CA1098548A

CA1098548A - Crystalline additive for magnesium alumina silicate

Info

Publication number: CA1098548A
Application number: CA303,952A
Authority: CA
Inventors: V. Durga N. Rao
Original assignee: Ford Motor Company of Canada Ltd
Current assignee: Ford Motor Company of Canada Ltd
Priority date: 1977-08-15
Filing date: 1978-05-24
Publication date: 1981-03-31
Also published as: DE2834103A1; DE2834103C2; GB1602590A

Abstract

CRYSTALLINE ADDITIVE FOR MAGNESIUM
ALUMINA SILICATE
ABSTRACT OF THE DISCLOSURE
A micro-crystalline material and method for making the same wherein a vitreous material known as magnesium aluminum silicate or cordierite glass is caused to undergo a high temperature solid state conversion to a crystalline material and wherein the crystallization is catalyzed by using a nucleating agent on the surface of the base material to trigger microscopic grain formation, the base material that is used being in the form of a fine frit or powder.

Description

t359L~3 r~he present invention relates gcnerally bo ~ormation o~
complex ceramic parts that are subjected to severe thermal stresses. It is particularly useful in forming gas turbine regenerator matrices where internal passages in the matrix are subjected alternately to hot exhaust gases and cooler inlet air for the combustor. In forming a re~enerator matrix, a polymer ~inder is mixed with a glass frit and rolled to form a flexible ribbon. One~side of the ribbon has ribs. The ribbon then is wound about itself to fo~m ~0 a cyLindrical structure, the ribs providing passageways extending in an axial direction.
In a gas turbine environment hot gases pass through the passages o the regenerator in one direction when ~he regenerator core is in one position, and cool ~ases pass through the same passages in the opposite direction when ; the regenerator core is angularly displaced. The core structure thus is subjected to repeated heating and cooling which creates thermal shock due to the severe changes in temperature~
Cordierite ox M~S material ~2MgO.~A12O3~5$iO2) is noted for its anistropic characteristics, and it is-useful in the manuacture of regenerators because of its low thermal expansion coefficient. ~he physical characteris~ics o~ ~ordierite and its crystallization properties have been described by Zdaniewski in a publication enti~led "Journal of Material Science", publlshed by Chapman & ~all, Ltd. in 1963, pa~es 192-202. A cordierite material is described

-2~

. ~ . . . . . . . .

also in U.S. patent No. 3,885,977. The so-called l~AS
material is used in a powder or frit form and in the f ring cycle, the particles fuse together.
The MAS glass material has a higher coefficient of thermal expansion than crystalline material and its higher elastic modulus does not permit it to absorb the thermal stresses characteristic Qf heat exchanger appli-ca~ion. Moreoverl the glass is unstable and readily converts to crystalline material when hea,t,ed to tempera-tures above 1600F. The MAS glass material, however, issubject to controlled crystallization and the crystalliza-tion is through nucleation primarily in the matrix.
In general, the bond strength and the bond area between th~ grains determines the'strength characteristics o~ crystalline ceramics. The finer the grain size, the larger is the area of bonding suriace and higher i.5 the strength. During normal sintering treatments, fewer nuclei are generated resulting in a coarse grained structure and, therefore, a weaker material. The strength is further rèduced by the aggravated thermal stresses resulting from the well known anisotropic thermal expansion of the cordierite c~ystal.
The present invention is directed to t~e production of a cordierite type crystalline ceramic of very Eine grain size starting with very ine particle size glass powder and promoting extensive nucleation at the particle surface during sintering by means of ferrous titanate as nucleating agent added to the po~der.
In,accordance with the present invention, there is provided a process for forming an anistropic polycrystallille part of very ~ine grain size, low thermal expansion and very low firing shrinkage comprising grinding a magnesium aluminum silicate glass material with 0.2 to 0.5% of ferrous titanate into a fine powder, mixing with the fine powder a quantity o~
crystalline powder of the same chemical composition as the glass material the quantity of which is less than approximately 60% and more than approximately 2~ by weight, ball-milling the mixture to very fine particle size in the 1/2 micron to 40 micron range, adding the mixture to an organic polymer binder to facilitate forming of the finished part, the binder being capable of being burned of~ at a relatively low temperature, and sintering the fabricated glass particles in a reducing or neutral atmosphere at temperatures of approximately 2000F
to effect bonding of the particle together and to trigger : crystall.ization of the particles, the crystallization occur-ring initially at the surface of the particles thereby in-creasing the bond strength o~ the pa.rticles and increasing the strength of the material and reducing the thermal e~pansion.
T.he invention is described further, by way of illus-tration, parkicularly with re~erence to the accompanyi~g drawings, in which:
Figure lA shows an anistropic crystal with arrow~
that designate the direction.of expansion or contraction;
- Figure lB is a diagxammatic representation of an anistropic bi-crystal with arrows indicating the directions o~ expansion or contraction; - .:
Figure 2 is a schematic representation of bonded crystal particles;
Figure 3 is a schematic representation of a crystal-line particle showing the directions of growth of indivi~ual ..
4 _ .

crystals in a matri~;
Figure 4 is a chart showing the ef~ect of additives to a cordierite glass material powder on the coefficient of thermal expansion of sintered bars;
Figure 5 is a chart similar to Figure 4 showing the effect of adding excess additives or additives that tend to enter solution with the glass during the sintering process; and Figure 6 is a chart showing the effect of additives on the degree o shrinkage of a regenerator matrix~
MAS materials which are of cordierite type ~2MgO.2A1203.5SiO2) are commercially available for the fabrication of regenerators or complex ceramic parts.
An organic binder is used in the fabrication of r~generators to retain MAS glass powder or glass frit. The mixture of the glass frit and the binder is rolled into a ribbon and the ribbon is wound about itself to form a cylindrical ~.~q~354~3 1 structure. The matrix then ls fired, and the binder is 2 burned off at a relati~ely low temperature leaving the

3 glass matrix. This process is described in a paper en-

4 titled "Regenerator Material, Processes and Properties"
presented at the Third Materials Conference - Turbine 6 Applications, Ann Arbor, Michigan, October 30, 1974.
7 It was authored by E. A~ Bush.
8 The glass frit used in the fabrication of ~he 9 regenerators is in the form of a fine powder. The glass frit is sintered after the fabrication procedure. The 11 ,~ S,ql,a,s,s m,ate,rial has ,a_re,lati,e~ _hiqheI coefic~
12 thermal expansion than the crystal and thermal stresses 13 are developed in a poorly crystallized glass which results 14 in cracking oE the material. The strength of the material is insuficient to accommodat:e the stresses developed by 16 the dif~erential thermal expcmsion.
17 The glass is ired t:o effect crystallization of 18 the glass in an effort to improve its strength. Completely `
19 crystallized glass has a higher strength, and it is also relatively stable. It is common practice during the iring 21 cycle for thermal expansion to occur in the order of 1,600 22 parts per million when heated from room temperatuxe to 23 800~ centigrade. The desirable thermal expansion for re-24 generator materials, howe~er, should be about son parts per million~ , .
26 During the fabrication proc~ss, the s~rength 27 capahilities o the crystalline material often are exceeded, 28 which results in cracking as mentioned earlier. In MAS

8~

1 materials, the crystallinity is such that the tharmal 2 expansion is anistropic; that is, each particle expands 3 at different rates in different directions as illustrated 4 in Figure lB. The bi-crystal of Figure lB contains two crystals of the type shown schematically in Figure lA
6 where the C axis is the direction of expansion or contrac-7 tion and the components of the expansion or contraction 8 can be illustrated by the axis Al, A2 and A3. ~he expan-~ sion along the C axis illustrated in Figure lB for one element o~ the bi-crystal ideally is equal to and of opposite 11 sign from the C axi5 expansion of the other member of the 12 bi-crystal so that the change in dimension is zero. ~his 13 ideal relationship does not ~lecessarily exist, but there is 14 a tendency for the change in one dimension of the bi-crystal to be counteracted by a chancre of opposite sign in another 16 part o the bi-crystal.
17 The organic binder is driven off at a temperature 18 of about 250 to 350 centrigrade. Sintering o~ the glass 19 frit occur~ at about 1500-2,000 Fahrenheit although sin-2~ tering may occux at hi~h tempexatures in the case of certain 21 other-types of cordierite materials such as mineral ba~e 2~ matrices.
23 During the sintering operation, the particles tend ~4 to bond togetheL as illustrated in Figure 2. Th~ bond ~ interfa~e within two particles is illustrated in Figure 2 26 ~ by referen~e chaxacter 10. As the particles grow, the 27 voids between th~m are ab50rbed and the surfaces between 28 the particles grow together. Nucleatin~ agents may be 29 added to the glass material to effect extensive crystalli-zation to reduce the grain size and therefore increase the 31 strength and reduce the thermal expansion.

, 1 The negative expansion or contraction of each 2 particle in one axis may be offset by the positive expansion or 3 contraction in the direction of another axis so that in a 4 random distribution it is possible that the net expansion may be zero or the statistical average for the two axes over the 6 entire mass. This is shown, for example, in Figure 3. This 7 ideal condition is illustrated in ~he Figure lB. The bond 8 ~ between the individual crystals should be su~icient to absorb g the differential expansion below a critical crystal size.
Actually the bond surface wiil increase as:-the particle Si2~
~1 decreases for any given unit volume of material. ~he finer the 12 grain size ~he more the crystal will be capable of absorbing 13 the stresses generated by di~ferential expansion.
14 Figure 3 shows the direction o~ the growth of the particles beginning with a nucleus. The various crystals grow 16 until they reach a bond intarface, such as the interface shown 17 at 12 and 14. The nucleus for one crystal is shown at 16 and 18 eac~ of the other crystals shown in the matrix 20 has a similar 19 nucleus. The arr~ws indicate the direction o crystal growth.
If the grain size is small enough and the crystals o~ random 21 orientation, the differential expansion of one direction will 22 be counteracted by ~he differential expansion of one o~ the 23 other particles in the opposite direction to approach a zero 24 net expansion or a statistical average value of the two.
A fine grain size can be achieved by introducing .
26 nucleating agents into the glass itsel. As the glass is heated, 27 the nucleating agent precipitates out first, and will grow 28 as indicated in Figuxe 3. me glass parti d es deine a matrix 29 20. Upon reheating and before crystalliæation of the glass beginS~ the nucleating agent is precipitated out to form 31 nuFlei in large numbersO

1 The starting particles of glass may be 3Q microns in size. The nucleating agents are precipitated out to form 3 grains of three microns or less in size.
A principal ~eature of~ invention comprises the addition of a nucleating agent to the fine glass powder. The 6 mucleating agent, which is ferrous titanate FeO.TiO2 is added 7 to the surface of the particles. Normally the nucleating agents 8 that are added to the body of the material ~re sensitive to g heat treatment and the crystal growth is inflexible. Nucleating agents added to the surface, however, make possible a more 11 controlled grain grow~hO ~he surface is a very desirable site 12 for depositing nucleating agents since the excess ~ree energy 13 o the surface provides the act:ivation energy ~or the crystal 14 growth. The addition of a nucleating agent to the surface will cause the crystal to grow as soon as the particle is heated 16 above the critical temperatuxe ~Eor crystal growth. The presence 17 of the nuclei causes growth of the crystal through the two 18 adjoining grains, thereby increasing densi~ication and strength.
19 The nucleating agent, ~or example ferrous titanate, is ball-milled, preferably with a glass powder. The ferrous 21 titanate is softer tha~ the MAS material so that it will be 22 smeared on the surface of the MAS frit. When the particle is 23 h~ated at a temperat~re where crystallization of glass begins, 24 there is a tendency for nucleation at the surface wherever there is a seed crystal thus resulting in a fine sintered grain size.
26 If nucleation starts ~rom inside the particle and 27 the crystal growth i5 compIeted before sintering it may leave 28 voids. Xn the present invention, however, crystallization 29 is triggered at the sur~ace as soon as sintering begins.

~3135~8 Initial sintexing is needed to get bonding in the first instance to create the strength as needed, but at the same time crystallization is generated in such a way that a finer grain size is achieved, and the grain growth is initiated right at the bond surface. The ball mill media preferably are made of the same MAS material as the ~lass frit itself. In the normal ball-milling operation, an aluminum oxide ball is used, but such normal ball-milling might contaminate the mix. A
zirconium oxide ball will not contaminate the mix since zirconium oxide picked up during the ball-milling operation is harmless i~ less than 0.5% by weight. Indeed, to some degree the presence of zirconium oxide is desirable up to0.5~ by weight.
In Figure 4 there is illustrated the e;Efect of the addition of ferrous titanate to the ~S material. If 0.2~ to 0-4~ by weight of ball-milled ferrous titanate is added to the magnesium alumina silicate glass, the coefficient o~ thermal expansion of the cr~stalline cera~ic is reduced. The addition o magnesium flouride also may have a beneficial effect up to 0.1~. The addition of0.5% by weight of zirconium oxide to 0.2 errous titanate will result in an expansion coefficient of about 1050 parts per million at 800 Centigrade. On the other hand, i the additive is allowed to oxidize or if erric - -titanate is added to the mix, the coe~ficient of the thermal expansion i5 increased. In Figure 4 the addition of 0.2% by wei~ht of ferric titanate to the mix will cause an expansion rate of 1,700 parts per million~ This is-due to the fact that the ferric titanate may go into solution ~ith the glass~
Zirconiu~ oxide reduces expansion but not as much as ferrous titanate. Ferrous titanate can easily be oxidi7ed to 3Q ferric titanate, so steps must be taken to avoid that~

_ 9 _ ~' ' . .

. ~

1 Otherwise the coefficient expansion will deteriorate. If the 2 firing of the binder and the glass frit mixture occurs in a 3 closed furnace, the binder is allowed to burn off at a rela-4 tively low temperature, thereby reducing or exhausting the oxygen. I~ a closed furnace is used, magnesium ~ e and ~` zinc and zirconium oxide individually do not have a large 7 influence; but they do provide a slight improvement especially 8 if used with ferrous titanate.
9 Some glass rit contains barium oxide and a slight amount of V2Os. ~hat material has a lower coefficient of 11 thermal expansion to ~egin-with or even prior to the addition 12 of ~e ferrous titanate and the ferrous titanate will still 13 reduce the ~hermal expansion.
14 It is undesirable to allow the ferrous titanate to be added in excess of above0.4% or ~.5%. If an excess amount 16 o~ ferrous titanate is ball-milled with the powder, the ex~ess 17 is deposited on the surface of the glass frit. This excess may 18 go into solution with the crystals already formed at the 19 sintering temperature or it may be transformed into ferric titanate as explained earlier. Ferric titanate tends to 21 increase the coefficient of thermal expansion which i5 the 22 opposi~e result from what is desired. I ferrous titanate goes 23 into solution in large proportion it may cause a major change in 24 crystallinity of the material. This phenomenon is described in the base of L~S, lithium aluminum silicate, ~aterials by 26 Jesse Brown in a paper entitled "Solid State Reaction 27 Differential ExpansiQn of Ferrous Materials and Systems Contain 28 ing Zinc Oxide and Other Selected Oxides", which was published 29 in 1964 by Pennsylvania State University, Microfilm No. 656725.
If the amount o~ ferrous titanate that goes into solution is small, it may not cause a deleterious effect since there would be no fundamental chan~es in crystallinity. The presence of magnesium ~louride may have a desirable effect up to 0.1% by weight. The same is true of zirconium oxide in quantities of up to 0.5%. Beyond those percentages, in moderate amounts magnesium fluorideand æirconium oxide merely act as extra-neous materials but do not cause a deletarious effect on the quality of the crystalline structure, however, excessive amounts cause an increase in thermal expansion. The shrinkage that occurs during firing is a maximum for glass particles that are nonnucleated. The particles grow in size as the bonding surfaces between the particles are eliminated and as the voids between the particles are absorbed. Glass particles that are nucleated internally have a large degree o~ shrin~age, as indicated in Figure 6~ while ylass particles that are both internally nucleated and surface nucleated have the least amount o shrin~age. In the latter case r crystal growth occurs on the surface and proceeds internally generati~g a large nu~ber o~ crystals. The result is an article of a ~ery minimum grain size.
A crystalline powder of the same composition as the starting glass material is added to the ferrous titanate and the mixture is ball milled to produce a fine powder mix con-sisting of particles in a size from about l/2 to 40 microns.
The following table illustrates the effect of adding ferrous titanate, zirconium oxide and magnesium ~7uoride to the base MAS material or cordierite-TABLE
Tensile Strength &
Composition Thermal E~pansion - at 800C.

Base Material + 0.2~ FeO~TiO2 - 11,000 psi - 13,000 psi (2050F) 1,050 ppm - 1,250 ppm Base Material ~ 0.4% FeO.TiO2 - 8,000 psi - 10,000 psi (2050F3 1,200 ppm Base Material + 0.2% Fe2O3.TiO2 - 11,000 psi - 13,000 psi 1,600 ppm -.1,800 ppm Base Material + 0.4% Fe2O3.TiO2 - 11,000 psi - 13,00Q psi (2050F) 1,900 ppm Base Material -~ 0.4% ~ ~r~ - 11,000 psi (2050F) 1~550 ppm Base Material + 0.2% FeO.TiO2 - l:L~000 psi - 13,000 psi 0 4 ~ z~ ~
(2050F) 1"050 ppm - 1,200 ppm Base Matarial + 0.2~ MgF2 ~ 1:3,000 psi (Lower sintering temperature below 2050F) 1,400 - 1550 ppm Base Material ~ 0.2% FeO.TiO2 - 12,000 psi 0.4% ~ %~~ 1,050 ~ 1,150 ppm + 0.2% ~qF~
Base Material + Seed crystal - 10,000 psi~

5% ~20 micron size 1,250 - 1,450 ppm and ~5~ firing shrinkage - i2 -

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for forming an anistropic polycrystalline part of very fine grain size, low thermal expansion and very low firing shrinkage comprising grinding a magnesium aluminum silicate glass material with 0.2 to 0.5% of ferrous titanate into a fine powder, mixing with said fine powder a quantity of crystalline powder of the same chemical composition as the glass material the quantity of which is less than approximately 60% and more than approximately 2% by weight, ball-milling the mixture to very fine particle size in the 1/2 micron to 40 micron range, adding the mixture to an organic polymer binder to facilitate forming of the finished part, said binder being capable of being burned of at a relatively low temperature, and sintering the fabricated glass particles in a reducing or neutral atmosphere at temperatures of approximately 2000°F
to effect bonding of the particle together and to trigger crystallization of the particles, the crystallization occur-ring initially at the surface of the particles thereby in-creasing the bond strength of the particles and increasing the strength of the material and reducing the thermal expansion.

2. The process of Claim 1 wherein the addition of ferrous titanate is supplemented by the addition of approximately 0.5%
by weight of zirconium oxide and approximately 0.2% by weight of magnesium fluoride to increase the strength of the crystalline structure and to accelerate crystal growth.

3. The process of Claim 1 wherein the addition of ferrous titanate is supplemented by the addition of approximately 0.5%
by weight of zirconium oxide to increase the strength of the crystalline structure and to accelerate crystal growth.

4. The process of Claim 1 wherein the addition of ferrous titanate is supplemented by the addition of approxim-ately 0.2% by weight of magnesium fluoride to increase the strength of the crystalline structure and to accelerate crystal growth.