JPH02289461A - Preparation of dense sintered body of cordierite - Google Patents

Abstract

PURPOSE: To rapidly and inexpensively obtain a sintered body at a relatively low sintering temp. by compressing cordierite powder to form green ware body shaped articles and further condensing these articles by heating up to a specific final sintering temp.

CONSTITUTION: The sintered body is produced by compressing the cordierite having a substantially stoichiometrical particle compsn. and a grain size below 3 μ to form the green ware body shaped articles having relative density of at least 0.687 and sintering these articles at a temp. elevation rate (heating rate) below 5 K/min starting at 800°C until the final sintering temp. is attained in such a manner that the relative density attains ≥0.99 in the process for compressing the cordierite powder to form the green ware body shaped articles and further condensing these articles in the sintering stage at the final sintering temp. up to 1400°C. If the crystalline cordierite powder is used, the final sintering temp. may be confined up to 1400°C and the adequate final sintering is 1300 to 1360°C. There are thus advantages in that the lower temp. is necessitated, the saving of energy and the shortening of the sintering time are made position and the use of a stable sintering furnace is made possible.

COPYRIGHT: (C)1990,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for producing a dense cordierite sintered body. The present invention relates to a method for producing dense cordierite sintered bodies that condense in the sintering process at final sintering temperatures of up to .degree.

[Problems to be solved by the prior art and the invention] Stoichiometric crystalline cordierite (2Ag203.2M
g Oφ5Si02) has a low density, a low coefficient of thermal expansion, and also a low thermal conductivity and electrical conductivity. At the same time, a decrease in strength begins to occur only at 8 degrees above 1000 °C. Possessing such a combination of properties, cordierite contributes to the solution of certain technical problems that are only partially or not completely satisfied by other ceramic materials. Cordierite has held a remarkable position in its own right in the field of ceramic materials because of its ability to meet the requirements of ceramic materials. Therefore, cordierite has been widely used as a catalyst support in automobile exhaust gas purification or as a base material in electrical engineering. However, due to the low strength and fracture toughness of this material, its use as a component with a 6;I heavy load function has hitherto failed.

One prerequisite for improving the mechanical properties is an almost porosity-free molding, or a component with a homogeneous distribution of fine pores without significant fracture-initiating effects. However, the production of dense sintered bodies has hitherto been limited to a considerable extent by the low sintering activity of stoichiometric cordierite, and additional measures must be taken to obtain the desired results. No. In this regard, the following measures can be taken. That is, the introduction of sintering aids or additives (U.S. Patent Specification Box 4,
No. 495.300/U.S. Patent No. 4,745,09
2), or the possibility of sintering chemically modified cordierite to make it glassy and then crystallizing it (Rabinovich, E., M.
inovlch, E, M,): ``Cordierite glass ceramics produced by sintering'', Advances in Ceramics (Advances!n)
Ceralcs), Volume 4 American Ceramics Society (Aserlcan Ceralcs)
5ociety) 1982, pp. 327-333)
However, in either case, the use is limited to high temperatures and the strength increases only slightly.

Sintering pure stoichiometric cordierite to a high density has always been unsuccessful (U.S. Pat. No. 3,928,648), and it is No. 4.745,092, column 1, line 40, 1
' is also clearly emphasized.

Tests conducted within the disclosure of the cited patent ("Comparative Test 1") only resulted in cordierite products with a density of 0.96. US time 5'"Special old handwriting 4.540,
No. 621 describes the production of a substrate material from cordierite by a sintering method, the sintering being carried out in a sealing gas or reducing atmosphere,
Oxidation of the metal layers applied at the same time must be avoided.

In this context, an increase in the fracture strength and fracture toughness of the composite system produced can be achieved by dispersing a second F-order (strengthening component) that has a higher modulus and strength than the cordierite matrix. It is also known that it can be obtained by especially,
It is known to use ZrO2 powder as a reinforcing component, which can be modified from a tetragonal to monoclinic system of ZrO2 in the advancing region of the crack tip as an additional strengthening machine t+D by appropriate selection of particle size and addition of suitable stabilizers. This is because there is an additional possibility of carrying out a phase transformation into a crystalline system (DE 3,445.7-65).

21 Suzerii, Kei, (Nlcszcry, in,)
, Weiskopf, K. L., (W!iβkop1
．． ni, L,), Pctzow,
G.), Pannhorst, W.
Sintering and strengthening of cordierite with varying amounts of zirconia; P. Vincenzinl (editor), Illgh Tcch Cerμmi
cs);Materials Science Seven Graph 38
A (Materials 5science Mono
graph 38A), Amsterdam, Elsevier 1987, pp. 841-849, and Haussman, Kay, (tlaussma
"Improvement of sintering behavior and mechanical properties of cordierite matrix by mixing finely dispersed ZrO2 grains", PhD thesis, University of Stuttgart (19
8g), various cordierite/Z by crushing
It is known to produce rO2 mixing ratios. However, using grinding balls with a diameter of 2-3 mm, it is not possible to obtain a particle size of 90% less than 1.5 μm under the conditions described. For this you will need a fairly small grinding ball (
For example, a diameter of less than 1.5 mm is required. The 4#1 defined and indicated particle size distribution in the micrograph does not represent the true particle size distribution because only randomly distributed cross-sections of the particles are measured. With this method, the maximum diameter of the particles is relatively rarely /l! I+ not determined.

This means that the true particle size is much larger than the measured value.

A green density of 65% of the theoretical density produced by cold isostatic pressing of these mixtures is obtained after sintering, which is reached at a heating rate of at least 10 to 7 minutes. The final density of 0.97 is quite low, as is the lower limit shown. IOK
A heating rate of less than 1/min results in a highly porous product. Relative density 0.95 is pure unreinforced crystalline chemical m
reported as the maximum value for the sintering of theoretical cordierite.

In particular, DE 3,445,785 describes the sintering of cordierite/ZrO2 powder mixture moldings. High density value and high bending fracture strength value (22
In order to obtain
It is sintered at a temperature of 900-1400°C. In contrast, molded parts sintered only in air exhibit significantly lower density and flexural tensile strength. Strength values of less than 15 ON/μm2 have been reported for cordierite sintered in air.

Zirconium oxide 51 corresponding to Zr0230 volume 96
In order to produce a powder mixture of cordierite with a weight of 96 or more and a stoichiometric cordierite (Travltzky, F.A., (Travltzky, N.A., et al.); "Microstructure and Mechanical Properties", German Ceramics Association (DKG) Progress Report; Materials;
Methods, Applications, Volume 2, No. 3, 1986787, 51
- page 58), the corresponding powder mixture is ground in a mill, dried and finally cold isostatically pressed at 3,000 bar. To sinter the green molded article thus produced, a sintering rate of approximately 0°C/min and a
A maximum final temperature of 400°C is likewise recommended. As a result of this sintering process, structures with a grain size of 1 to 3 μm are present in the sintered body. Maximum strength is 200-270
N/m112.

European Patent Application Publication No. 0.255,023 states:
The use of non-stoichiometric cordierite chemically modified with P2O5 and B2O3, which is reinforced with Zr02 grains, has been described. By chemically modifying cordierite, the sintering zone is expanded and crystallization of cordierite is suppressed, so that cordierite can be sintered in a glassy state. However, here too the strength obtained is unsatisfactory, especially at relatively high temperatures.

The production of composite products from silicon nitride and cordierite by sintering at temperatures between 1400°C and 1800°C in an inert atmosphere is described in U.S. Pat.
2. It is described in No. 100. The cordierite component introduced as a sintering aid into silicon nitride is in this case generated in situ for the first time during the sintering process from the required individual oxide components, which have been premixed as powder. . In this case, the formation of cordierite takes place via an interphase, and the individual reactions proceed rapidly enough if only a small amount of ZrO2 is present.

This publication reports strength values only for hot press molded products. Therefore, for example, a molded article of about 40% by volume cordierite has a pressure of 200 N/mm2 to 30
It has a strength of 0 N/mm'. It is not mentioned that the strength of composite products sintered without pressure is always much lower.

It is therefore an object of the present invention that the method is particularly suitable for producing large volumes of cordierite products, that the sintering step is carried out without pressure, and that sintering in air is possible. An object of the present invention is to provide a method for producing a dense cordierite sintered body having a relative density of 99 or more.

[Means to solve the problem]

In order to achieve the above object, according to the present invention, cordierite powder is compressed into a green molded product, and
In the method of condensing in the sintering step at a final sintering temperature of up to , 67 green molded products, 800
Starting at a temperature of 5 °C until reaching the final sintering iR degree
Provided is a method for producing a dense cordierite sintered body, characterized in that sintering is performed at a temperature increase rate (heating rate) of less than 1/min until the relative density reaches 0.99 or more.

[Actions, effects and modes of the invention]

The cordierite powder used consists of cordierite grains with an almost stoichiometric composition.

That is, all cordierite grains have a nearly stoichiometric composition, and many previously known stoichiometric compositions apply only statistically to a large number of grains. This is in contrast to the method of Such powders are suitably produced via the glass phase, since in this case a particularly homogeneous mixture is obtained. The production of cordierite via the glass phase is well known, for example from German Patent Specification Section 2,517.
．． No. 743, Example 2, or U.S. Patent No. 3,
No. 849.145. Apart from small amounts of impurities, which are determined above all by the raw material and the melting crucible, the quality of the cordierite powder produced depends on the cordierite melting throughout the volume, which is achieved by melting the raw material and subsequent homogenization by stirring. Determined by the uniform composition of things. Melting causes the concentration gradient in the molten state to become horizontal. The uniform quality of cordierite cannot be obtained by mixing the individual components followed by a solid state reaction at elevated temperatures, as is commonly used in the production of cordierite materials. Sol/gel processes known per se can also be used to produce high quality cordierite powder.

The cordierite produced by
It is made into a powder with a particle size of less than μm. A particle size of 3 μm or more results in a sintered body with low density and low strength.

Cordierite powder is Z r 02 with a particle size of 3 μm or less.
It can contain up to 50% by weight of flour. 0.1μm
It is preferred that commercially available high-purity zirconium dioxide powder with a microcrystalline particle size of less than or equal to 100% is mixed with the cordierite powder and ground. Zirconium dioxide serves to further strengthen cordierite products in a manner known per se.

Surprisingly, conventional methods contain zirconium dioxide! It is known that the strength does not increase or decreases at 30% by weight or more, whereas 5Of
It has been found that a zirconium dioxide content of up to m% increases the strength of the product. In particular, when using zirconium dioxide powder for reinforcement, it is preferable to use cordierite powder as fine as possible. Advantageously, the finer the cordierite powder, the more frequently successive cracks impinge on the homogeneously distributed reinforcing zirconium dioxide component, ie the greater the reinforcing effect.

The powder or powder mixture thus obtained is then compressed into a green molded article having a relative density of at least 0.67. If this green density is not achieved, it is no longer possible to produce a product with sufficient strength. The powder can be uniaxially pre-pressed and then cold isostatic pressed into a plastic elastic jacket, or directly filled into a plastic (e.g. silicone rubber) elastic jacket and then cold isostatic pressed. Either molding or molding.

Even if the addition of suitable compression aids (binders and lubricants) such as magnesium stearate, carbowaxes, amide waxes etc. may help to obtain high green densities, or if the green moldings are Even if a suitable effect can be obtained by machining,
Addition of compression aids (binders and lubricants) is not necessary for compression molding.

For further consolidation, the cold isostatic press molding is subjected to a sintering process. Air is preferred as the sintering atmosphere, although any known sintering atmosphere can be used as the sintering atmosphere, such as an inert gas, vacuum, or a silicon-containing atmosphere. The sintering is carried out from the starting temperature of the green molding at 800° C. at a certain heating rate (heating rate of less than 5 K/min) until the final sintering temperature is reached. A high heating rate will accelerate the sintering process, i.e. the sintering rate (equal to the change in density per unit time) will increase, and in this connection, pores will be created in the sintered body. It will be mixed. This is a particularly significant obstacle, especially in the case of large-volume sintered bodies, since the low thermal conductivity of the molded material results in distinct temperature distributions within these sintered bodies. be. At high heating rates, this pronounced temperature distribution not only creates a significant pore volume inside the sintered body, but also thermal stresses, which considerably increases the cracking rate during sintering of relatively large products. do. A heating rate of less than 4 K/min is preferred, especially between IK/min and 4 K/min, but it is in this range that
This is because a good sintered body can be obtained in a satisfactory amount of time. Moreover, even with surprisingly small heating rates of less than 5 K/min, final sintering temperatures significantly lower than normal temperatures can be achieved.Heating rates of 0.5 K/min
minutes, since, on the one hand, the sintering time increases rapidly and, on the other hand, there is a risk of undesirable reactions occurring due to an unduly long residence in the high-temperature region.

Low heating rates should begin to be applied at a sintering temperature of 800°C. The temperature limit of 800°C is, if you use cordierite powder that is still in a glassy state.
Above all else, we must persist. For well-crystallized cordierite powder, a small heating rate of 1
It must be maintained only from temperatures of 000°C. Generally, maintaining a low heating rate within the indicated temperature interval is not necessary until the relative density of the product has increased to 0.75.

Particularly in mass production, taking care of this fact can lead to shortening of the production process.

In the production of unreinforced cordierite products from vitreous powders, a final temperature of 1050° C. is sufficient. If crystalline cordierite powder is used, the final sintering temperature can be up to 1400° C. in a manner known per se. However, a final sintering temperature of 1300°C to 1360°C is preferred. This temperature is the commonly used 1
The final sintering temperature is significantly lower than 400 °C, which not only has the advantage of saving energy, shortening sintering time and allowing the use of cheaper sintering furnaces, but also especially if powder mixtures of cordierite and zirconium dioxide If used, it also has further systematic advantages as described below.

After reaching the final sintering temperature, the product to be sintered can be kept at this temperature for a further period of up to 4 hours in a manner known per se to allow further shaping.

In the sintering method according to the invention, the
A holding time of ~1 hour is generally sufficient.

If the subsequent holding time is appropriate, the sintered body becomes 0.
．． When a relative density of 99 or greater is reached, the desired final sintering temperature has been reached. In the process according to the invention, this final temperature is 1050°C for unreinforced vitreous cordierite products and, if crystalline powders are used, between 1300°C and 13BO°C, in particular between 1300°C and 1350°C. be. If the heating rate is low, the final temperature will be in the lower part of the indicated range, and if the heating rate is high, the final temperature will rise to a higher value. Good results are obtained, for example, at a heating rate of IK/min.
A final temperature of 300° C. and a holding time of 1 hour are obtained, whereas a final temperature of 1350° C. and a holding time of 1 hour as well are good at a heating rate of 3/min. As is known per se, the cordierite powder can also contain ZrO2 powder in order to improve the mechanical properties of the final sintered body. ZrO2 grains content up to 50% by weight can increase the strength. The particle size of the Z r 02 powder should be about 3 μm or less.

In this case, pure Zr with a microcrystalline grain size of less than 0.1 μm
O2 powders, especially powders having a crystallite particle size in the range 0.06 μm to 0.02 μm, are suitable. ZrO2 grains having such a microcrystalline grain size are commercially available. Pure Z'02 powder, and MgO1CaO
, Y203, both tetragonally and/or cubically stabilized in a manner known per se, can be used.

However, the final sintering temperature depends not only on the chosen heating rate; if a mixture of cordierite powder and zirconium dioxide powder is used,
Depending on the mixing ratio of these two substances and, to a lesser extent, on the nature of the zirconium dioxide powder used, e.g. whether the zirconium dioxide powder is stabilized or not. is also determined by the particle size of the zirconium dioxide powder. The optimum final sintering temperature can be determined by anyone skilled in the art.
3, using a commercially available sintered dilatometer,
can be easily determined.

The maximum final temperature and holding time also have an effect on the type of microstructure produced in the composite product. When the final temperature is low, the mixed zirconium dioxide powder is 0.1~0.25μ
It has a microcrystalline grain size of m, and forms a network structure along the cordierite grain boundaries. This means that the initially introduced zirconium dioxide microcrystals were sintered together. Higher final temperatures and longer holding times promote the growth of zirconium dioxide grains, which eventually form almost regular particles between cordierite grains as individual grains up to 1 μm in size. until it is evenly distributed at intervals.

Using knowledge of the growth rate of zirconium dioxide crystallites as a function of temperature and time, the maximum particle size of zirconium dioxide can be adjusted in a controlled manner.

In sintering at high final temperatures and long holding times, as is currently practiced in the art, the reaction between the cordierite and the zirconium oxide is initiated to a progressively greater degree, resulting in the formation of a zirconium silicate. It should be noted that the strength is gradually lost as zirconium oxide and spinel are formed, but this is because the reinforcing component zirconium oxide is consumed. This reaction can be observed for samples heated to 1400° C. at a heating rate of IK/min. No such reaction could be detected in the samples where the final sintering temperature was in the preferred range of 1300°C to 1360°C, because the final temperature was significantly lower.

The achieved mechanical strength value is one important criterion for evaluating manufacturing methods. In this case, the strength value is influenced both by the porosity of the final sintered body and by the size of the mixed zirconium dioxide powder. The smaller the pore volume, the fewer defects there will be in the material and the greater the strength obtained. Previous experience has also shown that a strengthening mechanism is obtained as the size of the zirconium dioxide powder increases. The growth process of zirconium dioxide powder, which is dominated by diffusion, is clearly influenced by higher final temperatures and longer holding times, so higher final temperatures and longer holding times are associated with increased strength. do.

However, it should be noted that at higher temperatures and longer holding times, a stepwise reaction between cordierite and zirconium dioxide to form e.g. zirconium silicate is also observed, and this begins to reduce the strength. Must.

Increasing the zirconium dioxide content is also associated with increasing the strength of the sintered particles. What is surprising here is that, contrary to current knowledge, a significant strength increase of 40% by weight was observed in the samples produced according to the present invention.
This phenomenon still occurs even at the above zirconium dioxide powder content.

The advantages achieved by the present invention are, above all, high-strength cordierite sintered bodies or cordierite/
This means that zirconium dioxide sintered bodies can be produced by simple means. Due to the low heating rate, it is possible to produce even large volumes of sintered bodies, in which case the thermal stresses generated during sintering may cause product damage (M) or sintering There is no need to worry that the interior of the body may remain overly porous.

The relatively low final sintering temperature made possible by the low heating rate makes it possible to minimize deleterious reactions of zirconium dioxide with cordierite.

Furthermore, by suitably choosing the final sintering temperature and the holding time at the final sintering temperature, the grain size or growth of the zirconium dioxide microcrystals in the sintered body can be influenced in a controlled manner.

〔Example〕

EXAMPLES Hereinafter, the present invention will be described in more detail and specifically by way of examples, but it goes without saying that the present invention is not limited to the following examples.

Example 1 Cordierite powder or a powder mixture of cordierite and zirconium dioxide was ground with a grinding container made of hard porcelain, and a grinding ball and a stirrer of 85/12 weight ffi%.
+7) Grinding for several hours in an attrition mill consisting of AN 203/S i 02 using Improper Tool as grinding liquid. After pulverization, the pulverized slips were dried under mild conditions and the maximum particle size was measured. The primary results showed that up to about 30% of the cordierite was converted from a crystalline phase to a glassy or amorphous phase as a result of the milling process. 100 bar (ba) from powder
A rectangular block was first preformed in r), which was then cold isostatically pressed at 6000 bar to give a green molding of high relative density (green density). The green molded product was then surrounded by alumina matsuru and sintered in a chamber furnace at 3/min in a sintering range of 800°C to 1350°C. If time was allotted, a hold time was also taken following sintering. The thus produced sintered body was processed into a bending test bar with a size of 3.5 x 4, 5 x 50 mm3 (sic), and a three-point bending test was conducted using a testing machine. In this case, the distance between the fulcrums was 40 mm. The results are summarized in the table.

Example 2 A mixture as in Example 1 was milled for 16 hours with circulating milling in a stirred ball mill equipped with a hard plastic-coated milling vessel and a stirring device. The grinding liquid used was a mixture of 90% by weight water and 10% by weight isopropanol. The diameter of the crushed body used was 0.6 to 11 Im.
It was an alumina ball. After the milling step, the suspension was dried by lyophilization under mild conditions and further processed as in Example 1. The results are also summarized in the table.

Example 3 Composition close to stoichiometric cordierite (S
i 02 about 50.5% by weight, AfI20s about 34.3% by weight, MgO about 14.2% by weight), and 99% of the
The manufacturing method described in Example 1 was applied to a crystalline powder of less than μm.

The sintered body had a relative density of 0.99 or more. The bending strength values can be found from the attached table (Test 1).

Example 4 Glassy stoichiometric cordierite (99% grain size 40 μm (s i
c) and 30% by weight of Y2O3 stabilized zirconium oxide, cold-opened green moldings were produced. 8 of one green molded product at 2.5K/min
Heating was carried out in the sintering interval from 00 to 1400° C., and once the final temperature was reached, the furnace was switched off and cooled down.

At 800-900° C., the sample is sintered in a glassy state with a relative density of 0.68-0.80, and the sintering process ends with crystallization of the cordierite. The density of the crystalline product remains almost constant between 900 and 1200°C. Further heating to 1400°C followed by cooling results in a relative density of 0.995
sintered body exists at room temperature.

Example 5 A vitreous, ie, amorphous powder (99% less than 40 μm) having the composition according to Example 3 was subjected to the grinding and drying treatment described in Example 1.

After sieving, the ground material was isostatically pressed into a cylindrical shape in a silicone rubber mold under a pressure of 6000 bar. The green molding was then heated at 10° C./min to 500° C., kept at this temperature for 30 minutes, and finally heated at 4.5 K/min to 1050° C. and then cooled to room temperature. Products sintered in this manner exhibited relative densities of 0.99 or higher.

Test magnets 1 to 5, 11 and 12 were conducted according to Example 1, and tests NQ and 6 to 10 were conducted according to Example 2. All samples had 90% of the particles in the powder mixture 1.3μ
It shows a particle size distribution smaller than m. 50% value is 1μm
less than or equal to. The heating rate chosen for the sintering of the composite products starting at 800°C was always 3K1 min. The final densities obtained were always above a relative density of 0.99. Y2O to strengthen cordierite
Zirconium dioxide stabilized at 33 mol % was used without exception. The average bending strengths given in the table are based on measurements on 6 bending test bars in each case.

Claims (11)

[Claims] (1) In a method in which cordierite powder is compressed into a green molded product and further condensed in a sintering process at a final sintering temperature of up to 1400°C, the particle size has an almost stoichiometric particle composition. Cordierite powder of less than 3 μm is compressed into a green molded article with a relative density of at least 0.67, starting at a temperature of 800° C. and increasing the temperature by less than 5 K/min until the final sintering temperature is reached. A method for producing a dense cordierite sintered body, which comprises sintering at a heating rate until the relative density becomes 0.99 or more. (2) The method according to claim 1, characterized in that crystalline cordierite obtained by crystallizing raw material cordierite glass is used. (3) The method according to claim 1, wherein the cordierite powder is vitreous. (4) A method according to any one of claims 1 to 3, characterized in that the sintering is carried out at a heating rate of less than 4 K/min. (5) 5K/only when a relative density of 0.75 is reached.
5. Process according to claim 1, characterized in that the sintering is carried out at a heating rate of less than 1 minute. (6) When using crystalline cordierite powder, 1
Less than 5K/min or 4K only at a temperature of 100℃
6. A method according to any one of claims 1 to 5, characterized in that sintering is initiated at a heating rate of less than /min. (7) The method according to any one of claims 1 to 6, characterized in that sintering is carried out to a final sintering temperature of 1300°C to 1360°C. (8) Any one of claims 1 to 7, characterized in that 90% by weight of the cordierite powder has a particle size of less than 1.5 μm, and 50% by weight of the powder has a particle size of less than 1 μm. The method described in. (9) Cordierite powder is Zr with a particle size of 3 μm or less
9. A method according to any one of claims 1 to 8, characterized in that it contains up to 50% by weight of O_2 powder. (10) The method according to claim 9, wherein the ZrO_2 powder has a microcrystalline grain size of less than 0.1 μm. 11. Process according to claim 1, characterized in that, when the sintering temperature has been reached, the sample is maintained at this temperature for a further period of up to 4 hours, in particular for up to 1 hour.