WO2002021575A2

WO2002021575A2 - Method of producing silicon carbide and various forms thereof

Info

Publication number: WO2002021575A2
Application number: PCT/IL2001/000834
Authority: WO
Inventors: Gady Golan
Original assignee: Silbid Ltd.
Priority date: 2000-09-06
Filing date: 2001-09-05
Publication date: 2002-03-14
Also published as: WO2002021575A3; AU2001288020A1

Abstract

A method of producing silicon carbide by introducing into the interior of a furnace a quantity of relatively pure elemental silicon and a quantity of elemental carbon; subjecting the interior of the furnace to a vacuum; and heating the silicon and carbon to a temperature of 1500 °C-2200 °C to vaporize the silicon and to react it with the carbon to produce silicon carbide. Several embodiments are described for producing a heating or lighting element or high temperature sensor. The carbon is shaped with the aid of a binder and silicon particles are applied to the outer surfaces. A further embodiment for producing silicon carbide powder is also described.

Description

METHOD OF PRODUCING SILICON CARBIDE AND VARIOUS FORMS THEREOF

FIELD AND BACKGROUND OF THE INVENTION

The present invention relates to a method of producing silicon carbide

(SiC). The method is particularly useful for producing silicon carbide heating and

lighting elements, high-temperature sensor elements, and finely-divided particles

of silicon carbide, (e.g., for use as abrasives, for hardening surfaces, etc.), and the

invention is therefore particularly described below with respect to these

applications. However, it will be appreciated from the description below that the

novel method could also be used for producing silicon carbide for many other

applications, such as semi-conductor substrates, hard coatings for turbine blades,

high power switching devices, cosmic radiation protectors, etc.

Silicon carbide (SiC), sometimes referred to as carborundum, is a hard,

clear, green-tinged or yellow-tinged crystalline compound, which is normally

insulating but which becomes conductive when properly heated at a high

temperature; for example, when heated to 2000°C, it is as conductive as graphite.

This material, therefore, is frequently classified as a semiconductor. It is presently

used in a wide variety of applications, including abrasives, heating elements,

illuminating elements, high-temperature sensors and semiconductor substrates.

Because of its highly unique properties, particularly hardness, heat resistance, semiconductivity, thermal and electrical stability, and corrosion resistance, it is

commonly considered as the material of the future.

Silicon carbide is generally manufactured, according to one known

method, by heating pure silica sand and carbon in the form of coke in an electrical

furnace.

According to another known method, a graphite heating element in a

cylinder bar is covered with mixture of carbon powder and quartz and high

electrical current is passed through it to create a temperature of up to 3000°C.

At this temperature, the quartz (Sj0₂) is broken down to pure silicon, which reacts

with the carbon powder and creates the required SiC. At a lower temperature zone

distant from the heater, the SiC begins crystallizing in the shape of small scales.

These scales are ground to form a powder of the required size. This process of SiC

powder synthesis, which takes place in a vacuum

(10^" Torr), requires a long period of time, in the order of 36 hours, as well as high

electrical currents. Moreover, it is difficult to obtain a powder of the required grain

size with this process.

Approximately 45 years ago a new concept was proposed by Lely for

growing silicon carbide crystals of high quality; and approximately 20 years ago, a

seeded sublimation growth technique was developed (sometimes referred to as the

"modified Lely Technique"). The latter technique lead to the possibility for true

bulk crystal preparation. However, these techniques are also relatively expensive

and time-consuming, such that they impose serious limitations on the industrial potential of this remarkable material. In addition, silicon carbide heating or

lighting elements prepared in accordance with these known techniques generally

vary in resistance with temperature, and/or lose power with age, thereby requiring

extra controls, special compensations, and/or frequent replacement.

OBJECTS AND BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a new method of producing

silicon carbide having advantages in one or more of the above respects.

According to a broad aspect of the present invention, there is provided a

method, of producing silicon carbide (SiC), comprising: introducing into the

interior of a furnace a quantity of elemental silicon and a quantity of elemental

carbon; subjecting the interior of the furnace to a vacuum; and heating the silicon

and carbon to a temperature of 1500°C-2200°C to vaporize the silicon and to

convert the carbon to silicon carbide. By elemental silicon is meant the silicon

element, as distinguished from the silicon dioxide compound (e.g., sand, glass,

quartz). Preferably, the silicon is relatively pure except for possible traces of

impurities or dopants, such as present in silicon semiconductor substrates. In fact

particularly good results were obtained, as described below, when the silicon used

was the wastage in the manufacture of silicon semiconductor substrates.

Preferably, the carbon is either lignite carbon or anthracite carbon ground

to a fine talc or power form. During this heating process, the silicon vaporizes, diffuses into the- carbon,

and converts it to silicon carbide (SiC). Since silicon carbide has a green-tinged or

yellow-tinged color, depending on impurities or dopants therein, the formation of

such a color during the above-described heating process indicates that the resulting

product is indeed silicon carbide.

Since the novel method utilizes elemental silicon, rather than Si0₂ (as in

sand, glass or quartz), it does not require the high temperatures (e.g., the order of

3000°C), or the long heating time (e.g., the order of 36 hours), required in the prior

art process as described above.

As will be described more particularly below, the method may be used in

a wide variety of applications for producing shaped articles of silicon carbide, or

for producing finely-divided particles of silicon carbide. Thus, the shaped articles

of silicon carbide produced in accordance with the invention could be used for

manufacturing heating elements, illuminating devices, high-temperature sensors,

semiconductor substrates, and the like. The silicon carbide particles made in

accordance with the invention could be used for producing abrasives, for

hardening surfaces such as turbine blades, for producing bullet-proof materials,

and the like.

According to another aspect of the present invention, there is provided a

method of producing a shaped article of silicon carbide (SiC), comprising:

preparing a mixture of a quantity of carbon in the form of finely-divided particles

mixed in a binder; shaping the mixture according to the desired shape of the article; applying finely-divided particles of silicon over the outer surface of the

shaped article; introducing the shaped article with the finely-divided particles of

silicon thereover into the interior of a furnace; subjecting the interior of the

furnace to a vacuum; and heating the interior of the furnace to a sufficiently high

temperature and for a sufficiently long period of time until a resulting product is

produced having a green-tinged or yellow-tinged color.

In the preferred embodiments of the invention described below, the

quantity of silicon is in excess of the quantity of carbon by weight to assure

relatively complete conversion of the carbon to silicon carbide, with the excess

silicon being removed by removing the silicon vapors during the diffusion process

to prevent or minimize condensation of the silicon vapor on the outer surface of

the silicon carbide.

Where heating or lighting elements are to be produced, the initial

composition preferably includes relatively pure silicon but having traces of a

dopant, such as zinc, aluminum, tellurium, or another element in the third or fifth

column of the periodic table, in the ratio of about 1 :10^" . The vacuum is preferably

from 10^"1 to 10^"3 Torr, and the heating temperature is preferably 1550-1600°C.

Such a process produces silicon carbon heating (or lighting) elements which are

green-tinged in color, and have a relatively low internal resistance in the order of

tens to a few hundreds of Ohm-cm.

On the other hand, where high-temperature sensors are to be produced, the

initial composition preferably includes at least 10% more silicon than carbon, with the silicon being relatively free of dopants; and the heating is preferably effected at

a vacuum of higher than 10^"4 Torr and at a temperature of about 1700 - 1800°C in

order to assure a Si:C ratio of 50:50 and to remove the extra silicon vapors. This

technique produces high-temperature sensors having relatively high internal

resistance, in the order of hundreds of Kilohm-cm, and a yellow-tinged color.

In some described preferred embodiments, the mixture is prepared by mixing

the finely-divided particles of carbon in a water solution of sucrose, and in other

described preferred embodiments, the mixture is prepared by mixing the

finely-divided particles of carbon in polyvinyl acetate. In both cases, the carbon

mixture is prebaked at about 500°C in order to harden the sample. It will be

appreciated, however, that other binders, preferably organic binders, may be used.

According to yet another embodiment of the invention described below, the

method produces finely-divided particles of silicon carbide (SiC), by: introducing into

the furnace the silicon and carbon in the form of layers of finely-divided particles

separated from each other by a layer permeable to silicon vapor.

According to further features in the described preferred embodiments, the

carbon and silicon are both contained in a graphite crucible when heated within the

furnace. The crucible is at least partly open at its upper end to the interior of the

furnace to permit excess silicon vapors to escape to the interior of the furnace, and

thereby to prevent or minimize condensation of silicon vapors on the outer surface

of the silicon carbide. Further features and advantages of the invention will be apparent from the

description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, with reference

to the accompanying drawings, wherein:

Fig. 1 diagrammatically illustrates one form of apparatus for use in

preparing shaped silicon carbide articles in accordance with the method of the

present invention;

Fig. 2 diagrammatically illustrates one form of apparatus for use in

preparing silicon carbide powder in accordance with the method of the present

invention; and

Figs. 3 and 4 are results of X-ray diffraction tests performed on the

materials produced according to the described method.

DESCRIPTION OF PREFERRED EMBODIMENTS

Producing Shaped SiC Articles (Fig. 1)

Fig. 1 illustrates one form of apparatus for use in producing shaped

articles of SiC, such as heating elements, lighting elements, high-temperature

sensor elements, etc. The apparatus illustrated in Fig. 1 includes a furnace, generally designated

2, whose interior 3 is heated by a plurality of planar electrical heating elements 4.

A pump (not shown) communicates with the interior 3 of the furnace via gas

outlets 5, for producing a vacuum therein. The interior of the furnace is lined with

graphite walls 6 for heat isolation.

Disposed within the interior 3 of the furnace is a table 7 for supporting a

crucible 8 to receive the work materials which, when subjected to heat and

vacuum as described below, produce articles of silicon carbide. Crucible 8 is of

hardened graphite. Its upper end is covered by a graphite lid 9 formed with

openings 10 to provide communication between the interior of the crucible and the

interior 3 of the furnace 2, as will be described more particularly below.

The work materials to be treated are introduced into the furnace via an

insertion pipe 11. Pipe 11 includes the main gas outlet 5 connected to the vacuum

pump (not shown), and also a vacuum valve 12. The furnace 2 further includes an

electric feed-through 13 for supplying the electrical current to the heating

elements.

Such electrical furnaces are well known, and therefore further details of its

structure and the manner of operating it are not set forth herein.

In the examples to be described below, the shaped workpiece of silicon

carbide to be produced is a rod, wire or electrode, to be used in the manufacture of

electrical resistor elements, light source elements, or high-temperature sensor

elements. Fig. 1 illustrates the workpiece, therein generally designated 15, of the desired shape disposed within the crucible 8. This workpiece is prepared from a

mixture of carbon in the form of finely-divided particles mixed in a binder to

produce a doughy mixture which can be shaped as desired, in this case according

to a rod, wire or electrode. Preferably, the carbon is either lignite carbon or

anthracite carbon ground to a fine talc or power form. The carbon-binder mixture

is pre-baked in order to harden the workpiece.

Finely-divided particles of relatively-pure elemental silicon 16 (as

distinguished from silicon dioxide, as in, e.g., sand or quartz) are applied over the

complete outer surface of the shaped workpiece 15 before the latter is placed in the

crucible 8. The crucible is then covered by the lid 9 and placed on table 7 in the

interior of the furnace.

The interior of the furnace, with the crucible 8 and workpiece 15 therein,

is subjected to a vacuum via gas outlets 5, and is heated by electrical heating

elements 4. This heating of the interior of the furnace 3 is at a sufficiently high

temperature, and for a sufficiently long period of time, to vaporize the silicon and

to cause its vapors to diffuse and to react with the carbon to produce silicon

carbide. Thus, the heating may be continued until the workpiece 15 exhibits a

green-tinged or a yellow-tinged color, thereby indicating that the silicon particles

16 applied over the carbon-containing body 15 have converted the carbon to

silicon carbide.

Crucible lid 9 is provided with the openings 10 to permit the silicon

vapors to escape during the heating process into the interior 3 of the furnace. This prevents or reduces the condensation and deposition of silicon on the outer surface

of the workpiece 15. If such a deposition is produced, it can be removed by a

suitable silicon etchants.

Following are several examples for producing silicon carbide heating and

lighting elements:

Example 1

In this example, the carbon particles used for making the shaped

workpiece 15 are finely-divided particles of charcoal having a particle size of 50 -

250 microns; and the silicon particles 16 applied over the shaped workpiece 15 are

finely-divided particles of the waste of silicon wafers, both the mono-crystalline

and the poly-crystalline type, resulting from the production of semiconductor,

devices, also ground to a fine particle size.

The initial composition preferably includes 54% silicon and 46% carbon by

weight, with the silicon being relatively pure but including traces of dopants, such as

zinc, aluminum, and/or tellurium, in the ratio of about 1:10^"6. Such dopants reduce

the internal resistance of the produced silicon carbide composition.

The carbon particles are mixed in a binder of white sugar (sucrose)

dissolved in soft water (one kilogram of white sugar with a few liters of water),

which water was subsequently evaporated. The carbon particles are

homogeneously mixed in the sugar solution by means of a blender, pre-baked at

about 500°C to harden the workpiece to a doughy consistency, and then shaped to

the desired configuration (e.g., a rod). The shaped workpiece 15 (consisting of carbon particles in the binder) is

covered by finely-divided particles of the silicon powder 16, and is then placed

within the crucible 8 and covered by the lid 8. The interior of the oven 3 is then

evacuated to a pressure of 10^"3 Torr and heated to a temperature of 1550°C -

1600°C for a period of 45 minutes. During this period, the silicon powder 16

vaporizes and diffuses into the carbon of the workpiece 15, converting it to silicon

carbide. This is manifested by a green-tinged color.

Upon completion of the heating process, the workpiece is retained in the

oven for a period of approximately 3 -hours after the heating elements have been

de-energized, to permit a gradual cooling of the workpiece in an annealing

process. The workpiece may then be removed from the oven.

The so-produced material was used for making wire elements of 0.3 mm

diameter, wound into a wire helix of 2 mm diameter, having a length of 400 mm.

The 400 mm wire reached the maximum temperature of 1600°C in less than

3 -seconds after switching to 220 volts; the current consumption was 9A.

The so-produced material was also used to make thin round elements of 1

cm in length and 4 mm in diameter. Such elements reached the maximum

temperature of 1600°C in less than 3 -seconds after switching to 60 N, with current

consumption of 10A.

The so-produced material was also used to make round elements 12 cm in

length and 10 mm in diameter. Such elements reached the maximum temperature of 1600°C in less than 3 -seconds after switching to 60 V. They were left to operate

continuously for 16 hours and had a current consumption of 30A.

The ohmic resistance of all the foregoing elements remained substantially

the same after 16 hours of operation.

Two samples of the fabricated SiC elements, one in the form of a thin rod

and the other in the form of ground particles, were sent for X-ray diffraction

measurements in order to verify their structure. The obtained results appear in Fig.

3 (thin rod) and Fig. 4 (ground particles) which clearly show that the produced

material is indeed SiC according to the obtained peaks.

The following two tables further confirm that the produced product was

SiC, Table 1 being a diffractometer analysis of the thin rod produced according to

the above example, and Table 2 being a diffractometer analysis of SiC.

Example 2

This example was the same as in Example 1, except that the finely-divided

particles of carbon are mixed in a binder of polyvinyl acetate, in an amount of 0.5

kg polyvinyl acetate to one kg. of carbon, instead of the sugar solution. The

process is otherwise the same as in Example 1. Example 3

This example is also the same as in Example 1, except that the sample is

heated to a higher temperature of 1800°C for a shorter period of time, 30 minutes.

The rest of the procedure is substantially the same as in Example 1.

Example 4

This example is also the same as Example 1, except that the sample is

heated to an even higher temperature, 2200°C, for an even shorter period of time,

15 minutes. The remainder of the procedure is the same as in Example 1.

Silicon carbide heating elements and lighting elements can thus be made

according to the foregoing examples to have some or all of the following

advantages: stable thermal and electrical performance over time and numerous

heating operations; vibration and shock proof while heating; operable in an open

air environment without oxidizing and without releasing poisonous gasses; capable

of operating in corrosive and aggressive conditions without degradation in

performance; capable of varying the temperature almost linearly with voltage up to

1600°C; lower manufacturing cost compared to conventional SiC elements; easily

structured in various sizes and shapes (variable wattage); and extremely radiation

hard and therefore protective against nuclear radiation; Following are several examples for producing silicon carbide

high-temperature sensor elements:

Example 5

In this example, the carbon particles used for making the shaped

250 microns; and the silicon particles 11 applied over the shaped workpiece 15 are

and the poly-crystalline type, resulting from the production of semiconductor

devices, also ground to a fine particle size. The silicon component, however, is

relatively free of dopants and impurities in order to obtain a high internal

resistance in the produced sensor element. In addition, the quantity of the silicon

should exceed by at least 10% the quantity of the carbon by weight, in order to

provide an excess of silicon vapor during the heating process, as described more

particularly below.

The carbon particles are mixed in a binder of white sugar (sucrose)

and homogeneously mixed by means of a blender. The mixture is pre-baked at

500°C to harden it to a doughy consistency, and then shaped to the desired

configuration (e.g., a rod).

The shaped workpiece 15 (consisting of carbon particles in the binder) is

covered by finely-divided particles of the silicon powder 16, and is then placed within the crucible 8 and covered by the lid 8. The interior of the oven 3 is

evacuated to a pressure higher than 10^"4 Torr and heated to a temperature of

1700°C - 1800°C for a period of 30 minutes. During this period, the silicon

powder 16 vaporizes and diffuses into the carbon of the workpiece 15, converting

it to silicon carbide. This is manifested by a yellow-tinged color.

Upon completion of the heating process, the workpiece is retained in the

de-energized, to permit a gradual cooling of the workpiece in an annealing. The

workpiece was then removed from the oven.

Since the vapor pressure of silicon is higher than that of carbon, the

relatively high heating temperature (1700°C - 1800°C), and the relatively high

vacuum, (higher than 10^"4 Torr) cause the excess silicon to evaporate until the

required equal amounts of 50/50 of siliconxarbon is obtained. In addition, the use

of silicon free of dopants and impurities in the initial material produces a silicon

carbide body of high internal resistance, in the order of hundreds of Kilohm-cms

and higher.

Example 6

This example is the same as Example 5, except that the finely-divided

particles of carbon are mixed in a binder of polyvinyl acetate, in an amount of

0.5 kg of polyvinyl acetate to one kg. of carbon, instead of the sugar solution. The

process is otherwise the same as in Example 4. Example 7

This example is also the same as Example 5, except that the sample is

heated to an even higher temperature of 2200°C in the furnace for a period of

about 15 minutes, rather than a temperature of 1700°C - 1800°C for 30 minutes as

in Example 5. The remainder of the procedure is otherwise the same as in Example

5.

Silicon carbide high-temperature sensor elements can thus be made

according to the foregoing examples to have some or all of the following

advantages: stable thermal and electrical performance over time and numerous

operations; vibration and shock proof; operable in an open air environment

without oxidizing and without releasing poisonous gasses; capable of operation in

corrosive and aggressive conditions without degradation in performance; lower

manufacturing cost compared to conventional SiC elements; easily structured in

various sizes and shapes; and extremely radiation hard and therefore protective

against nuclear radiation;

Preparation of Silicon Carbide Powder (Fig. 2)

Fig. 2 illustrates apparatus for producing silicon carbide powder, for

example in preparing abrasives, hardened surfaces of cutting tools, hardened

surface of turbine blades, bullet-proof tiles, etc. The apparatus illustrated in Fig. 2 includes a furnace, generally designated

22, whose interior 23 is heated by a plurality of planar electrical heating elements

24. A pump (not shown) communicates with the interior 23 of the furnace via gas

outlets 25, for producing a vacuum therein. The interior of the furnace is lined

with graphite walls 26 for heat isolation.

Disposed within the interior 23 of the furnace is a table 27 for supporting a

crucible 28 to receive the work materials which, when subjected to heat and

vacuum as described below, produce silicon carbide powder. Crucible 28 is of

hardened graphite. Its upper end is covered by a graphite lid 29 formed with

openings 30 to provide communication between the interior of the crucible and the

interior 23 of the furnace 22, as will be described more particularly below.

The work materials to be processed are introduced into the furnace via an

insertion pipe 31. Pipe 31 includes the main gas outlet 25 connected to the vacuum

pump, and also a vacuum valve 32. The furnace 22 further includes an electric

feed-through 33 for supplying the electrical current to the heating elements.

Such electrical furnaces are well known, and therefore further details of its

structure and the manner of operating it are not set forth herein.

Crucible 28 includes the silicon component in the form of finely-divided

particles 40 placed at the bottom of the crucible. The carbon component is in the

form of finely-divided particles shown at 41, separated from the silicon particles

40 by a layer 42 which is permeable to the silicon vapors produced during the

heating process, to enable such vapors to rise and react with the carbon particles 40 to produce the silicon carbide particles. Preferably, the carbon is either lignite

carbon or anthracite carbon ground to a fine talc or power form. Layer 42 is

preferably a hardened graphite cloth placed on the silicon particles with the carbon

particles on the graphite cloth, such that the silicon vapors penetrate and diffuse

with respect to the carbon particles to convert them to SiC.

The interior of the furnace 22, with the crucible 28 and the silicon

particles 40, carbon particles 41, and permeable graphite cloth layer 42 therein, is

subjected to a vacuum via gas outlets 25, and is heated by electrical heating

elements 24. This heating of the interior of the furnace 23 is at a sufficiently high

temperature, and for a sufficiently long period of time, until the particles within

the crucible exhibit a green-tinged or yellow-tinged color, thereby indicating that

the silicon particles 40 have vaporized, diffused into the carbon particles, and have

converted the carbon particles to silicon carbide particles.

As indicated earlier, crucible lid 29 is provided with openings 30. This

permits the silicon vapors to escape during the heating process into the interior 23

of the furnace, and thereby prevents or reduces the condensation and deposition of

silicon vapors on the outer surface of the carbon particles.

Following are several examples for producing silicon carbide particles or

powder using the apparatus of Fig. 2; Example 8

In this example, the carbon particles 41 are finely-divided particles of

charcoal having a particle size of 50 - 250 microns; and the silicon particles 40

introduced in the bottom of the crucible 28 are finely-divided particles of

relatively pure silicon obtained from the waste of silicon semiconductor wafers,

both the mono-crystalline and the poly-crystalline type, resulting from the

production of semiconductor devices and ground to a fine particle size. This

example used a 10% excess of silicon particles by weight over the carbon

particles, namely 1.0 kilogram of carbon particles and 1.10 kilogram of silicon

particles. The silicon is relatively pure elemental silicon but may include traces of

dopants or impurities as present in silicon semiconductor wafers.

The interior of the oven 23 is evacuated to a pressure of 10^" Torr and

heated to a temperature of 1550°C - 1600°C for a period of 30 minutes. During

this period, the silicon particles 40 vaporize, diffuse through the graphite layer 42

and convert the carbon particles to silicon carbide powder which is manifested by

a green-tinged or yellow-tinged color.

Upon completion of the heating process, the workpiece is retained in the

de-energized, to permit a gradual cooling of the sample in an annealing process.

The workpiece may then be removed from the oven. Example 9

This example is the same as in Example 8, except that the sample is heated

to a higher temperature of 1600°C for 45 minutes, rather than a temperature of

1800°C for 30 minutes. The rest of the procedure is substantially the same as in

Example 8.

Example 10

This example is also the same as Example 8, except that the sample is

heated to a temperature of 2200°C in the furnace for a period of about 15 minutes,

rather than a temperature of 1800°C for 30 minutes as in Example 8. The

remainder of the procedure is the same as in Example 8.

Example 11

According to a modification of Example 8, the carbon particles may be

placed in the bottom of the crucible, and the silicon particles placed thereover,

without the graphite sheet, to first liquefy the silicon to wet the carbon particles,

and then to vaporize the silicon and to react the vapor with the carbon particles, to

produce the SiC particles. The remainder of the procedure may be according to

any of Examples 7 - 10. Example 12

The following example relates to the production of SiC powder for use in

producing SiC tiles for bullet-proof materials to be installed in bullet-proof vests:

Finely-divided particles of carbon are prepared by grinding graphite grains

of 98% purity and 95% purity, respectively, to an average diameter of

10 - 50 microns in a steel ball grinder in which the steel balls have a diameter of 35

mm. Finely-divided silicon particles are also produced by grinding silicon grains of

pure, single-crystal silicon to an average grain diameter of 70 - 90 microns in a

similar steel ball grinder. The two types of particles are then mixed according to the

atomic weights of the two elements (carbon 12, silicon 28) - 1:2.4, i.e.,

100 grams of carbon powder with 240 grams of silicon powder to prepare a

homogenous mixture.

The mixture is then inserted into a vacuum chamber of a furnace, the

vacuum being at least 10^"3 Torr, and the temperature being at least 1800°C. After

achieving the required vacuum, the furnace is turned on for 1-hour until reaching

the diffusion temperature of 1800°C.

At this point, a shutter is removed to initiate the diffusion process of

silicon vapors into the carbon. At the end of a diffusion time of about 1-hour, the

heaters are turned off, but the obtained SiC powder is retained within the vacuum

chamber for 2 more hours before the chamber is opened, to anneal the SiC powder

in order to prevent thermal and mechanical stresses. This so-produced SiC powder is used for making SiC tiles for bullet-proof

materials to be installed in bullet-proof vests, in the following manner:

A mold is filled with the SiC powder produced as described above, and

with additional graphite powder in a 1:1 ratio. Alternatively, the mixture could

include 60% SiC powder and 40% graphite powder, or 70% SiC powder with 30%

graphite powder. The final goal is to achieve an SiC molecule with a specific

weight of 3.1 gr/cm³.

This mixture of SiC powder and graphite powder is then pressed by a

15 ton press for a few seconds in order to remove air gaps within the tile.

Another diffusion process is then effected by inserting the tile into a

vacuum chamber of a 1800°C furnace having a minimum vacuum of 10^"3 Torr.

After achieving the required vacuum, the furnace is turned on for 1 hour until

reaching the diffusion temperature of 1800°C. At this point a shutter is removed,

and the diffusion process of silicon vapors from the SiC powder is initiated into

the carbon. After about 1 hour of this diffusion process, the heaters are turned off,

but the obtained SiC tile is still kept in the vacuum furnace for two additional

hours before the chamber is opened, to anneal the so-produced tile in order to

prevent thermal and mechanical stresses.

While the invention has been described with respect to several preferred

examples, it will be appreciated that these are set forth merely for purposes of

illustrating the invention, and that many other variations, modifications and

applications of the invention may be made.

Claims

WHAT IS CLAIMED IS:

1. A method of producing silicon carbide (SiC), comprising:

introducing into the interior of a furnace a quantity of elemental silicon,

and a quantity of elemental carbon;

subjecting the interior of the furnace to a vacuum;

and heating the silicon and carbon to a temperature of 1500°C-2200°C to

vaporize the silicon and to convert the carbon to silicon carbide.

2. The method according to Claim 1, wherein said silicon is relatively pure

except for possible traces of impurities or dopants.

3. The method according to Claim 1, wherein the quantity of silicon by

weight is in excess of the quantity of carbon, and the excess silicon is removed by

removing excess silicon vapors.

4. The method according to Claim 1, wherein said carbon is in the form of a

mixture of finely-divided particles of carbon in a binder, and said silicon is in the

form of finely-divided particles of silicon applied to the outer surface of said mixture.

5. The method according to Claim 4, wherein said mixture is deformable

and is shaped according to a desired configuration before the finely-divided

particles of silicon are applied to its outer surface.

6. The method according to Claim 4, wherein said mixture is prepared by

mixing said finely-divided particles of carbon in a water solution of sucrose, and

pre-baking said mixture.

7. The method according to Claim 4, wherein said mixture is prepared by

mixing said finely-divided particles of carbon in polyvinyl acetate and pre-baking

said mixture.

8. The method according to Claim 1, wherein said carbon is in the form of

a layer of finely-divided particles, and said silicon is in the form of a layer of

finely-divided particles separated from said layer of finely-divided particles of

carbon by a layer peπneable to silicon vapor.

9. The method according to Claim 8, wherein said silicon- vapor

permeable layer is a sheet of graphite.

10. The method according to Claim 1, wherein said carbon and silicon are

both contained in a graphite crucible within said furnace.

11. The method according to Claim 10, wherein said crucible is at least

partly open at its upper end to the interior of the furnace to permit excess silicon

vapors to escape and thereby to avoid deposition of silicon on the outer surface of

the silicon carbide.

12. The method according to Claim 1, wherein the resulting product, after

being heated, is gradually cooled to room temperature over a period of time

substantially longer than the heating time, before the resulting product is removed

from the furnace.

13. A method of producing a shaped article of silicon carbide (SiC),

comprising:

preparing a mixture of a quantity of carbon in the form of finely-divided

particles mixed in a binder;

shaping said mixture according to the desired shape of the article;

applying finely-divided particles of silicon over the outer surface of said

shaped article;

introducing said shaped article with the finely-divided particles of silicon

thereover into the interior of a furnace;

subjecting the interior of the furnace to a vacuum; and heating the interior of the furnace to a sufficiently high temperature

and for a sufficiently long period of time until a resulting product is produced

having a green-tinged or yellow-tinged color.

14. The method according to Claim 13, wherein the silicon is relatively

pure elemental silicon except for possible traces of impurities or dopants.

15. The method according to Claim 13, wherein the interior of said

furnace is heated to a temperature of 1500°C-2200°C.

16. The method according to Claim 13, wherein the quantity of silicon by

removing excess silicon vapors.

17. The method according to Claim 14, wherein said mixture is prepared

by mixing said finely-divided particles of carbon in a water solution of sucrose,

and pre-baking said mixture.

18. The method according to Claim 13, wherein said mixture is prepared

by mixing said finely-divided particles of carbon in polyvinyl acetate and

pre-baking said mixture.

19. The method according to Claim 13, wherein said carbon and silicon

are both contained in a graphite crucible when heated within said furnace.

20. The method according to Claim 19, wherein said crucible is at least

vapors to escape and thereby to avoid condensation of silicon vapors on the outer

surface of the silicon carbide.

21. The method according to Claim 13, wherein the resulting product,

after being heated, is gradually cooled to room temperature over a period of time

from the furnace.

22. The method according to Claim 13, wherein said mixture is shaped

into the shape of a wire, bar or wafer before the finely-divided particles of silicon

are applied over its outer surface.

23. Silicon carbide produced according to the method of Claim 1.

24. A method of producing silicon carbide (SiC) high-temperature sensor

elements, comprising:

mixing a quantity of finely-divided particles of carbon in a binder; applying finely-divided particles of elemental silicon over the carbon

particles in the binder;

and heating the silicon and the carbon in the binder, in a furnace subjected

to a vacuum, to vaporize and diffuse the silicon and to react the silicon vapor with

the carbon in the binder to convert the carbon to silicon carbide;

said silicon particles being substantially free of dopants to produce a silicon

carbide high-temperature sensor element having a high internal resistance of at least

hundreds of kilohm-cms.

25. The method according to Claim 24, wherein said heating is at a

temperature of at least 1700°C for a period of time until the heated product

assumes a yellow-tinge color.

26. The method according to Claim 25, wherein said silicon is present,

before heating, in an amount which is in excess of the carbon by weight.

27. The method according to Claim 26, wherein said heating temperature

and vacuum are sufficiently high to vaporize the excess silicon and to produce a

50:50 ratio of the silicon and carbon in the resultant product.

28. The method according to Claim 26, wherein said silicon is present,

before heating, in an amount which is in excess of the carbon by at least 10% by

weight.

29. The method according to Claim 26, wherein said heating is effected

while the heated product is under a vacuum of at least 10^"4 Torr.

30. The method according to Claim 24, wherein the finely-divided

particles of carbon are mixed in a water solution of sucrose and the mixture is

pre-baked to harden it, before the finely-divided particles of silicon are applied

thereover.

31. The method according to Claim 24, wherein said carbon particles are

mixed in polyvinyl acetate.

32. The method according to Claim 24, wherein the silicon particles, and

the carbon particles in the binder, are contained in a graphite crucible when heated

within the furnace.

33. The method according to Claim 32, wherein said crucible is at least

partly open at its upper end to the interior of the furnace to permit excess silicon vapors to escape to the interior of the furnace, and thereby to suppress deposition

of silicon on the outer surface of the resulting product.

34. The method according to Claim 24, wherein the heated product, after

being heated, is gradually cooled to room temperature over a period of time

substantially longer than the heating time before being removed from the furnace.

35. The method according to Claim 24, wherein the binder containing the

finely-divided particles of carbon is shaped before the finely-divided particles of

silicon are applied over its outer surface.

36. The high-temperature sensor element produced according to the

method of Claim 24.

37. A method of producing silicon carbide (SiC) heating and lighting

elements, comprising:

mixing a quantity of finely-divided particles of carbon in a binder;

applying finely-divided particles of elemental silicon over the carbon

particles in the binder;

and heating the silicon and the carbon in the binder, while subjected to a

vacuum, to vaporize and diffuse the silicon and to react the silicon vapor with the

carbon in the binder, to convert the carbon to silicon carbide; said silicon particles including a dopant to reduce the internal resistance of

the produced silicon carbide to a value of up to a few hundred Ohm-cms.

38. The method according to Claim 37, wherein said dopant includes an

element from the third or fifth column in the periodic table.

39. The method according to Claim 37, wherein said dopant is present in a

concentration of the order of 1:10^"6 by weight.

40. The method according to Claim 37, wherein said heating is from

1550 - 1600°C for a period until the heated product assumes a green-tinged color.

41. The method according to Claim 40, wherein said heating is effected

while the heated product is under a vacuum of 10^" to 10^" Torr.

42. The method according to Claim 37, wherein said silicon is present,

before heating, in an amount which is in excess of the carbon by weight.

43. The method according to Claim 37, wherein the silicon and carbon are

present before heating in the ratio of 54% silicon and 46% carbon by weight.

44. The method according to Claim 37, wherein the finely-divided particles

of carbon are mixed in a water solution of sucrose, and the mixture is pre-baked to

harden it, before the finely-divided particles of silicon are applied thereover.

45. The method according to Claim 37, wherein said carbon particles are

mixed in polyvinyl acetate.

46. The method according to Claim 37, wherein the silicon particles, and

within a furnace.

47. The method according to Claim 46, wherein said crucible is at least

vapors to escape to the interior of the furnace, and thereby to suppress deposition

of silicon on the outer surface of the resulting product.

48. The method according to Claim 46, wherein the heated product, after

being heated, is gradually cooled to room temperature over a period of time

49. The method according to Claim 37, wherein the binder containing the

silicon are applied over its outer surface.

50. An electrical heating or lighting element produced according to the

method of Claim 37.

51. A method of producing finely-divided particles of silicon carbide

(SiC), comprising:

introducing into a furnace a cmcible containing a layer of finely-divided

particles of carbon and a layer of finely-divided particles of elemental silicon;

subjecting the interior of the furnace to a vacuum;

and heating said cmcible to a sufficiently high temperature for a sufficiently

long period of time to vaporize and diffuse the silicon and to react the silicon vapor

with the carbon particles to convert them to silicon carbide particles.

52. The method according to Claim 51, wherein the quantity of silicon,

before heating, exceeds the quantity of carbon by weight.

53. The method according to Claim 51, wherein said carbon and silicon

layers are both contained in a graphite cmcible when heated within said furnace

and are separated by a layer permeable to silicon vapor.

54. The method according to Claim 53, wherein said cmcible is at least

of silicon on the outer surface of the resulting product.

55. The method according to Claim 51, wherein the interior of said furnace

is heated to a temperature of 1600 - 1800°C for a period of 30 - 60 minutes.

56. The method according to Claim 55, wherein the interior of said

furnace is subjected to a vacuum of approximately 10^" Torr.

57. The method according to Claim 55, wherein the resulting product,

from the furnace.

58. The method according to Claim 51, wherein the silicon particles are

over the carbon particles in the cmcible when heated.

59. The method according to Claim 58, wherein the heating is effected at a

temperature of 1600°C - 1800°C for 30 - 40 minutes.

60. The method according to Claim 59, wherein the vacuum is

approximately 10^"3 Torr.

61. The method according to Claim 51, wherein said layer permeable to

silicon vapor is a graphite sheet.

62. The method according to Claim 51, wherein the heating is effected at a

temperature of 1600°C - 1800°C for 30 - 40 minutes and the vacuum is

approximately 10^"3 Torr.

63. The method according to Claim 51, wherein the carbon particles are

over the silicon particles in the cmcible when heated.

64. The method according to Claim 51, wherein the produced

finely-divided SiC particles are used for producing SiC tiles by:

mixing said produced finely-divided SiC particles with finely-divided

particles of carbon;

compressing said mixture to remove air therefrom; and

introducing said mixture into a furnace subjected to a vacuum of at least

10^" Torr at a temperature of at least 1800°C to cause the silicon from the SiC

particles to diffuse into the carbon.