' and as a result produce a more efficient light bulb filament when compared to conventional polycrystalline metallic tungsten wire filaments.

Although the present invention is described herein by reference to a preferred embodiment of the invention, it will be understood that various modifications, alterations and substitutions which may be apparent to one of ordinary skill in the art may be made without departing from the essential invention. Accordingly, the present invention is defined by the folJowing claims.