JPH0218348A - Ceramic powder, dispersion thereof and control of ceramic body - Google Patents

Abstract

PURPOSE: To obtain a ceramic powder providing a ceramic substance prevented a binder from being confined in the inside of the ceramic in a dewaxing and densifying process, that is, having no residual carbon inside the ceramic by applying a specific covering material on the ceramic powder having internal volume.

CONSTITUTION: A ceramic powder, which is suitable for preparing a mixture for suitably molding a green ceramic substance by being dispersed in a binder, is obtained by coating a ceramic powder (e.g. Si₃N₄) having an internal surface and internal volume corresponding thereto with a covering material (e.g. poly(vinyl pyrrolidone)) substantially decreasing the infiltration of the binder (e.g. paraffin wax, PE wax) for the dispersed body to the internal volume of the powder.

COPYRIGHT: (C)1990,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION FIELD OF INDUSTRIAL APPLICATION This invention relates to dispersions of ceramic powders that are starting materials for ceramic objects that are directly useful. More specifically, the present invention provides additives that can make the shaping of the dispersion easier, dispersions made with the dispersion, methods of making and using the dispersion, and Concerning products made using dispersions.

PRIOR ART AND PROBLEM SOLVED BY THE INVENTION One of the most common and economical methods of making ceramic objects is to - form a ceramic powder dispersion containing a temporary binder. This includes the process of attaching. This binder holds the shaped powder together at the forming temperature and also generally at room temperature if it is different from room temperature.
It serves to keep objects of a cohesive but usually brittle type known as raw objects or raw articles. (Ceramic objects are often referred to as "green" or "green articles" even at later stages of processing when they are still substantially more brittle than after final densification. ) The raw material is directly heated by controlled heating, which both drives off the binder and binds together the previously separated particles of the ceramic powder by volatilization, combustion, or other chemical reactions. Can be turned into ceramic objects that are useful.

The method of dislodging the binder is herein designated as "dewaxing" (this is a common term in the art, derived from the fact that the binder is often a wax); (Applicable even if the binder has other chemical properties.) Any method of converting the ceramic components from separated or only weakly self-bonded powders into a strongly cohesive body - is applicable here for boat fishing. In this case, it will be displayed as "densification". The term therefore refers to sintering (here, heating without the application of external pressure at a temperature high enough to bond the ceramic particles together by some method that reduces the porosity of the object formed from the powder). (used to mean), - encompasses separate types of processes known as axial compression molding, isostatic compression molding, etc.

Dewaxing and densification sometimes proceed simultaneously, but more often little or no densification occurs until dewaxing is complete, and densification almost always occurs after dewaxing is completed and the dewaxed object is removed. is started or continued by heating to a higher temperature.

Depending on the ceramic fraction of the dispersion and the time and temperature used for densification, it is possible to produce products that are impermeable or have controlled porosity.

Particularly if an impermeable final product is desired, it is advantageous to have as high a ceramic powder content in the dispersion as is practical.

This is because the time required for densification is thereby reduced, as is the shrinkage of the object during densification. - In terms of grading, ceramic powders with small particle sizes are preferred because finer powders tend to densify more easily.

Despite the recognition that both small particle size and high powder content of the dispersion are advantageous, small particle size and high content result in highly viscous dispersions;
There are practical limits to enhancing both of these properties of a dispersion. 2000 measured with a capillary rheometer
Viscosity higher than poise is considered impractical for commercial injection molding of complex shapes, and 400 to 10
A viscosity range of 0.000 poise is commonly employed for this molding technique. Note that a lower viscosity is employed for slip casting.

It is customary in the industry to measure the specific surface area of ceramic powders as a rough indication of powder particle size. As a result of the mathematical fact that the ratio of surface area to volume of any particular solid shape, such as a sphere, increases as the size of this solid decreases, the surface area of very fine powders should naturally be large. However, it is well known in the art that a portion of the specific surface area of an actual powder is not directly related to the powder size, but rather to internal porosity or surface depression. dimple,
That portion of the powder surface area that is attributable to the presence of pores, or the like, is herein designated as the "internal surface," while all such hollow spaces are filled to the surface of each powder particle. The portion that remains when a surface is made into a smooth, flat or convex surface corresponding to the outermost portion of its actual surface is designated herein as the "external surface."

The internal surface and associated volume of ceramic molding powders is usually completely undesirable without some compensating benefit. Pores in the powder will result in porosity, which is usually undesirable for the final product, or unexpected and random mechanical failure, which is always undesirable. The internal surface adsorbs surfactants that may be added to the powder or dispersion to reduce viscosity. These surfactants are usually expensive, so the need to use more of them as a result of adsorption on internal surfaces presents a series of economic obstacles to the use of powders with high internal surfaces. It's possible.

Another problem is the twist-shaped internal hole (ca.
vities) that binder may be trapped if the only exit from the hole is blocked by local densification before all the binder is driven away. Such entrapped binder can result in residual carbon in the ceramic and/or mechanical failure of the object. Sometimes the risk of binder entrapment can be avoided by increasing processing time, but this also increases costs. Although dewaxing times of one week or more are occasionally reported, the actual use of such long times is extremely rare for commercial applications.

Another practical challenge well known in the art is that inconsistencies in molding results often occur between one loft of powder and another. Since the amount of internal surface is subject to variation from factors that are not always well controlled during the production of practical powders, formulations for molding dispersions optimized for a given type of powder. The table will often be shown to be inappropriate for subsequent lots manufactured under supposedly identical conditions. This variability is believed to be due to the varying amount of internal surface from one powder loft to another.

Although all of these internal surface and volume disadvantages in ceramic molding powders have long been known in the art, practical methods of producing molding powders for most ceramic compositions have always resulted in significant Few, if any, means are known in the art for providing powders with a large amount of internal surface and volume and heretofore overcoming the disadvantageous effects of such powders.

One practically important example of a dispersion suitable for injection molding is silicon nitride powder dispersed with paraffin, polystyrene, and/or polyethylene binders. In the prior art, the maximum powder content that is possible for such dispersions without exceeding practical viscosity limits for injection molding is such that the average particle size and specific surface area of the silicon nitride are approximately 1 and 1, respectively.
2 to 15 m"/g, it was mentioned that it was about 80% by weight. Powder contents in the range of 85 to 87% by weight were used using powders with a larger particle size and a specific surface area of 8 m"/g. However, such powders do not sinter as well as those with a specific surface area of at least 12 m''/g. It has been found that it is impractical to achieve a final product density higher than 98% of the theoretical maximum density for silicon nitride when using thicker silicon nitride powders. For the final ceramic product, the highest possible density is generally preferred.

Storm, U.S. Pat. No. 4,207,226, issued June 10, 1980, discloses that in order to reduce viscosity and increase the practical loading of fine silicon carbide powder in organic binders, The use of titanate coupling agents is described. The teachings of this Storm patent are directed only to the processing of sinterable metal carbides. (See Storm, column 1, lines 11-13 and column 3, line 41.) Similarly, 1
No. 4,056,588 to Banel, issued Nov. 1, 977, teaches the use of coupling agents for oxide and carbide ceramics.

-Due to the peculiarities of surface effects in boat chemistry, it is unlikely that teachings on the treatment of carbides and/or oxides will reveal the ability of these materials to improve the treatment of silicon nitride. .

Powder dispersions primarily rely on acid-based interactions between powders,
The above is particularly true as it is believed to be dependent on the dispersion medium and any dispersion aids used, and also because the surface of silicon carbide is generally acidic whereas the surface of silicon nitride is generally basic. It is certain.

[Means and Effects for Solving the Problems] The difficulty arising from the presence of internal surfaces and volumes in ceramic powders is that the dispersion is coated with
It has been found that significant improvements can be made by adding appropriate substances labeled as ``Ver material''. This coating material is capable of reducing the specific surface area of the ceramic powder with which it is used by at least 10% of its original value, and by densification a ceramic object is created which is directly useful. It is completely driven off during dewaxing from the ceramic powder used at the same time, so that no significant amount of residue from this material is present in the ceramic object made from the dispersion afterwards. It should be a substance that can be used. The coating material is preferably added to the dispersion by coating the ceramic powder with it before dispersing it in a conventional binder.

An important advantage of coating powders in accordance with the present invention is that after application of the appropriate amount of coating material, the dispersion has different amounts of powder on the internal surface that do not have the coating taught herein. This means that it behaves more similarly than powders do. Thus, powders coated according to the invention can be processed with significantly better consistent results.

The coating materials described herein are applicable to any ceramic powder that has a significant amount of internal surface, and
It is suitable for use with any such ceramic powder. The coating material should have sufficient adhesion to the ceramic powder to be coated, but generally the minimum amount of adhesion required is relatively small and easily obtained.

The loading of silicon nitride powder and zirconia powder in the thermoplastic binder can be determined by using suitable organometallic coupling agents in the dispersion without thereby increasing the viscosity of the dispersion to impractical levels. It has also been found that , can be significantly increased. A coupling agent is defined here as a substance which does not significantly reduce the specific surface area of the ceramic powder when dispersed therein, and which may be present in a dispersion of ceramic powder up to about 5% by weight of the ceramic. When present in an amount that reduces the viscosity of the dispersion to a level below that which a dispersion of the same formulation would otherwise have except that the coupling agent replaces an equivalent weight of other components of the dispersion medium. Defined as a substance that lowers

The coupling agent according to the invention must be selected with knowledge of the powder and the binder to be used. This is because the beneficial effect of the coupling agent appears to depend on the type of adsorption to the powder, which depends on the specific chemical interaction with the powder, and on the nature of the binder.
This is because, for example, if the binder competes as an adsorbent, it may be suppressed.

The average particle size of the silicon nitride powder that can be processed according to the invention is generally less than or equal to two digits. Preferably, the average particle size of the silicon nitride powder ranges from 0.7 to 2 JIF@, more preferably about 1.2-.

To carry out injection molding, the amount of silicon nitride powder of 1 size in a conventional binder containing paraffin and polyolefin is mixed with organosilanes, organotitanates, or It can be increased from 80% to 87% by weight by adding 0.3 to 3.0% by weight of organozirconates. As a result of the higher loading, the time to dewax the green body and the possibility of shrinkage and green body cracking during conversion of the green body to the final silicon nitride ceramic body are all reduced. Similar benefits can be obtained with zirconia powder using various organometallic coupling agents.

If a suitable coupling agent is known, it is usually advantageous to use both a coating material and a coupling agent to make a dispersion from any particular powder.

When slip casting is used instead of injection molding to form green objects, the total loading level is lower for slip casting than for injection molding, as is known in the art; Similar benefits can be achieved using coating materials and/or coupling agents.

If the coating material is to be applied in essentially pure form,
The coating material should be liquid and at a suitable coating temperature so that it can easily fill at least the outer portion of the pores and/or pores on the surface of the powder particles. Viscosity and spreading wetting for powders
The angle should be relatively small. Alternatively, the coating material may be dissolved in a volatile solvent and applied to the powder from solution. Any solvent that may be used to apply the coating material should be easily driven out of the hole during drying, dewaxing, and/or densification without leaving any significant amount of residue. , nor should it have any significant tendency to chemically react with the ceramic to be coated.

The amount of coating material used can be readily determined by experimentation with each type of powder.

That is, only enough coating material should be used to reduce the specific surface area of the powder by at least 10%, and generally enough coating material should be used to reduce this specific surface area as much as possible. Most preferably, it is used.

The most suitable coating materials have molecular weights of 100 and 10.000.
and at least 1 with a molecular weight of 100
It is an organic substance that has 500 oxygen atoms. An example of this is
Includes oligomers and polymers of ethylene oxide, vinylpyrrolidone, and diethylene glycol, vinylpyrrolidone, and diethylene glycol.

A preferred organometallic coupling agent for silicon nitride powder or zirconia powder in a paraffin-polyolefin binder may be represented by the general formula R-Z (O-Xb-QC) a. In this formula, R and Q are organic groups, typically alkyl groups having 1 to 25 carbon atoms,
represents an alkoxyl group or an aminoalkyl group, and R and Q may be the same or different; Z
represents a silicon atom, a titanium atom, or a zirconium atom, X represents an inorganic atom or atomic group, and a represents 1 and 4.
, d is an integer between 1 and 4, and a+
d=4, b is 0 or 1, and C is an integer between 1 and 4. Preferably, 0-x is selected from the group consisting of phosphate, phosphite, pyrophosphate, or sulfate.

For silicon nitride powders, R in the above general formula should be a group that is easily substituted by reaction with protons. This is because this additive causes proton transfer cleavage (proto-transfer cleavage) of this bond.
lytic cleavage) and the formation of alternative bonds to the particles, which are believed to provide strong adhesion to the silicon nitride powder. The proton source for this reaction is
It is believed to be a surface compound that naturally forms on silicon nitride powder in the presence of humid air. Q preferably has at least 6 carbon atoms so as to provide substantially free volume for internal lubricity through van der Waals force entanglement and good processing properties for the dispersion.

These organometallic coupling agents have a specific surface area of about 1 to
15 m'/g, but most useful for powders with a specific surface area of 8 to 15 m'/g.

The organometallic coupling agent may be dissolved in the molten binder, but is preferably dispersed onto the surface of the silicon nitride powder before it is added to the binder. This dispersion can be accomplished by dissolving the coupling agent in a volatile solvent, spraying the solution onto the powder, or slurrying the powder with the solution, and then driving off the solvent.

For injection molding purposes, the amount of silicon nitride powder free in the paraffin-olefin binder is preferably about 82-85% by weight. To perform slip casting,
Lower powder loadings of up to about 60% by weight are preferred. At a given loading level, the viscosity of the casting slip was reduced by approximately one third from the value obtained without any coupling agent.

The use of both a coupling agent and a coating material in accordance with this invention is generally highly preferred. Coupling agents are often capable of reducing viscosity to a greater extent than the coating material reduces viscosity when used alone. However, coupling agents do not have the beneficial effect of making the behavior of powders of different lofts more uniform than do coating materials. Additionally, the use of coating materials allows the use of less coupling agent to obtain the same amount of viscosity reduction, which is an important economic factor given the high cost of most coupling agents. It is.

The method of this invention may be advantageously applied to silicon nitride powder made by the diimide method. The chemical purity of such powders is higher than more common silicon nitride powders made by nitriding powdered silicon. However, powders obtained from the diimide method have been difficult to use in injection molding.

This difficulty has been attributed to both its small surface area and relatively uniform grain size, the former making sintering difficult and the latter inhibiting high packing densities. Applying the materials and methods of this invention gives much better results for these powders obtained from the diimide process.

This invention can also be advantageously applied to zirconia powder. For example, the viscosity of a typical 92% by weight formulation of easily sinterable zirconia powder to a paraffin-type binder was greater than 4000 poise; The viscosity of a dispersion of the same powder applied at 1% by weight is 140°C.
It was only 345 poise. This dispersion was successfully injection molded to a width of 0.2 mm.
It was possible to produce parts with slots in two orthogonal directions that were as narrow as 3 mm deep, as well as other complex shapes that were generally difficult to mold. The zirconia capacity of the above 92% by weight zirconia dispersion was 62% by volume. It is believed that no previously known injection molded Qin'M object of zirconia has a zirconia content greater than 88% by weight.

The scope and practice of the invention may be better appreciated from the examples provided below.

〔Example〕

Example 1 A silicon nitride powder with a nominal average particle size of 1.2 J- and a specific surface area of about 12 m''/g is mixed with a 2% methanol solution of poly(vinylpyrrolidone) having a molecular weight of 10.000, and this solution is added to the above powder. The powder was treated by spraying and then dried in an oven at about 80° C. for 24 hours. The specific surface area decreased to about 10.5 m"/g after treatment and the powder was treated with a conventional binder of paraffin and polyethylene wax. A readily injection moldable mixture having approximately 82% by weight silicon nitride was obtained.

This example is the same as Example 1, except that the coupling agent neoalkoxytris(dodecylbenzenesulfonyl)zirconate was applied to the powder after the coating material described above was dispersed onto the powder surface as described in Example 1. Met. The viscosity of the dispersion formed was lower than the level achieved in Example 1, and a dispersion with a viscosity of less than 1000 poise at 140° C. with 85% by weight silicon nitride dispersed in the binder was obtained. , and successfully injection molded.

Example 3 In this example, a zirconia powder with a specific surface area of about 7 m''/g was used which also contained about 4.5% by weight of ittria. Diethylene glycol was used as the coating material and Example 1
applied in the same way. The surface area decreased to approximately 4 m'/g. Using vinyltris(2-methoxyethoxy)silane as the coupling agent, a dispersion with 92% by weight zirconia in a conventional paraffin-polyolefin-stearic acid binder with sufficient injection molding properties was obtained.

Claims (7)

[Claims] 1. A ceramic powder having an internal surface and a corresponding internal volume that is generally suitable for dispersion in a binder to create a mixture suitable for forming green ceramic objects; Ceramic powder characterized in that it has a coating material on its surface which substantially reduces the penetration of the binder into said internal volume of the powder when dispersed in the powder. 2. In a binder comprising primarily paraffin wax, polyethylene wax, or mixtures thereof, particles having an average particle size of 2 μm or less and a specific surface area of at least 1
A dispersion comprising at least 85% by weight of silicon nitride powder of 1 m^2/g and having a viscosity at 140°C of 1000 poise or less. 3. It comprises at least 89% by weight of zirconia powder with a specific surface area of at least 6 m^2/g in a binder mainly comprising paraffin wax, polyethylene wax, or a mixture thereof, and has a viscosity at 140°C of 1000 poise or less. A dispersion. 4. Comprising 60-90% by volume of ceramic powder,
Aggregated bodies whose remaining volume consists essentially of organic binders. 5. The steps of: (a) coating a ceramic powder having an internal volume with a coating substance that substantially reduces penetration of the binder of the dispersion into the internal volume of the powder; (b) (a) dispersing the coated powder from a thermoplastic binder to make a dispersion suitable for molding; (c) molding and optionally compressing this dispersion to form the final ceramic material; (d) driving off substantially all of the thermoplastic binder from the green body and converting the green body into a cohesive green body having the shape and approximate size desired for the body; 1. A method for preparing a ceramic object of predetermined size and shape, comprising: exposing the green object to heat and a suitable fluid atmosphere to convert it into a unitary, strongly cohesive, refractory ceramic object. 6. The steps of: (a) preparing a silicon nitride powder having an internal surface and an internal volume comprising primarily paraffin wax, polyolefin wax, or mixtures thereof, and also having the formula R
_a-Z-(O-X_b-Q_c)_d (In this formula, R and Q represent an alkyl group, an alkoxy group, or an aminoalkyl group having 1 to 25 carbon atoms, and R and Q are the same O represents an oxygen atom, Z represents a silicon atom, a titanium atom, or a zirconium atom, b is 0 or 1, and b
is 1, O-X is selected from the group consisting of phosphate, phosphite, pyrophosphate, and sulfate, and a is an integer between 1 and 4, and d is 1
and 4, where a+d=4 and c is an integer between 1 and 4). 0.
dispersing in a thermoplastic binder comprising from 3% to 5% by weight to make a dispersion suitable for injection molding; (b) injection molding the dispersion into a final ceramic object; (c) driving off substantially all of the thermoplastic binder from the green body and converting the green body into a unitary, cohesive green body having a desired shape and approximate size; 1. A method for preparing a ceramic object of predetermined size and shape, comprising: exposing the green object to heat and a suitable fluid atmosphere to convert it into a strongly cohesive refractory ceramic object. 7. The steps of: (a) dispersing a zirconia powder having an internal surface and internal volume in a thermoplastic binder comprising primarily paraffin wax, polyolefin wax, or mixtures thereof, suitable for injection molding; (b) injection molding the dispersion to form a cohesive green body having the desired shape and approximate size of the final ceramic object; (c) exposing the green body to heat and a suitable fluid atmosphere to drive off substantially all of the thermoplastic binder from the green body and convert the green body into a unitary, strongly cohesive, refractory ceramic body; A method of preparing a ceramic object of predetermined size and shape comprising the steps of of the formula R_a-Z-(O-X_b
-Q_c)_d (In this formula, R and Q represent an alkyl group having 1 to 25 carbon atoms, and R and Q may be the same or different, O represents an oxygen atom, and Z Represents a silicon atom, a titanium atom, or a zirconium atom, b is 0 or 1, and when b is 1, O-X is selected from the group consisting of phosphate, phosphite, pyrophosphate, and sulfate. and a is an integer between 1 and 4, d is an integer between 1 and 4, a+d=4, and c is an integer between 1 and 4). A method characterized in that it contains a coupling agent consisting essentially of one or more compounds.