JPH0218311A - Hexagonal silicon carbide platelet and preform, and its production and use - Google Patents

Abstract

PURPOSE: To obtain crystalline silicon carbide excellent in heat resistance, suitable for a reinforcing material or the like and having a specific crystal structure by heating a porous silicon carbide precursor composition containing silicon and carbon in the presence of a hexagonal system crystal growth controlling additive to a high temp.

CONSTITUTION: In a porous silicon carbide precursor composition containing silicon and carbon brought into close contact with each other about 0.3-5% hexagonal system crystal growth controlling additive selected from boron, Al or the like to the weight of silicon in the composition is added. Next, heating is carried out at 2,100-2,500°C for about 3 min to 24 hr under a non-reactive atmosphere. As the result, the crystalline silicon carbide, which consists essentially of α-silicon carbide, ≥90 wt.% of which is formed from plural hexagonal system crystal lattice, ≥80 wt.% of which has parallel basic plane parts opposed to each other apart from a 0.5-20 μ distance, in which ≥2 adjacent crystals with an angle of 120° has a 2.5-150 μ distance between the angles and which has <10 visible flaws per 100 crystals under a magnification of 200, is obtained.

COPYRIGHT: (C)1990,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to silicon carbide for use as a reinforcing material in other materials, more specifically silicon carbide in a previously unknown shape that can be used for the above and other purposes; The present invention relates to a new method for producing silicon carbide, a method for using the same, and a new reinforced product obtained.

BACKGROUND OF THE INVENTION There is currently a worldwide research effort to utilize silicon carbide voice cars and fibers as reinforcement materials. Currently known voice ker, i.e. single crystals, usually with high aspect ratios, are mainly composed of silicon carbide in the β-phase or cubic structure, so that the β-phase fractionated silicon has limited thermal stability. It is unsuitable for manufacturing ceramic composite materials that require treatment at temperatures of 1800° C. or higher.

Despite this, attempts have been made to use such β-phase ratio silicon as a reinforcing material.

Examples of such prior art include the following.

Brennan, et al., U.S. Pat. No. 4,41
No. 0,635 discloses a ceramic composite reinforced with discontinuous silicon carbide fibers, which starts with a ceramic matrix material in a glassy state, which is transferred from the glassy state after densification of the composite. It is formed by converting it into a ceramic state.

U.S. Pat. No. 4,399,23 to Prewo et al.
No. 1 discloses a glass composite reinforced with discontinuous silicon carbide fibers, in which the silicon carbide fibers are oriented in a substantially in-plane random direction through the use of silicon carbide paper.

U.S. Pat. No. 4,387,0 for Latta et al.
80, 4.467.647 and 4,467.
No. 042 is a metal or non-metal element such as Si, B
discloses the production of flaky β-phase silicon carbide from organosilicon polymers containing Ti, Fe, Al, Zr, Cr, and the like. The organosilicon polymer is formed into a thin sheet, which is then treated to prevent melting. This sheet is later treated with a non-oxidizing gas such as N2, ■1□, N
Heated to high temperature in H3, Ar and CO gas atmosphere. This heat treatment is carried out at a temperature not exceeding 1800°C. The resulting product is flake-like β-phase complexed silicon. The infusible sheet of organosilicon polymer can be cut into small flake-like pieces, each of which has a thickness of
100 times longer and wider, and these pieces are β-
Can be converted to silicon carbide flakes. Further, flaking can also be carried out after heat treatment of a large sheet. composite materials,
The use of thin sheet or flake materials for thermal stress distribution in, for example, rubber, plastics, metals, and concrete, has also been disclosed. Flaked β-silicon carbide with a width and length ranging from 10 to 100 times the thickness is used as a resi-silicon carbide extruder.
sting breakage). However, such β-silicon carbide does not have heat resistance.

Alien U.S. Patent No. 3,661.662
No. 1, No. 1, No. 1, No. 1, No. 1, No. 1, No. 1, No. 1, 2003, 2003, discloses a method for making a sheet material in which flakes of silicon carbide or boron carbide are suspended on a pool of liquid metal that is inert to the flakes and aggregated on the surface of the pool. The flakes are bonded together to form a sheet and the sheet is removed from the pool surface. The binding material is an organic resin.

α-Silicon carbide including α-silicon carbide having a hexagonal crystal structure is known. However, such materials are not particularly suitable for use as reinforcement. This is because it has heretofore been impossible or at least not practicable to obtain silicon carbide with a structure of sufficient purity and integrity to function as a consistently good reinforcement.

Separate large, typically O01-3C11, usually intergrowth hexagonal crystals often spontaneously form Acheson (A
cheson) seen when synthesizing silicon carbide using the electric resistance furnace method. However, such crystals are generally too large and too small in quantity compared to the pre-silicon carbide formed in the furnace to be used as a reinforcing material. Even if such crystals are collected separately and crushed into small sizes, they do not provide good reinforcement. This is because such a crushing operation causes a large amount of damage to the final crushed material particles, as well as particles of undesirable shape and size.

US Pat. No. 3,962,406 to Knippenberg et al. discloses a method for making resistivity silicon carbide crystals. In this method, a core of silicon dioxide is embedded in granular silicon carbide or a material forming silicon carbide. The temperature at which silicon dioxide vaporizes this lump, that is, about 1
When heated to 500° C. or higher, a cavity surrounded by silicon carbide is formed. After the cavity is formed, heating is continued at a temperature of about 2500° C. or higher. At this temperature, flat silicon carbide crystals are formed in the cavity walls.

Another patent disclosing the production of hexagonal silicon carbide crystals is US Patent No. RE 26,941 to Lowe. This patent describes the preparation of thick (and highly pure) crystals for electronics applications, such as rectifiers and transistors, by a time-consuming and painstaking vapor deposition process. The crystals are up to 0.75 inches in diameter. So, the thickness is 1~1
00 mils (25-2540μ) (e.g.
(See column 5, lines 59-61). Such materials are generally too bulky for many reinforcement applications.

American Matrix Inc., Knaxville, Tennessee
+Inc, +Former Phoenix International (P
hoenixIn terna t 1ona 1)
) recently reported the availability of α-silicon carbide particles for reinforcing composite materials. However, the method for making this material has not been disclosed. When these particles are viewed under a microscope, they are found to be crushed large particles. The product appears to be a mixture of various structures of various shapes: needles, powders and fragments (including some hexagonal crystalline material). These particles have a large number of defects and are of low purity according to analysis.

In general, those skilled in the art will know that α- or β-
It is known that there are many ways to prepare -type silicon carbide. However, a thin single-crystal hexagonal plate tray made of more stable α-type silicon carbide
There is no suggestion or teaching that platelets could be intentionally formed, nor how this could be accomplished if desired, or whether such platelets would have any unexpected utility. There is no teaching.

Furthermore, the base face is 0.5 to 20.
There is no suggestion of a porous silicon carbide matrix with a small hexagonal crystal structure separated by μ, nor is there any suggestion or suggestion of the use of such a matrix.

(Problems to be Solved by the Invention and Means for Solving the Problems) According to the present invention, crystalline silicon carbide is provided, and at least 90% by weight of the silicon carbide is composed of a plurality of hexagonal crystal lattices. At least 80% by weight of the crystals formed from the lattice include at least opposing parallel elementary plane portions separated by a distance of 0.5 to 20μ.

The silicon carbide may be in the form of discrete particles and at least 70% by weight of the particles may be composed of a single crystal lattice, usually platelet-like.

Additionally, the silicon carbide may have a monolithic porous structure including intergrowth. This porous structure may be either continuous or closed pore system. Preferably, the porous structure has a porosity in the range of 5 to 80% by volume and an average pore size in the range of 1 to 100 microns.

The present invention also encompasses articles formed by impregnating a reinforced matrix and porous structure into particles of the present invention. This matrix and impregnated material
It can be glasses, ceramics, metals, and polymers, including both amorphous glasses and glass-ceramics. After impregnation or initial formulation, some amorphous glasses can be converted into fine-quality ceramics. Examples of metals that can be used are various steels, aluminum and metal alloys. The reinforced matrix and impregnated products of the invention are characterized by good strength, fracture resistance and improved thermal conductivity. Furthermore, if the matrix material is a high temperature material such as sintered alpha-silicon carbide, the product is characterized by exceptionally high stability.

The silicon carbide of the present invention is prepared in an inert atmosphere, e.g. nitrogen, inert (rare It is prepared by heating a porous silicon carbide precursor composition comprising silicon and carbon in intimate contact under an atmosphere of gases and mixtures thereof for a time sufficient to form the crystals.

The growth control additive is typically a metal from Group 1 of the Periodic Table of the Elements, generally selected from boron, aluminum and mixtures thereof. Boron and aluminum can be used in boron compounds and aluminum compounds, such as AlaCs, AIN
, 84C or BN, the control additive is not the compound, but the aluminum or boron in the compound.

The growth control additive is typically present in an amount of 0.3 to 5% by weight (based on silicon in the precursor).

The most useful precursor compositions are β-silicon carbide powder and Si
It is selected from a mixture of particulate silica and carbon or a mixture of particulate silicon and carbon in the amount stoichiometrically required to form C. The average particle size of the precursor material is usually about 0.
．． 005-5μ, but larger particle sizes, for example 100μ, can be used if the precursor material, such as silica, forms gaseous products before reaction.

Coarse powder precursors are used when loose particles are desired, and agglomerated precursors are used when an interlocked porous structure is desired.

The novel products of this invention are useful in improving the strength of a variety of materials, including improving the stiffness of key metals and the toughness of ceramics. In addition, materials found to be useful in improving the heat transfer properties of various materials provide improved thermal shock resistance to ceramics, function as insulators, and improve the corrosion resistance of many materials. It can be improved. The products of the invention also have good oxidation and chemical resistance.

The silicon carbide for the purpose of the present invention is α-silicon carbide as opposed to β-type silicon carbide. Alpha-silicon carbide is preferred in the present invention due to its high heat resistance and good structural integrity.

In particular, the present invention includes silicon carbide of the type formed from a "hexagonal crystal lattice", that is, a crystal lattice that, when grown freely, forms hexagonal silicon carbide crystals. There are several types of crystal lattices of this type, including mixtures of lattice types (polytypes), which can be utilized in the present invention, but the most common are those of the 6H and 4H crystal forms. When grown freely and completely, these hexagonal silicon carbide crystals defined by a hexagonal crystal lattice form "late" crystals at their ends.
It is characterized by two parallel hexagonal shaped "base faces" connected by a "ral face".

Silicon carbide crystals formed from a hexagonal crystal lattice are not always perfect. That is, although the basic planes are not always perfectly hexagonal, the crystal has at least partially opposing parallel basic planes, and the basic planes are between 0.5 and 20
It is characterized by being separated by a distance of μ. In this disclosure, such imperfect crystals are nevertheless referred to as “
"hexagonal crystal structure" and the crystals are called "hexagonal crystals". The silicon carbide product of the invention always contains a plurality of such crystals, i.e. always more than 1000, usually 10 .000 or more such crystals.

The silicon carbide of the present invention contains loose hexagonal crystal grains,
Alternatively, it may be an interlocked structure including interlocked hexagonal crystals.

If the silicon carbide comprises discrete particles, at least 70% by weight of the particles are in a plate trend shape. These platelets are single crystal grains with parallel elementary planes, where on average the longest distance across each of said elementary planes is at least 6 times, usually at least 8 times, the distance between said elementary planes. be.

In platelets, the ratio of the largest dimension to the smallest dimension of the cardinal surface is usually between 1 and 3. Each of the cardinal faces of this platelet has at least two adjacent 120' corners (c
It is desirable that the crystal structure contains a hexagonal crystal structure.

Such corners are typically separated by a distance of 2.5 to 150 microns.

The granular crystalline silicon carbide of the present invention is characterized by an even lower damage rate or failure. When observed under a magnification of 200 with an optical microscope, the 100 base planes observed
) less than 10, usually less than 2, cranks or damage were observed per ). Shapes resulting from incomplete crystal growth are not considered damage, breakage, or cranks.

When silicon carbide has a monolithic porous structure, it includes a plurality of interlocking crystals formed from a hexagonal crystal lattice. For purposes of this invention, an intergrown crystal is considered to be another crystal. The portion of the crystal that has no detectable damage remains at 0.
It is characterized by having at least opposing parallel basic surface portions separated by a distance of 5 to 20 microns. This internally porous structure typically has a porosity of 5 to 80, preferably 35 to 65% by volume and an average pore size of 1 to 100 microns.

The pore structure may be either continuous or independent, depending on the intended application.

The silicon carbide of the present invention is characterized in that it has a high purity, ie a purity of more than 95%, often more than 99%, with a weight proportion of characteristic impurities of less than 0.5% iron, 0.5% of the sum of boron and aluminum. 03-3%, free silicon 0.5%, free carbon and all other impurities total less than 0.5%. Most of the boron and aluminum impurities are due to the addition of these elements as growth control additives. Much of the remaining boron and aluminum is surface contamination from internal boron and aluminum impurities, which are below the solid solution limit in silicon carbide, ie, less than about 3.000 ppm for boron and less than about 4,000 ppta for aluminum.

The silicon carbide products of this invention are useful in a variety of reinforced structures. This product can impart strength, fracture toughness and impact resistance while improving or maintaining thermal stability. Free platelets can be incorporated into various matrices as reinforcing aids.

Such matrices include glasses, including amorphous glasses and glass ceramics, crystalline ceramics, metals and polymers. Examples of amorphous glasses include silicate glasses and glasses that can be substantially converted into glass ceramics. Examples of ceramics to be strengthened include silicon carbide, alumina and zirconia. Essentially all metals can be strengthened, especially alloys of iron (steel) and aluminum, as well as other metal alloys. Such alloys may be high temperature alloys, ie superalloys. Polymers that can be reinforced with the platelets include both thermoplastics and thermosets,
Examples include polyolefins, vinyl resins, nylons, polycarbonates, and epoxy resins. When such platelets are used for reinforcement, they are usually present in an amount of about 10 to about 70% by volume.

Intergrown porous structures can be impregnated with certain materials to form strong and impact resistant articles. Examples of such materials that can be impregnated with this porous structure include amorphous glasses and polymers, including glasses that can be substantially converted into glass-ceramics, and metals as mentioned above. Ceramics can be incorporated into the matrix by steam impregnation.

The method for producing silicon carbide of the present invention involves forming a porous carbide containing silicon and carbon in close contact at a temperature of 2100 to 2500°C in the presence of a hexagonal crystal growth control additive and in a non-reactive atmosphere. heating the silicon precursor composition. here,
A non-reactive atmosphere is a vacuum or any atmosphere containing a gas that does not significantly adversely affect hexagonal crystal growth of alpha-silicon carbide, excluding gaseous reaction products. Suitable gases include nitrogen and noble gases. Oxidizing atmospheres are generally undesirable. This reaction is allowed to occur for a sufficient time to result in the formation of silicon carbide crystals.

The precursor composition may be a liquid or particulate polycarbosilane or a chemical combination of silica and carbon, such as silicon carbide. Silicon carbide is always in granular form, including crystallites, whiskers, needles, or powder, and is generally (but not always) β-silicon carbide. The precursor may also be a close blend of reactants capable of forming silicon carbide, with an appropriate stoichiometry of silicon to yield alpha-silicon carbide containing multiple hexagonal crystal lattices. and carbon or a blend of silica and carbon. where at least 80 of the crystals formed by the lattice
The weight percent includes at least opposing parallel basic surface portions separated by a distance of 0.5 to 20 microns. The silica used may be in granular form or in the form of a sol. Without intending to be bound by a particular theory, it is believed that when silica, silicon, or carbon is used, silicon carbide passes through the β-form before forming an α-hexagonal crystalline structure.

The precursor materials provide a mixture of reactants within 10 mole percent of the stoichiometric amount according to the following formula:

1, β-5iC → α-5iC 2, Si+C → β-5iC → α-5iC3, SiO
To satisfy the stoichiometry of the three formulas such as 2+3C-β=SiC+2CO-I α-3i, the precursor material contains about 58 to about 66% by weight silicon and 34 to 42% by weight.
of carbon, preferably about 60 to about 65 weight percent silica and about 35 to about 40 weight percent carbon.

This precursor material must be extremely fine and tightly blended. The average particle size of this precursor material is typically 0.
It is in the range of 0.005 to 2μ. If the precursor material is silica, larger particles can be used, such as 100 microns or more. The reason for this is not completely understood but may be related to the formation of silicon-containing gaseous products at the reaction temperature.

This precursor material is desirably of high purity and can be obtained from any suitable source.

The most common carbon source is colloidal-sized carbon (carbon black or lamp black). Typical particle sizes for carbon blanks range from about 0.01 to 1 micron.

The silica used can be fumed silica or can be obtained by incinerating seed husks, especially rice husks, in air. Other seed shells can also be used, such as babassu coconut shells. Silica can have particle sizes as low as 0.005 microns. Therefore, the particle size of the silica can vary over a wide range, for example from 0,005 to 100 microns or more.

A mixture of silica and carbon in a weight ratio of about 1=1 can be obtained by pyrolyzing seed husks, for example rice husks, at 800° C. under an inert atmosphere. Various other silica-carbon ratios can also be obtained by varying the pyrolysis temperature in the range of 400-1000°C.

In this method, the silicon and carbon in the precursor must be in intimate contact. When p-silicon carbide is used as a precursor, this contact is at the molecular level. If carbon is used as part of the precursor, it must have a very small particle size, eg less than 1 micron, and must be well mixed with the silicon source material. The silicon source material may be of larger than normal particle size. This is because it is believed that they are at least partially evaporated at the reaction temperature and come into contact at the molecular level.

This reaction must be carried out in a non-reactive atmosphere, for example in nitrogen or an inert gas such as argon. It should be understood that this reaction can be carried out under reduced or superatmospheric pressure. If β-silicon carbide is used as precursor, a high vacuum can be used, the use of gas being excluded.

β-silicon carbide powders and whiskers are known to those skilled in the art, such as those taught in U.S. Pat. No. 3,340.020, U.S. Pat. can be obtained by the following method.

Desired beta-silicon carbide precursors have an average particle size of 0.1 to 2 microns.

The precursor must be porous and loosely packed, and if individual crystal grains, such as platelets, are desired, the precursor must also be individual particles. If an intergrowth structure is desired, the precursor material should be agglomerated, which can be accomplished, for example, by mixing the materials in a liquid, then removing the liquid and drying the product. If necessary, to increase cohesion, −
A small amount of a temporary binder, such as a resin, may be included in the liquid. The porosity of the starting material should be in the range 20-70% by volume, and the average pore size is usually 1-70% by volume.
It is within the range of 100μ.

A hexagonal growth control additive is any additive that promotes the growth of alpha-silicon carbide hexagonal crystal platelets. Such additives are usually metals selected from Group 11IA of the Periodic Table of the Elements, in particular boron, aluminum and mixtures thereof. If the growth control additive is present in the precursor, it is in an amount of 0.3 to 5% by weight, preferably 1 to 3.5% by weight (based on the amount of silicon in the precursor composition). It is.

The growth control additive can be a fine, e.g. 0.4-5 micron powder that is blended into the precursor material, or it can be vaporized during heating, e.g. from an impregnated crucible, or a growth control additive that is not blended with the precursor material. An amount of control additive can be provided in the gas phase by vaporization. When using steam, aluminum or boron undergoes mass transfer from the atmosphere to the silicon carbide produced, resulting in at least 3
00 ppm at sufficient vapor pressure to form a solid solution.

This growth control additive is placed at the bottom of the crucible, for example,
and can be vaporized via the precursor material at the top of the heated core crucible. The growth control additive is initially a compound of boron or aluminum, e.g.
It can be provided as IN, B4C, BN, or as metallic boron or aluminum. Regardless of how the additive is included, it should be understood that the growth control additive is boron or aluminum and not a compound thereof. Unless stated otherwise, calculations herein are therefore based on the weight of boron or aluminum.

Any suitable non-contaminating, heat resistant crucible can be used, for example crucibles made of graphite, SiC, 84C or BN.

Heating temperature is 2100-2500°C. Its preferred range is 2150-2400°C. When using nitrogen gas, higher temperatures are usually required, such as 2250° C. or higher, while lower temperatures can be used with inert gases.

The time required to form a given crystal structure according to the method of the invention is usually between 3 minutes and 24 hours, and most often at least 15 minutes. The most common heating times are 10 minutes to 1 hour.

A specific method for preparing silicon carbide of the present invention includes the following steps.

A. Mixing together finely divided silica, finely divided carbon and finely divided boron or a boron-containing compound. However, the amount of SiO□ relative to C is the reaction s SiO□+3C=SiC+2
It is about 90-110% of the stoichiometric amount of CO, and the amount of elemental boron is 0.35-3.5% based on silicon. . and B, heating the mixture in argon at 2100-2500''C for a time sufficient to convert the mixture into hexagonal platelets of silicon carbide, wherein the crystals initially may or may not be intergrowth depending on whether the starting materials consist of separate particles or are aggregated.

This mixture is most commonly heated using a heat resistant crucible, such as a graphite crucible.

A specific example of fabricating a rigid and porous SiC article is 100
Parts by weight of finely ground β-silicon carbide powder; 0.25 to 2 parts by weight of elemental boron or aluminum of a hexagonal crystal growth control additive selected from elemental boron, elemental aluminum, boron-containing compounds, aluminum-containing compounds and mixtures thereof. ．．
30 to 50 parts by weight of denatured ethanol, 5 to 8 parts by weight of polyvinyl alcohol, 1 to 2 parts by weight of oleic acid, and the resulting dry mixture; 100 to 200 parts by weight of a blend of deionized water. The entire mixture is blended in a high shear mixer and the resulting homogeneous composition is then spray dried. This dry mixture is then compressed to 30-50% of the theoretical density of silicon carbide and then heated in argon at a temperature of 2150-2400 °C for 10 minutes to
Bake for 1 hour. The resulting product will be an open-pore ceramic article consisting of intergrowth crystals (of alpha-silicon carbide crystals in a hexagonal crystal lattice). Bulk density is 30-70% of theoretical density
It is. If this procedure is carried out in the absence of polyvinyl alcohol and oleic acid, and if the premix is dispersed at low bulk density before calcination, individual hexagonal crystals will be obtained.

Seed husks, such as rice husks or babassu coconut husks, can be used to form single crystal platelets of alpha-silicon carbide in a hexagonal crystal lattice. For example, rice husk in particular has a
It is possible to obtain amorphous SiO□ by ashing at 00°C, and rice husks can be thermally decomposed in an oxygen-deficient atmosphere.
This is about s o o ”c and about 1:1 weight ratio of C and Si.
It gives a residue containing O□.

Therefore, rice husk can be used as a source of silica and carbon. When using a 1:1 weight ratio of silica to carbon, the stoichiometry requires additional silica. Additional silica is
It can be provided by any suitable source, including the ashed rice husk described above. In such cases, approximately 30 to 36
Parts by weight of ashed rice husks will be ground together with lOO parts by weight of pyrolyzed rice husks.

This mixture is then heated in an inert gas containing boron or aluminum vapor or in nitrogen at a temperature of about 2150 to about 250
Heat at a temperature of 0° C. for a time sufficient to form discrete platelet crystals of α-silicon carbide.

(Example) EXAMPLES The present invention will be explained in more detail with reference to Examples below.

Example 1 120g of β-silicon carbide powder with an average particle size of 0.3μ is 2% in width
It was filled into a graphite crucible measuring 9% inch long and 1 inch deep. The powder contains less than 0.06% total iron, less than 0.28% free silica and 0.44% free carbon.
Including less than. The powder was sieved through a 60 mesh cloth before charging. The coarse powder was charged to a depth of about 1 inch. 0.5% crystal growth control additive based on charged β-5iC,
Added to the bottom of the crucible before adding the silicon carbide powder. This crystal growth control additive contains 0.61 g of one 325 mesh metal boron and 0.81 g (7) 100/t'J
It consisted of a mixture with Affi4C: + powder.

The capped vessel containing the charge was passed through a 6 inch 10 graphite tube furnace with a 2300°C hot zone in the center under a nitrogen atmosphere. The residence time in the high temperature region was about 1 hour.

The resulting product was a coarse powder, which under an optical microscope was hexagonal silicon carbide platelets with an average aspect ratio (largest dimension divided by thickness) of about 12 and an average largest dimension of 130μ. Ta.

The yield of hexagonal crystals was 95% by weight or more.

FIG. 1 shows a 400x SEM optical micrograph of a typical platelet made according to this example. This platelet is characterized by exhibiting extremely low fragmentation rate and high purity.

Example 2 Approximately 77 g of 1325 mesh aluminum oxide powder was
A 16 ounce plastic container was charged with a 0.5 inch diameter 50 aluminum oxide cylindrical grinding media and 0.39 g of submicron magnesium oxide. Distilled water was added to fill the container to 374 ml, and the mixture was milled in a roller mill overnight.

The mixture was removed from the mill and poured into a glass dish, which was placed on a cold hot plate while stirring to prevent the aluminum oxide from settling.

20% by weight of the platelets obtained in Example 1 were gradually added to thicken the slurry. When the slurry became too viscous to stir, the dish was removed from the hot plate and placed in an oven for final drying at approximately 70°C overnight. The product was then cooled and sieved through a 40 mesh sieve.

This product was then heated at 1550°C 12000psi
It was hot pressed in a 2% inch graphite malt. The product was easily removed from the malt and was very uniform and compact. This final product sample has a thickness of 0.
It had a diameter of 311 inches and a diameter of 2.49 inches. The density was about 98% of the theoretical value.

Example 3 Silicon carbide platelets as prepared in Example 1 were placed into malt. The platelets were in a rough condition and had a porosity of about 57% by volume. Approximately 765°
The molten aluminum alloy of C (89% Al 9% Sis and 2% Mg) has an impregnated width of about 17
, 500 psi was applied. The resulting product was characterized as essentially porous, ie, containing from about 0.5 to about 31% by volume silicon carbide. The Young's modulus of this composite material was approximately 20,000,000 psi, compared to approximately 10,000 psi for unreinforced metal.

Example 4 150g of β-silicon carbide powder with an average particle size of about 0.3μ was
．． It was mixed in a beaker with 5 g of submicron boron powder.

25% polyvinyl alcohol aqueous solution 9.5g, oleic acid 1.5g, denatured ethanol 50cc and water 200c
c was stirred by hand. The mixed solids were then added to the liquid mixture in a blender and mixed for 3 minutes.

This blended slurry was then spray dried. 75 g of this spray dried powder was compressed to a volume of 63.5 cc.

The density of this green material was approximately 37% of theoretical. This compacted powder was fired at 2150"C in argon for about 1 hour. The density of the resulting preform after firing was about 45% of the theoretical density of SiC, rich in hexagonal crystal forms, about 5
It had a porosity of 5%. A 260x SEM optical micrograph of a typical preform obtained substantially in accordance with this example is shown in FIG.

Example 5 A preform prepared substantially according to Example 4 was
9 wt% Al1, 9 wt% Si, 2 wt% M
at about 765° C. and under pressure up to about 17,50 Qpsi.

The resulting product is characterized by almost zero porosity and contains about 51% alloy by weight and about 49% by weight
of silicon carbide.

Example 6 Approximately 60 g of silica (approximately 240 mesh), 36 g of carbon black, and 0.96 g of submicron boron powder were
Stirred in 0 cc of acetone for 20 minutes.

This mixture was dried in air overnight. The dry mixture was stirred by hand and sieved through a 100 mesh stainless steel sieve. The sieved powder was then placed in a graphite crucible, covered, and heated at 2150°C in argon.
It was fired at.

The conversion rate of raw material to SiC platelets is approximately 100% of the theoretical value. The average aspect ratio is in the range 10 to 15.

FIG. 3 shows a 500x SEM optical micrograph of a -400 mesh fraction of platelets made substantially according to this example. The other halves show similar morphology and distribution.

Example 7 125 g of 325 mesh silicon powder was ground in 200 cc of hebutane and 8 cc of denatured ethanol using 2200 g of cylindrical 0.5 inch diameter x 0.5 inch (height) silicon carbide grinding media. It was ground in a vibrating mill for 24 hours.

This ground slurry was dried overnight. The loss of grinding media was 3.02 g. The average particle size after pulverization was 0.75μ.

Three batches using the above ground material were made using different growth control additives as shown in Table 1 below.

-1111 Si metal carbon black acetone growth control additive 1° 35g + 1g grinding media wear amount 0 cc 1.02 g submicron boron powder λ 35 g + 1 g grinding media wear amount 0 cc 1.02 g aluminum powder (-400 mesh ) 3° 35 g + 1 g Grinding media wear 0 cc 0.51 g Boron powder 0.54 g Aluminum powder Each of the above batches was mixed for 20 minutes using a magnetic stirrer and dried overnight.

These samples were fired in a graphite tube furnace at 2100-2150°C under argon for about 1 hour.

The yield is more than 90% free and interlocked hexagonal crystals. 1750x SEM optical micrographs of Samples 1, 2, and 3 are shown in Figures 4, 5, and 6, respectively.

These samples were analyzed by X-ray diffraction to determine the crystal form. As shown in Table 2, these are almost completely 4H
and 6H hexagonal crystal.

Member - Mataichimyo Sample Nα crystal form α-5iC6H main α-3iC4H low α-5iC15R low Example 8 Mix the rice husks in a blender, Lord Lord Lord Lord low low low 20 mesh sieve I passed it.

50g of the rice husk was placed in a crucible and heated at 800° in a nitrogen stream.
A mixture of amorphous Sing and carbon (approximately 1
=1 ratio) was obtained.

The pyrolyzed rice husks were ground to approximately -200 mesh in a vibratory mill.

36 g of ground, pyrolyzed rice husks and 12 g of fumed silica were mixed in a blender for 1 minute.

In a cylindrical graphite container with an inner diameter of 3 inches and a depth of 2.5 inches, 0.4 g of submicron boron powder and 0.55 g of 1100 mesh aluminum carbide powder, 0.02 g
50 cc of a growth control additive coating suspension containing polyvinyl acetate and 50 cc of ethanol was charged. This suspension was dried to form a film on the bottom of the crucible.

14.5 g of the pyrolyzed rice husk-fumed silica mixture was then charged to the crucible.

The crucible was covered and heated for 1 hour at 2250-2300°C in the main heating zone in a tube furnace under nitrogen.

The yield of silicon carbide platelets was about 75% of theory. Plate red prepared substantially in accordance with this example is shown in FIG. 7 as an optical micrograph at 400x magnification.

The platelets have an average diameter of less than 150μ and an aspect ratio (maximum dimension/thickness) of about 15-20.

Example 9 3.55 g of silicon carbide platelets containing a -60/+200 mesh size fraction were mixed with 20 g of submicron α-
3iC powder, 0.13g B, C and 1 as carbon source
% of phenolic resin.

Then, it was dried.

Discs with a diameter of 1 to 1 inch were heated to 20.00 g of the mixture.
Prepared by compacting in Opsi to a raw density of 55% of theoretical. Pressureless sintering at 2150℃,
This was done by holding the temperature for 2+A hours under an argon atmosphere. The density of the obtained sintered material was 82.
It was 7%. The fracture surface of this example was examined using SEM, but
The results showed that the platelets were well preserved in the matrix.

[Brief explanation of drawings]

FIG. 1 is a 32M optical micrograph showing a 400 times magnification of the crystal structure of a typical example of the plate obtained in Example 1, and FIG. 2 shows a 260 times magnification of the crystal structure of a typical example of the preform obtained in Example 4. FIG. 3 is a 500x SEM optical micrograph of the crystal structure of the −400 mesh fraction of the platelet obtained in Example 6, and FIGS. 4 to 6 are photographs of Example 7, respectively. FIG. 7 is a 32M optical micrograph at 1750 times magnification of the crystal structure of Samples 1 to 3, and FIG.
This is an optical micrograph at 00x magnification. Procedural amendment (method) % formula % ■, Indication of the case 1988 Patent Application No. 142735 3, Person making the amendment Relationship with the case

Claims (27)

[Claims] (1) At least 90% by weight of the crystalline silicon carbide is formed from a plurality of hexagonal crystal lattices, and at least 80% by weight of the crystals formed from the lattices are arranged oppositely at a distance in the range of 0.5 to 20μ. these crystals comprising at least parallel elementary plane portions and having at least two adjacent 120° angles, with a distance between the corners ranging from 2.5 to 150 μ;
Crystalline silicon carbide, characterized in that the crystals have less than 10 visible flaws per 100 crystals under 200 magnification. (2) The crystalline silicon carbide according to claim 1, wherein the silicon carbide is in the form of separate particles, and at least 70% by weight of the particles are platelet-shaped single crystals. (3) The silicon carbide contains a hexagonal crystal growth control additive selected from the group consisting of boron, aluminum, or a mixture thereof, and the growth control additive is 0.03 to 3% by weight based on the silicon carbide. 2. The crystalline silicon carbide of claim 1, wherein the crystalline silicon carbide is present in an amount of . (4) The crystalline silicon carbide according to claim 3, wherein the number of visible cracks in the basic planes is less than 2 per 100 basic planes when observed with an optical microscope at a magnification of 200. (5) The silicon carbide has an integral porous structure including intergrowth, and the structure has a porosity in the range of 5 to 8% by volume, and the average pore diameter is in the range of 1 to 100μ. The crystalline silicon carbide according to claim 1. (6) The silicon carbide according to claim 5, wherein said structure has a porosity of 35 to 65% by volume. (7) The silicon carbide has a purity of 95% or more, and
．． The silicon carbide of claim 1, comprising less than 5% free silicon, less than 0.5% silica, and less than 0.5% free carbon. (8) A reinforced article comprising a silicon carbide reinforced matrix according to claim 1 above. 9. The article of claim 8, wherein said matrix is selected from the group consisting of ceramic materials, metals, organic polymers, glasses, or glass ceramics. (10) Claim 9 wherein the ceramic material is silicon carbide.
Items listed. (11) The article according to claim 9, wherein the metal is aluminum. (12) A reinforced article comprising silicon carbide according to claim 5, impregnated with a material selected from the group consisting of metals, ceramics, polymers, or glasses. (13) The silicon carbide according to claim 1, wherein the hexagonal crystal lattice is mainly of 6H type, 4H type, or a mixture thereof. (14) at least 95% pure α-silicon carbide, including at least 1,000 α-silicon carbide crystals, at least 90% by weight of the crystals being formed from a hexagonal crystal lattice; at least 8 of the crystals formed from
0% by weight contains at least opposing parallel elementary plane portions separated by a distance of 0.5 to 20μ, and those crystals with at least two adjacent 120° angles have a distance between the corners of 2.5 to 150μ. a porous silicon carbide precursor comprising silicon and carbon in intimate contact; The precursor composition comprises a hexagonal crystal growth control additive selected from the group consisting of boron, aluminum and mixtures thereof in an amount of from about 0.3 to about 5% by weight of silicon in the precursor composition. in the presence of a non-reactive atmosphere for a time sufficient to cause the formation of said crystals for about 3 minutes to 24 hours. Method. (15) the growth control additive is present in an amount of 1 to 3.5% by weight of the silicon in the precursor composition, and the precursor composition is β-carburized with an average particle size of 0.1 to 2μ; silicon powder,
and the composition comprises about 58 to about 66% by weight silica and about 3% by weight silica.
Claim 1: A dense mixture with 4 to 42% by weight of carbon.
The method described in 4. (16) the mixture comprises about 60 to about 65% by weight silica and about 35 to about 40% by weight carbon, the silica has an average particle size of about 0.005 to about 100μ, and the carbon 15. The method of claim 14, having an average particle size of about 0.01 to about 1 micron. 17. The method of claim 14, wherein said precursor comprises a mixture of submicron silicon and carbon in a weight ratio of 70% silicon and 30% carbon. 18. The method of claim 14, wherein the precursor is a porous aggregate and the resulting silicon carbide product is in a porous structure comprising interlocked silicon carbide crystals with a hexagonal crystal lattice. 19. The method of claim 14, wherein the precursor is a coarse powder and the resulting silicon carbide product is in discrete particles, and at least 70% by weight of the particles are single crystals. 20. The method of claim 14, wherein the precursor has a porosity of 30 to 90% by volume, and the average diameter of pores in the precursor ranges from 1 to 100 microns. (21) Claim 14, wherein the growth control additive is in a gas phase.
Method described. 22. The method of claim 14, wherein said growth control additive is in the gas phase and said heating time is about 5 minutes to 24 hours. (23) Fine β-silicon carbide powder is heated under an inert gas, nitrogen gas, or vacuum at a temperature of at least about 2150 °C and 2
heating at a temperature not exceeding 400° C. for at least 15 minutes to convert substantially all of the β-silicon carbide powder into single crystal platelets of α-silicon carbide;
Method for forming separate platelets of α-silicon carbide. (24) The β-silicon carbide has an aluminum or boron vapor pressure sufficient to cause mass transfer from the atmosphere to Si to provide a solid solution containing at least 300 ppm boron or aluminum or a mixture thereof in the silicon carbide. 24. The method of claim 23, wherein the method is heated in the atmosphere containing (25) The fine β-silicon carbide powder contains boron, aluminum or a mixture thereof to form combined boron and aluminum in an amount of 0.3 to 3.5% of the silicon weight in the β-silicon carbide. 24. The method of claim 23, wherein the method is blended with a powder containing an amount sufficient to . (26) 0.3 to 3.5% of the silicon weight in α-silicon carbide
The β-silicon carbide whiskers are heated to a temperature of at least 2100° C. in an inert gas or in nitrogen to a temperature of at least 2250° C. in the presence of an additive selected from boron, aluminum and mixtures thereof. At the temperature
1. A method of forming single crystal platelets of alpha-silicon carbide comprising maintaining for a sufficient time to convert into hexagonal platelets of alpha-silicon carbide. (27) A, finely ground silica, finely ground carbon and finely ground boron or a boron-containing compound are mixed together, where the amount of SiO_2 relative to C is the reaction: SiO_2+33C=SiC
approximately 90-110% of the stoichiometric amount of +2CO and the amount of elemental boron is 0.35-3.5% based on silicon; and B, at a temperature of 2100-2500 °C in argon. ,
1. A method for producing hexagonal platelets of alpha-silicon carbide, the method comprising: heating the mixture for a period of time sufficient to convert the mixture into hexagonal platelets of silicon carbide.