GB2228932A - Process for producing dense-sintered cordierite bodies - Google Patents

Description

t Process for oroducing dense-sintered cordierite bodies

The invention relates to a process for producing dense-sintered cordierite bodies according to the preamble of Patent Claim 1.

Stoichiometric crystalline cordierite (2A'20, X 2MgO x 5SiO2 ) has a low density, a low coef f icient of thermal expansion and a low thermal and electrical conductivity. At the same time, a loss of strength starts only at temperatures above 1000C. Owing to the combination of the said properties, cordierite can thus contribute to the solution of certain technical problems or meet demand criteria which can only be partially satisfied or not at all by other ceramic materials, so that cordierite has retained a remarkable position on its own in the field of ceramic materials. Thus, cordierite has been widely used as a catalyst support in the exhaust gas purification of motor vehicles or as a substrate material in electrical engineering. The use as a com ponent with a load-bearing function has, however, failed hitherto because of the low strength and fracture tough ness of the material. I A prerequisite for an improvement of the mechanical properties is, however, an almost pore-free moulding or a component having a homogeneous distribution of minute pores without a critical fracture-initiating effect. The production of dense-sintered bodies has, however, hitherto been restricted to a considerable degree by the low sintering activity of stoichiometric cordierite, so that additional measures must be taken in order to achieve the desired result. Regarding this point, the following may be mentioned: the introduction of sinter aids or additives (US Patent Specification 4,495,,300 / US Patent Specification 4,745,092) or the possibility of sintering chemically modified cordierites until glassy and then to crystallize them (Rabinovich, E. M.: Cordierite glass-ceramics produced by sintering. Advances in Ceramics, volume 4, American Ceramics Society 1982, pages 327333) which in both cases restricts use at high temperatures and leads only to slight increases in strength.

The sintering of pure stoichiometric cordierite up to high densities has here always failed (US Patent Specification 3,926,648) which is explicitly stressed again in, for example, US Patent Specification 4,745, 092 in column 1, line 40. An experiment carried out within the scope of the cited patent ("Comparative Experiment 111) leads only to a cordierite body having a density of 0.96. US Patent Specification 4,,540,621 describes the production of substrate materials from cordierite by a sintering process which must take place in a blanketing gas or a reducing atmosphere in order to avoid oxidation of the metallic layer which is applied at the same time.

In this connection, it is also known that an increase in the fracture strength and fracture toughness of the composite system produced can be obtained by dispersing a second phase (reinforcing component) having a higher modulus of elasticity and a higher strength than the cordierite matrix. In particular, it is known to use ZrO2 particles as a reinforcing component, since there is the additional possibility in this case of effecting a phase transformation in the Zr02 from tetragonal to monoclinic in the process zone of a crack tip as an additional reinforcing mechanism by appropriate selection of the particle size and by appropriate stabilizing additives (German Offenlegungsschrift 3,445, 765).

From Nieszery, K.; Weifikopf, K.-L.f Petzow, G.; Pannhorst, W.: Sintering and strengthening of cordierite with different amounts of zirconia. In: P. Vincenzini (editor): High Tech Ceramics; Materials Science Monograph 38A. Amsterdam: Elsevier 1987, pages 841-849) and (Haussmann, K.: Verbesserung des Sinterverhal tens und der mechanischen Eigenschaften einer Cordieritmatrix durch Einlagerung feindisperser Zr02-Teilschen [Improvement of the sintering behaviour and mechanical properties of a 1 cordierite matrix by incorporation of f inely dispersed Zr02 particles], doctorate thesis, Stuttgart University (1988) it is known to produce various cordierite/Zr02 mixing ratios by grinding. However, the grinding balls of 2-3 mm diameter used do not allow a particle size of 90 % < 1.5 pm to be obtained under the conditions described. Considerably smaller grinding balls (for example smaller than 1.5 mm diameter) are necessary for this purpose. The indicated grain size distribution, measured on a micrograph, does not indicate the real grain size distribution, since only randomly distributed section faces of the grains are measured. The largest diameter of a grain is relatively rarely measured in this method. It follows from this: the real particle size is considerably above the measured values.

The green density of 65 % of the theoretical density, produced by coldisostatic pressing of these mixtures, is comparatively low, as is the indicated lower limit of the final density of 0.97 achieved after sintering, which is reached at a rate of temperature rise of at least 10 K x min-'. At a rate of temperature rise of less than 10 K x min-', significantly more porous bodies are obtained. A relative density of 0.95 is reported as the maximum value for the sintering of pure unreinforced crystalline stoichiometric cordierite.

German Offenlegungsschrift 3,,445,765 describes, inter alia, the sintering of compacts of cordierite/ZrO. powder mixtures. To obtain high densities and high bending fracture strength values (220-310 N/MM2), the coldisostatic ally pressed mouldings are sintered therein in an Si- containing atmosphere at temperatures between 900 and 14000C. Mouldings sintered only in air show, by contrast, a significantly lower density and bending tensile strength. Strength values of less than 150 N/mm2 are reported as the strength values of cordierite sintered in air.

To produce powder mixtures of more than 51 % by weight of zirconium oxide, corresponding to 30 % by volume of Z ro?. I and stoichiometric cordierite (Travitzky, N. A. et al.: Microstructure and mechanical properties of a cor.dierite-Zr02 dual composite. Fortschrittsberichte der DKG: Werkstoffe, Verfahren, Anwendung; [German Ceramic Society progress reports: Materials, Processes, Application]; volume 2, issue no. 3 (1986/87, pages 51-58), corresponding powder blends are ground in an attritor mill,, dried and finally pressed cold- isos tatically under 3,000 bar. For sintering the green compacts thus produced. a sintering rate of about 10C/minute and a maximum final temperature of 1400C are likewise recommended. As the result of the sintering process, a structure with a grain size of between 1 and 3 pm is present in the sintered body. The maximum strength values are between 200 and 270 N/MM2.

In European Published Application 0,255,023. the use of a nonstoichlometric cordierite chemically mod!f ied by means of P.05 and B203 is described, which is reinforced by ZrO2 particles. The chemical modification of the cardierite serves for extending the sintering interval and suppressing the crystallization of the cordierite, so that the cordierite can be sintered in the glassy state. Here again, however, the attainable strengths are not satisfactory,. especially at higher temperatures.

The production of composite bodies from slicon nitride and cordierite by sintering in an inert atmosphere at temperatures between 1400C and 1800C is described in US Patent Specification 4,542,109. The cordierite component introduced as a sinter aid into the silicon nitride body is here produced only during the sintering process in situ from the required individual oxide constituents which are premixed as a powder. The formation of the cordierite here takes place via intermediate phases. the individual reactions proceeding sufficiently fast only,if small quantities of ZrO, are present. In this publication. strength values are reported only for hot-pressed mouldings. Thus, for example. a moulding with about 40 % by volume of cordierite has a strength of between 200 NImm? and 300 N/mm2.

4 The always considerably lower strength properties of composite bodies sintered without pressure are not mentioned.

The invention is based on the object of providing a process for producing dense-sintered cordierite bodies which have a relative density greater than or equal to 0.99. which process should in particular be suitable for producing large-volume cordierite bodies and in which the sintering step should proceed without pressure and sintering in air should be possible.

Claims (11)

This object is achieved by the process described in Patent Claim 1. The cordierite powder used Is a powder consisting of cordierite particles of largely stoichiometric composition, that is to say that every cordierite particle should largely have the stoichiometric composition, in contrast to many hitherto known processes in which the stoichiometric composition applies only statistically across a large quantity of particles. Such a powder is appropriately prepared via the glass phase since, in this case, particularly homogeneous mixtures can be obtained. The preparation of cordierite via the glass phase is well known and described, for example, in German Patent Specification 2.517,,743, Example 2, or in US Patent Specification 3,849,145. Apart from the small quantity of impurities which are determined above all by the raw materials and the fusion crucible, the quality of the cordierite powder produced is determined by the homogeneous composition of the cordierite melt throughout the entire volume, which is accomplished by melting the raw materials and subsequent homogenization by stirring. As a result of the melting. concentration gradients in the molten state are levelled out. An equivalent cordierite quality cannot he obtained by mixing the individual components and a subsequent solid-state reaction at elevated temperatures, as normally used for the production of cordierite materials. The sollgel process known per se can also be used for producing high- quality cordierite powder. The cordierite thus produced is then ground to a powder having a particle size of less than 3 Am, about 90 % by weight of the powder having a particle size of less than 1.5 pm. A grain size of more than 3 pm leads to sintered bodies of lower density and strength. The cordierite powder can contain up to 50 % by weight of ZrO, powder of a grain size smaller than or equal to 3 pm. Highly pure, commercially available zirconium dioxide powder having a crystallite size of less than 0.1 pm, which is mixed together with the cordierite powder and ground, is preferred. The zirconium dioxide serves in a manner known per se for further reinforcement of the cordierite bodies. Surprisingly, it was found that, up to a zirconium dioxide content of 50 % by weight, the strength of the resulting bodies increases, whereas no increase in strength or decrease in strength is to be found in the conventional processes at zirconium dioxide contents of more than 30 % by weight. Especially when zirconium dioxide powder is used for reinforcement, it is of advantage to use a cordierite powder which is as fine-grained as possible. The smaller the cordierite grains, the more often a continuing crack strikes the homogeneously distributed zirconium dioxide reinforcing component, that is to say the greater is the reinforcing effect. The powder thus obtained or the powder mixture is then compressed to a green compact having a relative density of at least 0.67. If this green density is not reached, a body having adequate strength properties can frequently no longer be obtained. The powder is either pre-pressed uniaxially and then compacted coldisostatically in a resilient envelope of plastic, or a resilient envelope of plastic (for example of silicone rubber) is filled directly and then pressed cold-isostatically. The addition of pressing aids (binders and lubricants) is not necessary for the compression, even though the addition of appropriate pressing aids, for example magnesium stearate. carbowax. amide wax or the like, can be helpful in achieving high green densities or has a favourable effect on a machining treatment of the green compacts after the cold-isostatic pressing. For further compaction, the cold- isostatical ly pressed specimens are subjected to a sintering process. The known sintering atmospheres such as inert gases, vacuum or a silicon-containing atmosphere can be used as the sintering atmosphere, but air is preferred as the sintering atmosphere. The sintering is carried out from a starting temperature of the green compact of 800C up until the final sintering temperature is reached at a rate of temperature rise (heating rate of less than 5 K x min-'). Higher heating rates lead to an accelerated sintering process, that is to say to higher sintering rates (equal to change of density per unit time) and, connected with this, to an inclusion of pores in the sintered body. Especially in the case of large-volume sintered bodies, this is a particularly noticeable interference, since a pronounced temperature profile arises in these bodies due to the low thermal conductivity of the material which is to be compacted. At high heating rates, this pronounced temperature profile not only leads to a considerable pore volume in the interior of the sintered body, but also to thermal stresses, whereby the proportion of fractures.during the sintering of relatively large bodies increases considerably. Heating rates of less than 4 K x min-', especially of 1 K x min-' - 4 K x min-', are preferred, because good sintered products are obtained in this range with the time required still acceptable. Furthermore,, the low heating rates of less than 5 K x min-' surprisingly also allow a final sintering temperature which is markedly below the usual one. The heating rates should not be less than 0. 5 K x min-' since, on the one hand, the sintering time increases too sharply and, on the other hand, there is a risk of undesired reactions occurring due to an unduly long stay in the region of high temperature. The low heating rate should be applied starting - a - at a sintering temperature of 8000C. The temperature limit of 800C must be adhered to above all if a cordierite powder isused which is still in the glassy state. In the case of a cordierite powder which is fully crystallized, the low heating rate must be adhered to only from a temperature of 1000C. In general, adherence to the low heating rates within the indicated temperature interval is not necessary until the relative density of the body has risen to 0.75. Especially in mass production, allowance for this fact can lead to shortening of the production process. In the production of unreinforced cordierite bodies from glassy powder, a final temperature of 10500C is sufficient. If crystalline cordierite powders are used, the final sintering temperature can be up to 1400C, in the manner known per se. However, a final sintering temperature which is between 1300C and 1360C is preferred. This temperature, which is significantly lower than the final sintering temperature of 1400"C generally used, has not only the advantage of saving energy, a shorter sintering time and the fact that less expensive sintering furnaces can be used, but has also further systematic advantages which are described below, especially if a powder mixture of cordierite and zirconium dioxide is used. After the final sintering temperature has been reachedr the body to be sintered can, as is known per se, also be held at this temperature for up to a further four hours, in order to achieve further compaction. In the sintering process according to the invention, a holding time of less than two hours, in particular of 0 - 1 hour, is in general sufficient. The desired final sintering temperature has been reached when the body which is to be sintered, if appropriate with a subsequent holding timer has reached a relative density of 0.99 or more. In the process according to the invention, this final temperature is 1050C for unreinforced glassy cordierite bodies, and between 1300% and 1360C, in particular between 1300C and 17 1350C, if crystalline powder is used. At low heating rates, the final temperature is in the lower part of the indicated range, and it rises to higher values at higher heating rates. Good results are obtained, for example, at a heating rate of 1 K x min-1 with a final temperature of 1300C and a holding time of one hour, whereas a final temperature of 1350% likewise with a holding time of one hour, is preferred at a heating rate of 3 K x min-'. In a manner known per se, the cordlerite powder can also contain Zr02 particles for improving the mechanical properties of the finished sintered body. Up to a content of 50 % by weight of ZrO2 powder, an increase in strength can be achieved. The grain size of the Zr02 powder should be about 3 pm or less. The powder of pure ZrO2 having a crystallite size of less than 0. 1 pm. in particular a powder having a crystallite size in the range from 0.06 pm to 0.02 pm, is preferred here. ZrO, powders having such crystallite sizes are commercially available. Both pure Zr02 powders and those which have been stabilized tetragonally and/or cubically in a manner known per se by small additions of MgO, CaO, Y.03 can be used. However, the final sintering temperature depends not only on the selected heating rate but. if a mixture of cordierite powder and zirconium dioxide powder is used. also on the mixing ratio of these two substances and, to a smaller extent, also on the nature of the zirconium dioxide powder used, for example on whether the zirconium dioxide powder is stabilized or unstabilized, and on the particle size of the zirconium dioxide powder. The optimum final sintering temperature can easily be determined by any person skilled In the art by means of a few experiments. for example with the aid of a commercially available sintering dilatometer. The maximum final temperature and the holding time also affect the type of the microstructure produced in the composite bodies. At low final temperatures, the incorporated zirconium dioxide powders form networklike structures along the cordierite grain boundaries, with a crystallite size of between 0.1 and 0.25. It follows from this that the zirconium dioxide crystallites introduced originally have sintered together. Higher final temperatures and longer holding times promote the growth of the zirconium dioxide grains until finally they are homogeneously distributed as individual grains of a size of up to 1 micrometer at largely regular spacings between the cordierite grains. Using the knowledge of the rate of growth of the zirconium dioxide crystallites as a function of the temperature and time, the maximum zirconium dioxide particle size can thus be adjusted in a controlled manner. In the sintering at high final temperatures and long holding times, as in the state of the art, it is to be noted that increasingly a reaction between cordierite and zirconium oxide starts, with the formation of zirconium silicate and spinel, which leads to a loss in strength, since the reinforcing component zirconium dioxide is consumed. This reaction can be observed on samples which were heated to 1400C at a heating rate of 1 K x min-. In the specimens, the final sintering temperature of which was in the preferred range of 13000C - 1360C, such a reaction was not detectable, the reason being the markedly lower final temperature. The achievable mechanical strength values are an important criterion for evaluating the production process. The strength values are here affected by both the porosity of the finished sintered bodies and the size of the incorporated zirconium dioxide grains. The lower the pore volume, the smaller is the number of defects in the material and the higher is the achievable strength. Previous experience has also shown that the reinforcing mechanism gains with increasing size of the zirconium dioxide grains. Since the diffusion-controlled growth process of the zirconium dioxide grains is positively affected at a higher final temperature and a longer holding time,. higher final temperatures and a longer holding time involve an increase in strength. However, 1 it is to be noted that, at higher temperatures and longer holding times, a progressive reaction between the cordierite and the zirconium dioxide to form, for example, zirconium silicate is also to be observed, and this in turn leads to a decrease in the strength properties. An increasing zirconium d ioxide content also involves an increase in the strength of the sintered grains. It is surprising here that a marked increase in strength still occurs in the specimens, produced according to the invention, at a content of zirconium dioxide powder of more than 40 % by weight. contrary to existing knowledge. The advantages achievable by the invention are above all that cordierite sintered bodies or cordierite/ zirconium dioxide sintered bodies of high strength can be produced by simple means. Due to the low heating rate. it is possible to produce even large-volume sintered bodies, without having to fear that, during sintering, the bodies would be damaged by thermal stresses arising or that the interior of the sintered bodies would remain unduly porous. The relatively low final sintering temperatures made possible by the low heating rates have the result that the detrimental reaction of the zirconium dioxide with the cordierite very largely does not take place. Furthermore, by a suitable choice of the final sintering temperature and of the holding time at the final sintering temperature, the grain size or the growth of the zirconium dioxide crystallites in the sintered body can be influenced In a controlled manner. For the further development of the process described so far,. the invention is described in more detail below with reference to examples, without these being intended to restrict the scope of protection. Example Cordierite powder or a mixture of cordierite and zirconium dioxide powder was ground for several hours, using isopropanol as the grinding fluid, in an attritor mill whose grinding vessel consisted of hard porcelain and whose grinding balls and stirrer consisted of 85/12 % by weight of A1203/S'02, After grinding, the ground slip was dried under mild conditions and the maximum particle size was determined. As a secondary result, it was found that up to about 30 % of the cordierite was transformed from the crystalline phase into the glassy or amorphous phase as a result of the grinding process. A rectangular block was first prepressed from the powder at 100 bar, and this was then compressed coldisostatically by about 6000 bar to give a green compact having a high relative density (green density). Subsequently, the green compact was, enclosed in an alumina muffle, sintered in a chamber furnace in the sintering interval between 8000C and 13500C at 3 K x min-1. If appropriate, the sintering was followed also by a holding time. The sintered bodies thus produced were worked into bending bars having dimensions of 3.5 x 4.5 x 50 MM3 (sic) and subjected to a three-point bending test in a testing machine. The support distance was here 40 mm. The results are summarized in the table. Example 2: Mixtures as in Example 1 were ground f or 16 hours by circulating grinding in a stirred ball mill having a grinding vessel coated with hard plastic and a stirrer unit. The grinding fluid used was a mixture of 96 % by weight of water and 10 % by weight of isopropanol. The grinding bodies used were alumina balls having a diameter of between about 0.6 and 1 millimetre. After the grinding process. the suspension was dried under mild conditions by freeze-drying and then processed further analogously to Example 1. The results are also summarized in the table. Example 3: A crystalline powder of 99% < 60 having a composition approximating that of stoichiometric cordierite (about 50.5 % by weight of Si02, about 34.3 % by weight of A1203; about 14.2 % by weight of MgO) was 1 subjected to the production process Example 1. described in The sintered bodies had a relative density greater than 0.99. The values for the bending strength can be taken from the attached table (Test 1). Example 4: Cold-pressed green compacts were produced as described in Example 1 from a mixture of 70 % by weight of glassy stoichiometric cordierite (particle size 99 % < 40 qm (sic)) and 30 % by weight of Y.03-stabilized zirconium oxide. One green compact was heated in the sintering interval between 800 and 140CC at 2.5 K x min"l and, after the final temperature had been reached, cooled by switching the furnace off. Between 800 and 90CC, the sample sinters in the glassy state from 0.68 to 0.80 relative density, the crystallization of the cordierite terminating the sintering process. The density of the now crystalline body remains almost constant between 900 and 1200C. After further heating to 1400C and subsequent cooling, a sintered body of about 0.995 relative density is present at room temperature. Example 5: A glassy, that In to say amorphous, powder (99 % < 40 m) having a composition according to Example 3 was subjected to the grinding and drying process described In Example 1. Af ter screening,, the ground material was compressed cold- isos tatical ly In a silicone rubber mould under a pressure of 6000 bar to give a circular cylinder. The green compact was then heated at 10CIminute to 500% held there for 30 minutes at this temperature and finally heated at 4.5 K x min-1 to 1050C and then cooled to room temperature. The body sintered in this way showed a relative density greater than 0.99. The Tests No. 1 to 5. 11 and 12 were carried out according to Example 1. and the Tests 6 to 10 according - 14 to Example 2. All the specimens have a grain size distribution in which 90 % of the particles of the powder mixture are smaller than 1.3 um. The 50 % value was smaller than or equal to 1 pm. The heating rate chosen for the sintering of the composite bodies, starting at 800C, was always 3 K x min-'. The final densities achieved were always above 0.99 relative density. To reinforce the cordierite,. zirconium dioxide stabilized with 3 mol % of Y203 Was used without exception. The averages of the bending strength, indicated in the table, are based on the measurement of 6 bending bars in each case. 1 1 Test no. CordieritelZrO, weight ratio (%) 1 1001 0 2 90110 3 70130 4 60140 solso 6 70130 7 70130 8 70130 9 70/30 70130 11 70/30 12 60/40 Zr02 unstabilized T A B L E Green density (%) 70 70 71 71 70 70 69 69 70 73 73 Final temperature CC) 1350 1350 1350 1350 1350 1350 1350 1350 1350 1360 1350 1350 Holding t ime (hQurs) 1 1 1 1 1 0 1 2 3 1 1 1 1 Strength Averagelmaximum (lq X RM-2) 1701195 217/249 245/287 297/308 309/346 2881322 271/300 2791297 2591291 273/290 2361269 3051319 1 %,A A Claims:

1. Process for producing dense-sintered cordierite bodies, cordierite powder being compressed to give a green body and being compacted in a sintering process at final sintering temperatures of up to 1400"C, characterized in that a cordierite powder having a largely stoichiometric composition of the particles and a particle size of less than 3 pm is compressed to give a green body of a relative density of at least 0.67 and is sintered, starting at a temperature of 800C, at a rate of temperature rise (heating rate) of less than 5 K x min-' until the final sintering temperature is reached, up to relative densities greater than or equal to 0.99.

2. Process according to Claim 1, characterized in that a crystalline cordierite from crystallization of a starting cordierite glass is used.

3. Process according to Claim 1, characterized in that the cordierite powder is glassy.

4. Process according to at least one of Claims 1 to 3, characterized in that sintering is carried out at a heating rate of less than 4 K x min-'.

5. Process according to at least one of Claims 1 to 4, characterized in that sintering is carried out atia heating rate of less than 5 K x min-' only when a relative density of 0.75 has been reached.

6. Process according to at least one of Claims 1 to 5,. characterized in that, if crystalline cordierite powder is used, sintering at a heating rate of less than 5 K x min-' or 4 K x min-' is started only at a temperature of 11000C.

7. Process according to at least one of Claims 1 to 61 characterized in that sintering it taken up to a f inal sintering temperature of 13000C to 13600C.

S. Process according to at least 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 um and 50 % - 17 by weight of the powder has a particle size of less than 1 JAM.

9. Process according to at least one of Claims 1 to 8, characterized in that the cordierite powder contains up to 50 % by weight of ZrO2 powder of a grain size of less than or equal to 3 pm.

10. Process according to Claim 9, characterized in that the ZrO2 powder has a crystallite size of less than 0. 1 pm.

11. Process according to at least one of Claims 1 to 10, characterized in that, when the sintering temperature is reached, the specimen is held at this temperature for up to a further four hours, in particular for up to one hour.

1 Published 1990 at The Patent Office. St&teRoure.86'71 High Holborn. London WClR4TF Further copies MaYbeObtsined from 77ie Patent Office Wes Branch, St Mary Cray. Orpington. Kent BR5 3RD. Printed by MWtiplex techmques ltd. St MarY CraY. Ktnt, Con 1,187