Compositions: ceramic – Ceramic compositions – Pore-forming
Reexamination Certificate
2000-10-25
2002-08-20
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Pore-forming
C501S084000, C501S102000, C501S104000, C501S121000, C501S123000, C501S135000
Reexamination Certificate
active
06436861
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to porous calcium zirconate/magnesia composites, and a method of producing the same, and in particular, relates to porous calcium zirconate/magnesia composites having a fine composite structure due to uniformly dispersed equimolar amounts of calcium zirconate [CaZrO
3
] and magnesia [MgO] and controlled grain growth, and a method of producing the same.
Porous CaZrO
3
/MgO composites with a uniform three-dimensional (3-D) network structure have been successfully synthesized using reactive sintering of highly-pure mixtures of natural dolomite, (CaMg(CO
3
)
2
), and synthesized zirconia powders with LiF additive. Equimolar dolomite and zirconia powders doped with 0.5 wt % LiF were cold isostatically pressed at 200 MPa and sintered at 1100-1400° C. for 2 h in air. Through the liquid formation via LiF doping, strong necks were formed between constituent particles before completion of the pyrolysis of dolomite, resulting in the formation of a 3-D network structure. During and after the formation of network structure, CO
2
was given off to form a homogeneous open-pore structure. The pore-size distribution was very narrow (with pore size: ~1 &mgr;m), and the porosity was controllable (e.g. ~30-50%) by changing the sintering temperature. The porous composites can be applied as filter materials with good structural stability at high temperatures.
The porous composites of the present invention are useful as highly corrosion-resistant materials that function as fluid-permeable filters, lightweight members used at super-high temperatures, catalyst carriers, insulation, sound-absorbing materials, and the like.
2. Description of the Related Art
Porous ceramics having a high porosity made of oxides are used as conventional fluid-permeable filters, and the like. The conventional products are those in which pores are dispersed throughout a sintered compact by lowering the molding density and sintering temperature (controlled sintering), those in which pores are made by burning an organic binder (organic binder removal method), those in which uniform pores are made at a relatively low temperature using chemical means, such as alkoxide decomposition and reaction, etc. (sol gel method), and the like. Moreover, most of the porous products made by any of these production methods are single-phase porous compacts made from one ceramic.
However, these products have the following disadvantages: in conventional porous compact materials made by the controlled sintering method, bond strength between the crystal particles themselves that comprise the porous compact is insufficient and material strength of the porous compact overall is also insufficient, and because pore diameter distribution is wide, fluid selectivity is insufficient. There is a problem with porous compacts made by the organic binder removal method in that harmful gases such as NO
x
, and the like are generated corresponding to the components of the polymer when it is burned. Moreover, there is a problem in that it is difficult to control microstructure of the porous compact because heat is generated when the polymer is burned. In addition, although it is possible to control structure to a relatively high degree with porous compact materials made by chemical means, beginning with the sol gel method, high cost is required and therefore, there are problems in terms of mass production. Furthermore, there is a disadvantage with porous compact materials made by any of these production methods in that when they are used at high temperatures of 1,000° C. or higher, sintering of the porous compact proceeds and structure thereof becomes coarser and pore diameter of the porous compact changes with time and as a result, properties thereof deteriorate.
Under these conditions, in light of the above-mentioned related art, the inventors have repeatedly performed intense research in order to develop new porous compact materials with which the above-mentioned problems can be solved, and successfully have completed the present invention upon developing porous calcium zirconate/magnesia composites having superior properties.
That is, the present invention provides porous calcium zirconate/magnesia composites whose structure is controlled to a high degree and which further have excellent heat resistance and corrosion resistance, and a method of producing these porous materials using a process that is advantageous in terms of cost in order to solve the above-mentioned disadvantages.
SUMMARY OF THE INVENTION
The present invention provides porous calcium zirconate/magnesia composites, and a method of producing the same.
The present invention relates to porous calcium zirconate/magnesia composites having a thermally and chemically stable porous structure, which consist of sintered compacts having a fine composite structure stable under high temperatures due to uniformly dispersed equimolar amounts of calcium zirconate [CaZrO
3
] and magnesia [MgO] and controlled grain growth, and also relates to a method of producing the same.
The present porous composites are useful as, for instance, highly corrosion resistant materials that function as fluid-permeable filters, lightweight members used at super-high temperatures, catalyst carriers, insulation, or sound-absorbing materials, and the like.
The present invention for solving the above-mentioned problems consists of the following technical means:
(1) Porous calcium zirconate/magnesia composites having a thermally and chemically stable porous structure,
said porous composites consist of sintered compacts having a fine composite structure stable under high temperatures due to uniformly dispersed equimolar amounts of calcium zirconate [CaZrO
3
] and magnesia [MgO] and controlled grain growth,
which are synthesized by using reactive sintering of equimolar mixture of dolomite and zirconia powders doped with liquid phase forming material.
(2) Porous calcium zirconate/magnesia composites according to above (1), wherein dolomite [CaMg(CO
3
)
2
] is used as the calcium source and magnesium source of the calcium zirconate and magnesia to achieve uniform mixing of the calcium and magnesium contained in the starting materials on an atomic level.
(3) A method of producing the porous calcium zirconate/magnesia composites defined in above (1) or (2), comprising:
uniformly crushing and mixing equimolar amounts of dolomite and zirconia powders doped with low-melting-point liquid phase forming material;
molding this mixture as needed; and
sintering the mixture to obtain the porous composites.
(4) A method of producing porous calcium zirconate/magnesia composites according to above (3), wherein natural dolomite ore of a high purity is used as the starting material and reacted with zirconia during the sintering to form an equimolar calcium zirconate/magnesia composite structure inexpensively and in a short time.
(5) A method of producing porous calcium zirconate/magnesia composites according to above (3), wherein the composites having a 3-dimensional network structure and high strength even though having a porosity of 40 to 60% are synthesized by using reactive sintering which comprises uniformly mixing equimolar amounts of dolomite and zirconia [ZrO
2
] with 0.5 to 2.0 wt % low-melting-point liquid phase forming material per the total amount of dolomite and zirconia, molding the mixture as needed, heating the mixture to promote intergranular dispersion via the formation of a liquid phase at a relatively low temperature (500 to 700° C.), thereby forming strong necks between the dolomite and zirconia during the process of pyrolysis of the dolomite, liberating CO
2
during the course of the subsequent rise in temperature, and sintering the mixture in atmosphere at 1,300 to 1,400° C. to obtain the porous composites.
(6) A method of producing porous calcium zirconate/magnesia composites according to above (3), wherein an alkali fluoride selected from LiF or NaF is used as the low
Morgan Peter E. D.
Ohji Tatsuki
Suzuki Yoshikazu
National Institute of Advanced Industrial Science and Technology
Sample David
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