Aluminium titanate structure and process for producing the same.

Background of the Invention and Related Art Statement

0001The present invention relates to an aluminum titanate structure having excellent thermal shock resistance and high actual break strain, as well as to a process for producing the structure.

0002A ceramic containing aluminum titanate as a base material has a low thermal expansion coefficient and a small Young's modulus. Hence, it has been used as an effective material for thermal shrinkage of metal, as in production, by casting with aluminum metal, of a port liner for the heat insulation of the inside of exhaust pipe of gasoline engine.

0003A port liner is required to have high thermal shock resistance because it is used at extremely high temperatures and under large heat gradients, and is further required to have excellent mechanical properties because it is used under severe conditions.

0004Because of such technical requirements for port liner, aluminum titanate has been considered as a ceramic for use in port liner, and various investigations have been made to develop an aluminum titanate material of high strength, in particular. For example, Japanese Patent Publication No. 3629/1982 discloses an aluminum titanate material of high strength comprising aluminum titanate as a main phase and 0̸.3-8% by weight of rare earth elements selected from Y, La and Ce.

0005Japanese Patent Application Kokai No. 164760̸/1989 discloses an aluminum titanate material which is made of starting materials of Al₂O₃ , TiO₂ and quartz (SiO₂), which contains unreacted TiO₂, which has no crack and which can be casted together with a melt of non-ferrous metal(s) at temperatures of 80̸0̸°C or less. Japanese Patent Application Kokai No. 257165/1989 discloses an aluminum titanate material which contains rare earth element oxide(s), mullite and iron oxide and which is stable at high temperatures.

0006Further, Japanese Patent Application Kokai No. 30̸5854/1989 discloses an aluminum titanate material which contains iron oxide and mullite, which has high mechanical strengths and which is usable at a temperature range of 1,0̸0̸0̸-1,30̸0̸°C.

0007The above materials, however, each have problems. The aluminum titanate material disclosed in Japanese Patent Publication No. 3129/1982 has a high strength because it comprises aluminum titanate as a main phase, but has drawbacks of small aspect ratio and small actual break strain. The aluminum titanate material disclosed in Japanese Patent Application Kokai No. 164760̸/1989 uses only untreated raw materials as starting materials and has no crack, and therefore has drawbacks of small aspect ratio and small actual break strain. The aluminum titanate material disclosed in Japanese Patent Application Kokai No. 257165/1989 uses a starting material composed essentially of a single phase of FeTiO₅ and Al₂TiO₅, contains no vitreous phase and has a high strength, and therefore has drawbacks of small aspect ratio and small actual break strain. The aluminum titanate material disclosed in Japanese Patent Application Kokai No. 30̸5854/1989 is produced by sintering at 1,30̸0̸-1,450̸°C and has a high strength, and therefore has drawbacks of small projected diameter, small aspect ratio and small actual break strain.

0008In order for an aluminum titanate (AT) material to have a large actual break strain, the material must consist of large grains and must contain many cracks at the grain boundaries. Such a material, however, has a drawback in that the cracks at the grain boundaries spread during the heat cycle, giving a reduced strength.

0009A port liner is required to have both actual break strain and heat cycle durability and must satisfy these two incompatible properties at high levels.

0010Hence, the objects of the present invention are to provide an aluminum titanate material which is free from the above-mentioned drawbacks of conventional aluminum titanate materials and which has excellent thermal shock resistance, large actual break strain and excellent heat cycle durability, and a process for producing said aluminum titanate material.

Summary of the Invention

0011According to the present invention, there is provided an aluminum titanate structure comprising, as a main phase, crystal grains having an average aspect ratio of 1.6 or more and an average projected diameter of 5-10̸ mm.

0012According to the present invention, there is further provided a process for producing an aluminum titanate structure, which comprises mixing an aluminum titanate cullet (AT cullet) (a) obtained by mixing a plurality of raw materials selected from the group consisting of an Al₂O₃ source, a TiO₂ source, a SiO₂ source, a Fe₂O₃ source, a MgO source, a rare earth element source and a mullite source, calcining the mixture so as to give an aluminum titanate proportion of 90̸% or more, and grinding the calcination product, with a plurality of raw materials (b) selected from the group consisting of an Al₂O₃ source, a TiO₂ source, a SiO₂ source, a Fe₂O₃ source, a MgO source, a rare earth element source and a mullite source, shaping the mixture as a starting material and sintering the shaped material.

Brief Description of the Drawing

0013Fig. 1 is a strain-stress curve when a ceramic is subjected to a 4-point bending strength test by JIS R 160̸1.

Detailed Description of the Invention

0014The present inventor made research and found that an aluminum titanate material having a large actual break strain and excellent heat cycle durability can be obtained by allowing an aluminum titanate material to comprise crystal grains having a high strength between the grain boundaries, many cracks inside the grains, an average aspect ratio of 1.6 or more and an average projected diameter of 5-10̸ µm.

0015Preferably, the above aluminum titanate structure further comprises a vitreous phase and mullite, or a vitreous phase and rare earth element(s), or a vitreous phase, mullite and rare earth element(s). Further preferably, the aluminum titanate structure has an actual break strain of 5x10̸⁻³ or more and a strength deterioration in heat cycle, of 0̸ or less.

0016When the average projected diameter of an aluminum titanate material is 5 µm or less, the strength is high and the actual break strain is 5x10̸⁻³ or less. When the average projected diameter is 10̸ mm or more, the actual break strain is 5x10̸⁻³ or more but the strength deterioration in heat cycle is 0̸ or more. When the average projected diameter is 5-10̸ µm and the average aspect ratio is less than 1.6, the actual break strain is 5x10̸⁻³ or less or the strength deterioration in heat cycle is 0̸ or more.

0017In the process for producing an aluminum titanate structure according to the present invention, it is preferable to use a AT cullet (obtained by mixing a plurality of raw materials selected from the group consisting of an Al₂O₃ source, a TiO₂ source, a SiO₂ source, a Fe₂O₃ source, a MgO source, a rare earth element source and a mullite source, calcining the mixture so as to give an aluminum titanate proportion of 90̸% or more, and grinding the calcination product) in an amount of 5% by weight or more, preferably 20̸% by weight or more.

0018In the process, it is also preferable to use a AT cullet (obtained by mixing in a liquid phase a plurality of raw materials selected from the group consisting of an Al₂O₃ source, a TiO₂ source, a SiO₂ source, a Fe₂O₃ source, a MgO source, a rare earth element source and a mullite source, calcining the mixture so as to give an aluminum titanate proportion of 90̸% or more, and grinding the calcination product) in an amount of 5% by weight or more, preferably 20̸% by weight or more.

0019According to the present process, there can be obtained an aluminum titanate material wherein mullite, rare earth element(s), a vitreous phase, etc. are uniformly dispersed at the grain boundaries and thereby the aluminum titanate grains are strongly bonded to each other at the boundaries, and wherein each grain has many cracks inside and has an average projected diameter of 5-10̸ µm and an average aspect ratio of 1.6 or more.

Examples

0020The present invention is hereinafter described in more detail with reference to Examples. However, the present invention is by no means restricted to these Examples.

0021Description is first made on the process for producing aluminum titanate materials, comprising a raw material preparation step, a shaping step and a sintering step. Then, description is made on the methods for measuring the properties of the sintered materials (aluminum titanate structures) obtained, as well as on the results obtained.

AT Cullet preparation step

0022There were used two methods for AT cullet preparation, i.e. a method (A) preparing a AT cullet by a liquid phase mixing and a method (B) preparing a AT cullet by powder mixing. In the method (A), the raw materials shown in Table 1 were weighed so as to give a desired composition when converted to respective oxides; they were mixed in an aqueous solution; and a necessary amount of an aqueous NH₄OH solution was added. Then, the resulting precipitate was washed to remove the impurity ions present therein; the precipitate after washing was calcined so as to give a desired aluminum titanate proportion; the calcination product was ground to obtain a AT cullet. In the method (B), the raw materials shown in Table 1 were weighed so as to give a desired composition; they were uniformly mixed by a dry method, or a wet method followed by drying; the mixture was calcined so as to give a desired aluminum titanate proportion; the calcination product was ground to obtain a AT cullet. <tables id="tabl0001" num="0001"><img file="EP0506475A2_D0001.tif" /></tables>

Shaping step

0023There were adopted three shaping methods, i.e. slip casting, press molding and cold isostatic press (CIP). In the slip casting, the AT cullet obtained in the above AT cullet preparation step and the raw materials shown in Tables 2 to 5 were weighed so as to give a desired composition; thereto were added 22% by weight of water and 0̸.5% by weight of a polyacrylic acid type deflocculant, and mixing was conducted in a pot mill for 5 hours; then, 1.5% by weight of a binder was added and vacuum degassing was conducted; the resulting mixture was cast into a gypsum mold to obtain a shaped material. In the press molding or the CIP, the AT cullet obtained in the above AT cullet preparation step and the raw materials shown in Tables 2 to 5 were weighed so as to give a desired composition; thereto were added 60̸% by weight of water and 2% by weight of a binder for pressing, and mixing was conducted in a pot mill for 5 hours; then, granulation was conducted using a spray dryer and the granulated powder were subjected to press molding or CIP using a mold.

Sintering step

0024The shaped material obtained in the above shaping step was kept at a temperature shown in Tables 2 to 5 for 4 hours to conduct sintering at the normal pressure.

0025Each AT cullet shown in Table 1 was measured for aluminum titanate as follows

Measurement of aluminum titanate proportion

0026A AT cullet was subjected to X-ray diffractometry in the range of 2ϑ = 25 28.5° using a CuKα line. There were measured a peak (d = 3.356, 2ϑ = 26.6) intensity of aluminum titanate (I_AT), a peak (d = 3.245, 2ϑ = 27.5) intensity of rutile (I_Ru) and a peak (d = 3.479, 2ϑ = 25.6) intensity of corundum (I_Co). The aluminum titanate proportion of the AT cullet was calculated from these intensities using the following formula.<maths id="math0001" num=""><math display="block"><mrow><msub><mrow><mtext>Aluminum titanate proportion = [I</mtext></mrow><mrow><mtext>AT</mtext></mrow></msub><msub><mrow><mtext>/(I</mtext></mrow><mrow><mtext>AT</mtext></mrow></msub><msub><mrow><mtext> + I</mtext></mrow><mrow><mtext>Ru</mtext></mrow></msub><msub><mrow><mtext> + I</mtext></mrow><mrow><mtext>Co</mtext></mrow></msub><mtext> /0̸.75)] x 10̸0̸</mtext></mrow></math><img file="EP0506475A2_D0002.tif" /></maths>

0027Each sintered material (aluminum titanate structure) obtained in the above sintering step was measured for proportions of crystalline phases, average aspect ratio, average projected diameter, actual break strain and strength deterioration in heat cycle (which shows thermal shock resistance), as follows.

Measurement of proportions of crystalline phases

0028A sintered material was subjected to X-ray diffractometry in the range of 2ϑ = 25 28.5° using a CuKα line. There were measured a peak (d = 3.356, 2ϑ = 26.6) intensity of aluminum titanate (I_AT), a peak (d = 3.245, 2ϑ = 27.5) intensity of rutile (I_Ru), a peak (d = 3.479, 2ϑ = 25.6) intensity of corundum (I_Co) and a peak (d = 3.428, 2ϑ = 26.0̸) intensity of mullite (I_Mu). The proportions of crystalline phases were calculated from these intensities using the following formulas.<maths id="math0002" num=""><math display="block"><mrow><msub><mrow><mtext>I = I</mtext></mrow><mrow><mtext>AT</mtext></mrow></msub><msub><mrow><mtext> + I</mtext></mrow><mrow><mtext>Ru</mtext></mrow></msub><msub><mrow><mtext> + I</mtext></mrow><mrow><mtext>Mu</mtext></mrow></msub><msub><mrow><mtext>/0̸.95 + I</mtext></mrow><mrow><mtext>Co</mtext></mrow></msub><mtext>/0̸.75</mtext></mrow></math><img file="EP0506475A2_D0003.tif" /></maths><maths id="math0003" num=""><math display="block"><mrow><msub><mrow><mtext>Proportion of aluminum titanate crystalline phase (%) = (I</mtext></mrow><mrow><mtext>AT</mtext></mrow></msub><mtext>/I) x 10̸0̸</mtext></mrow></math><img file="EP0506475A2_D0004.tif" /></maths><maths id="math0004" num=""><math display="block"><mrow><msub><mrow><mtext>Proportion of rutile crystalline phase (%) = (I</mtext></mrow><mrow><mtext>Ru</mtext></mrow></msub><mtext>/I) x 10̸0̸</mtext></mrow></math><img file="EP0506475A2_D0005.tif" /></maths><maths id="math0005" num=""><math display="block"><mrow><msub><mrow><mtext>Proportion of mullite crystalline phase (%) = (I</mtext></mrow><mrow><mtext>Mu</mtext></mrow></msub><mtext>/0̸.95/I) x 10̸0̸</mtext></mrow></math><img file="EP0506475A2_D0006.tif" /></maths><maths id="math0006" num=""><math display="block"><mrow><msub><mrow><mtext>Proportion of corundum crystalline phase (%) = (I</mtext></mrow><mrow><mtext>Co</mtext></mrow></msub><mtext>/0̸.75/I) x 10̸0̸</mtext></mrow></math><img file="EP0506475A2_D0007.tif" /></maths>

Measurements of average aspect ratio and average projected diameter

0029A sintered material was subjected to surface polishing; a photograph of the polished surface was taken using a scanning electron microscope at a magnification (50̸0̸-1,0̸0̸0̸ times) at which the grains of the sintered material could be seen under reflected electrons; the photograph was subjected to image analysis using an image analyzer (KONTORON manufactured by ZEISS of Federal Republic of Germany) to determine the aspect ratio and average projected diameter of the sintered material. In the analysis, when a grain had crack(s) inside and was divided thereby, the crack(s) was (were) regarded as grain boundary (boundaries). This method enables accurate measurement for grains of complex shape.

Measurement of actual break strain

0030There was adopted the same method as used for measurement of the 4-point bending strength test of ceramics specified by JIS R 160̸1. That is, the deflection amount "a" (mm) shown by a sintered material during the time period from the moment of load application to the destruction of the material was taken as a length in Fig. 1 from a point 1 (start of load application) to a point 4 which is an intersection between a base line 3 and a perpendicular drawn from a destruction point 2 to the base line 3; the thickness "t" (mm) of the specimen was also measured; the actual break strain of the specimen was calculated from "a" and "t" using the following formula.<maths id="math0007" num=""><math display="block"><mrow><mtext>Actual break strain = 6ta/10̸0̸0̸</mtext></mrow></math><img file="EP0506475A2_D0008.tif" /></maths>

Measurement of strength deterioration in heat cycle

0031A sample for 4-point bending strength test by JIS R 160̸1 was heated for 20̸ minutes in a 90̸0̸°C furnace, then taken out and cooled by air blow for 10̸ minutes; this procedure was repeated 60̸0̸ times; then, the resulting material was measured for 4-point bending strength by JIS R 160̸1; the strength deterioration in heat cycle, of the sample was calculated using the following formula.<maths id="math0008" num=""><math display="block"><mrow><mtext>Strength deterioration in heat cycle = [(4-point bending strength before heat cycle - 4-point bending strength after heat cycle)/4-point bending strength before heat cycle] x 10̸0̸</mtext></mrow></math><img file="EP0506475A2_D0009.tif" /></maths>

0032In Tables 2 to 5 are shown the average aspect ratio, average projected diameter, actual break strain and strength deterioration in heat cycle, of each sintered material obtained above by selecting the raw materials, composition, shaping method and sintering temperature. <tables id="tabl0002" num="0002"><img file="EP0506475A2_D0010.tif" /></tables><tables id="tabl0003" num="0003"><img file="EP0506475A2_D0011.tif" /></tables><tables id="tabl0004" num="0004"><img file="EP0506475A2_D0012.tif" /></tables><tables id="tabl0005" num="0005"><img file="EP0506475A2_D0013.tif" /></tables>

0033As is clear from Tables 2 to 5, the aluminum titanate structures containing crystal grains having an average aspect ratio of 1.6 or more and an average projected diameter of 5-10̸ µm are low in strength deterioration in heat cycle and high (5x10̸⁻³ or more) in actual break strain. In Tables 2 to 5, those with a * mark are Comparative Examples and none of them satisfies the requirements specified by the present invention. Incidentally, 27*, 28*, 29* and 30̸* are tests conducted for confirming Japanese Patent Publication No. 3629/1982, Japanese Patent Application Kokai No. 164760̸/1989, Japanese Patent Application Kokai No. 257165/1989 and Japanese Patent Application Kokai No. 30̸5854/1989, respectively.

0034As described above, the aluminum titanate structure according to the present invention is constituted of crystal grains having an average aspect ratio of 1.6 or more and an average projected diameter of 5-10̸ µm and therefore has excellent thermal shock resistance and high actual break strain. Hence, the aluminum titanate structure exhibits excellent effects when casted with metal(s) to produce a ceramic material for use in, for example, port liner. When used in engine head port, the aluminum titanate structure is effective because it shows no strength deterioration during heat cycle.

0035The aluminum titanate structure comprising crystal grains having the above-mentioned properties, as a main constituent phase, can be produced according to the process of the present invention which uses, as a starting material, at least a AT cullet whose aluminum titanate proportion is a certain level or below.