Title of Invention

A PROCESS OF MAKING LOW THERMAL EXPANSION LITHIUM ALUMINOSILICATE CERAMICS

Abstract

In the present invention there is provided a process for preparing low thermal expansion ceramics in the lithium aluminmosillicate system. The process includes the preparation of an intermediate product in the hydroxy-hydrogel form which is in a reactive state. The process involves incorporation of nucleating materials such as zirconia in the composition that not only increases the product quality but also reduces the overall sintering temperature. The coefficient of thermal expansion of the material ranges from-48.0 to +27.8X10-7/°C measured in the temperature range from room temerature to 800°C. The sintered product may find application in the area of electronic devices, heat engine components, spark plug or as hot plate tops, domestic cooking ware and such other engineering and technological applications where zero to a moderate thermal expansion coefficients and higher meachanical strength are required.

Full Text

The present invention relates to a process of making low thermal expansion lithium aluminosilicate ceramics. Ceramic articles have many uses including catalyst supports, dental porcelain, heat exchangers, turbine blades, substrates for integrated circuits. The particular ceramic which is used in a given application depends on the properties required for the particular application. For example, lithium-based ceramics such as beta-eucryptite and beta-spodumene have high thermal shock resistance and find applications as honeycomb substrates for supporting catalysts, electronic substrates and mirror blanks. Beta-eucryptite and beta-spodumene are lithium alumino silicate compositions. As noted in U.S. Patent no. 3,600,204, a preferred ceramic material for heat regenerator applications is a lithium aluminosilicate (LAS) glass powder which, upon firing, is converted by thermal crystallization to a low-expansion ceramic material. Such ceramics, sometimes termed glass-ceramics because they originate from glasses, comprise beta-spodumene or a beta-spodumene solid solution as the principal crystal phase. LAS powders and low-expansion beta-spodumene glass ceramics produced thereof offer a number of advantages for heat exchanger applications, such as ceramic turbine regenerators, wherein thermal cycling of the ceramic is frequent and severe. However, the cost of fabricating such regenerators by lay-up processes is still too high. Therefore, it has been proposed to produce similar products by the extrusion of high-cell-density ceramic honeycombs. Ceramic materials having negative coefficients of thermal expansion or which exhibit negative thermal expansion, i.e. which contract, rather than expand as their temperature increases, are known. In general, these materials have crystal structures with an-isotropic thermal expansion, where expansion along one crystallographic direction is negative and expansion along a different direction is positive. Examples of ceramic materials exhibiting negative thermal expansion include lithium aluminosilicate (LiAISi04), cordierite (2Mg0.2AI203.5Si02) and Ge-modified cordierite (2Mg0.2AI203.5Si02) compositions, aluminum titanate (AI2TiOs) based compositions, calcium strontium zirconium phosphate (Ca -1-x SrxZr4P6024) and similar compositions, Zr2P209l ZrV207, Ta2W08) and Nb2 05. In polycrystalline form, these ceramics exhibit a "net" negative thermal expansion because the contractive component of thermal expansion is larger than the expansive component. Of the various types of known negative thermal expansion ceramic materials, several ceramic compositions in the lithium aluminosiiicate family exhibit the largest negative thermal expansion values. There are three identified negative-expansion compounds in the lithium aluminosiiicate ceramic system: beta.-eucryptite (LiAISi04), spodumene (LiAISi206), and petalite (LiAISi4O10). All of these compounds exhibit anisotropy in thermal expansion, with negative expansion in one crystallographic direction. Sintered beta-eucryptite ceramics exhibit the most negative thermal expansion, with reported thermal expansion values ranging from -6 to -8 X 106/°C. The beta-eucryptite form of LiAISi04 is stable above 970°C whereas the alpha-eucryptite form is stable at lower temperatures. However sintered LiAISi04 ceramics which are processed above 970°C always have the beta-eucryptite structure, because the transformation from the alpha form to the beta form that occurs during calcination (or sintering) is irreversible. Thermal expansion anisotropy of beta.-eucryptite with a hexagonal crystal structure, is very large, with alpha in the a-axis direction is +8 x10-6/°C and alpha in the c-axis direction is about -17 x10-6/°C. The general methods of preparation of this class of ceramics include making of lithium aluminosiiicate glass powder which in turn is used as raw material for making desired ceramic material. In this regard reference may be made to Day; J. Paul in United States Patent No. 5,403,787, wherein extruded low-expansion ceramic honeycombs comprising beta-spodumene solid solution as the principal crystal phase and with less than 7 weight percent of included mullite are produced by compounding an extrusion batch comprising a lithium aluminosilicate glass powder and a clay additive, extruding a green honeycomb body from the batch, and drying and firing the green extruded cellular honeycomb to crystallize the glass and clay into a low-expansion spodumene ceramic honeycomb body. The extrusion of glass powders, however, presents several problems. Among these problems are the relatively low packing density of the glass powders, resulting in a need for higher levels of organic binders in glass-powder-based extrusion batches in order to successfully extrude complex shapes. As a consequence, attempts to produce products by the extrusion of glass powder batches have not been successful, due to the very high firing shrinkages incurred. High firing shrinkages would present particularly difficult problems in the production of thin-walled honeycomb structures by extrusion. Among such problems are a higher risk of distortion of the structure during sintering, and a higher incidence of cracking defects in the product. In addition, larger dies would be required to produce the oversize green honeycombs needed for the final product, and dies for the extrusion of large honeycombs are particularly difficult to fabricate. Alternatively aluminosilicate preform network structure such as zeolite may be used for making lithium aluminosilicate low expansion ceramics as is described by Bedard; Robert L. and Flanigen; Edith M. in United States Patent No. 5,179,051 titled: High density lithium-based ceramics from zeolites. This invention relates to a process for preparing a lithium containing ceramic article. The process provides a ceramic article that is substantially crack free and has as its principal crystalline phase a beta-eucryptite phase, a beta-spodumene phase, or a mixture thereof. The process comprises calcining a powder of a lithium-exchanged zeolite up to its collapse temperature for a time sufficient to collapse the zeolite framework and provide an amorphous powder. The amorphous powder is now formed into a shaped article and sintered at a temperature of about 700.degree. to about 1150.degree. C. for a time of about 1 to about 12 hours. The zeolites which can be used are those having a SiO.sub.2 Al.sub.2 O.sub.3 ratio of about 2 to about 8.5 and include zeolite B, zeolite ZK-19, zeolite W, phillipsite, hormotome, gismondine and mixtures thereof and have a sodium content less than about 1 weight percent. It is preferred that the lithium-exchanged zeolite have an average particle size of less than about 10 microns. The conventional manner of preparing lithium aluminosilicate ceramics is to sinter the respective oxides at high temperatures. Although this process can yield satisfactory ceramics, there is a need to form these ceramics at lower temperatures. The said inventors have developed a process which yields a high density ceramic article that is substantially crack free and has a crystalline phase of either beta-eucryptite or beta-spodumene at a substantially lower temperature than previous processes. The process involves calcining a lithium-exchanged zeolite up to its collapse temperature to give an amorphous powder, forming the amorphous powder into a desired shape and sintering at a temperature of about 700 to about 1150°C. This process produces ceramic articles which are substantially crack free. The prior art describes the preparation of ceramics from zeolites. For example, D. W. Breck in Zeolite molecular sieves, John Wiley & Sons, New York (1974), pp. 493-496 states that Mg-X can be heated to form cordierite. The disclosed process involves heating the Mg-X zeolite at 1500°C to form a glass and then heating the glass above 1000°C to form cordierite. Another reference which teaches the preparation of a cordierite based ceramic article is U.S. Patent no. 4,814,303. This discloses producing a monolithic anorthite, anorthite-cordierite or cordierite based ceramic article by heating the Ca, Ca/Mg and Mg forms of zeolites X, Y and A at a temperature of about 900°C to about 1350°C. It also discloses that one should maximize the removal of sodium present in the zeolites since sodium ions are known to significantly increase the dielectric constant and dielectric loss. Reference may be made to European Patent Publication Number 298,701, which describes the preparation of a ceramic article having an anorthite phase from a calcium zeolite. The process involves a calcination to form an amorphous product which can then be shaped into an article and sintered at tempereatures of about 850°C-950°C. Finally, the preparation of beta-spodumene from lithium zeolite-Y and beta-eucryptite from lithium zeolite-A is reported in Mat. Res. Bull., 21, (1986) 1525-1532 and J. Solid State Chem., 63, (1986) 46-51 respectively. However, neither of these references disclose the preparation of a crack free dense beta-spodumene or beta-eucryptite ceramic article. None of these references disclose or render obvious applicants' two step process for producing a lithium-based ceramic article. The said inventors have observed that the collapse temperature of each zeolite must be individually determined so that recrystallization does not occur. The importance of the collapse temperature is nowhere mentioned nor recognized in the prior art.Reference may be made to United States Patent No. 5,179,051, which describes "a process for preparing a lithium containing ceramic article". The process provides a ceramic article that is substantially crack free and has as its principal crystalline phase a beta-eucryptite, a beta-spodumene, or a mixture thereof. The process comprises calcining a powder of a lithium-exchanged zeolite up to its collapse temperature for a time sufficient to collapse the zeolite framework and provide an amorphous powder. The amorphous powder is now formed into a shaped article and sintered at a temperature of about 700°C to about 1150°C for a time of about 1 to about 12 hours. The zeolites which can be used are those having a SiO2Al2Os ratio of about 2 to about 8.5 and include zeolite B, zeolite ZK-19, zeolite W, phillipsite, hormotome, gismondine and mixtures thereof and have a sodium content less than about 1 weight percent. It is preferred that the lithium-exchanged zeolite has an average particle size of less than about 10 microns. A problem with known negative thermal expansion ceramic materials is that in many cases, the expansion anisotropy leads to microcracking, which reduces mechanical strength of sintered ceramics. There has been a need to develop suitable compositions and ceramic processing methods to allow the fabrication of high density and mechanically robust ceramic components of materials exhibiting negative thermal expansion behavior. Beta.-eucryptite ceramics can be readily formed using conventional ceramic methods of ball milling, calcination, and sintering. The LiAISi04 composition can be modified by additions of up to about 50 mol % of both AIP04 and LiAIGe04, without affecting beta-eucryptite phase formation or expansion behavior. With oxide starting materials, calcination temperatures of between about 1000 and about 1100°C and sintering temperatures of between about 1200 and about 1300°C have been used for the successful preparation of single phase beta-eucryptite ceramics. There also has been some work in the synthesis of beta-eucryptite using sol-gel methods for powder preparation. The sol-gel method involves the use of expensive metal-organic precursors, but has some advantages related to controlling U2O volatility and achieving high sintered densities. However, anisotropic thermal expansion has made it difficult to produce high strength LAS ceramics. A relatively low modulus of rupture or flexural strength value of 2000 psi (13.8 MPa) is typical for beta-eucryptite ceramics. Reference may be made to United States Patent No 6,066,585 by Swartz; Scoh L. in "Ceramics having negative coefficient of thermal expansion, method of making such ceramics, and parts made from such ceramics" wherein a ceramic material in the lithium aluminosilicate (LAS) system, having a negative coefficient of thermal expansion and improved mechanical properties were prepared. The ceramic material can be made by mixing silicon and aluminum oxides (Si02 and Al205) with lithium carbonate (Li2C03) and calcining the mixture. Alternatively, the ceramic material can be made by mixing silicon oxide (Si02), lithium aluminate (LiAI02), and, if desired, lithium carbonate (Li2C03), and calcining the mixture. Alternatively, the ceramic material can be made by mixing spodumene (an inexpensive mineral with a nominal composition of LiAISi206), lithium aluminate (LiAI02) and the required amounts of other constituents (Li2C03, Al203l or Si02), and calcining the mixture. Alternatively, the ceramic material can be made by mixing spodumene (nominally LiAISi2 Oe) and the required amounts of other constituents (Li2C03, Al203 or Si02) and calcining the mixture. The ceramics of this invention exhibit negative thermal expansion, and improved mechanical properties, which allow them to be used as components of thermostats and other products. The method described above was refined by Beall; Douglas M. and Beall; George H. of United States Patent No. 6,566,290 whose instant invention is founded upon the discovery of a predominately two-phase ceramic within the Li20-Al2 03-Si02 system which has high refractoriness, high resistance to thermal shock, and high heat capacity properties which make the inventive ceramic extremely desirable in high temperature applications, such as filters for diesel exhaust engines. The process as described in the above Patent entitled Lithium aluminosilicate ceramic is described below: A ceramic article which consists essentially, by weight on the oxide basis, of 10-25% Si02, 65-85% Al203, and 2-12% Li20 and comprises beta-eucryptite as a first phase having a negative component in thermal expansion and a melting point Tm-i, and a second phase having a positive component in thermal expansion which is higher than the component in thermal expansion of the first phase and a melting point Tm2, wherein Tnri2 >Tm1, wherein the first phase is at most 50% by weight of the ceramic, and wherein the ceramic is characterized by microcracking. Tm2 is at least 1800°C. The ceramic article exhibits a near zero coefficient of thermal expansion from room temperature to 800°C, a high refractoriness, and a high resistance to thermal shock properties which make the inventive ceramic extremely desirable in high temperature applications, such as filters for diesel exhaust engines. In our co-pending patent application No. NF 416/2000, we have described and claimed a process of preparation of lithium aluminosilicate material. In the same patent document several drawbacks of the known methods have been described with relevant references. A method of making powder precursor and sintering of the pressed powders made thereof is discussed. From the above discussion, it seems that tailoring of composition may produce sintered materials of desired thermal expansion characteristics. This tailoring is possible either by manipulation in the composition to synthesize biphasic material or the composition may be selected in such a way that a phase transition is effected during the densification producing desired thermal expansion. In all the previous works, the initial composition comprised of lithium oxide, alumina and silica that produce mostly beta-eucryptite, beta-spondumene and phases consisting of lithiumaluminate spinel, lithiumaluminate, corundum and combination thereof. Formation and separation of such phases requires considerably higher processing temperature and as a consequence, renders controlling the processing parameters more complex. In addition to the above formation and segregation of undesirable phases exerts undesirable influence on the desired properties. These difficulties or drawbacks may be overcome by taking broadly two measures namely, (a) Appropriately processing right type of raw materials to make a powder precursor which on further processing give the desired products. This may be done by forming a hydroxy-hydrogel type of intermediate of which details are given in our earlier patent No. NF 416/2000. (b) Using a nucleating material which catalyses the formation of desirable phases in the sintered composites. The main object of the present invention is to provide a process of making low thermal expansion lithium aluminosilicates ceramics. Another object of the present invention is to have precise control over the composition of reactants to make the desired product. Yet another object of the present invention is to increase the reactivity of the reactants to reduce sintering temperature. In the present invention there is provided a process for preparing low thermal expansion ceramics in the lithium aluminmosilicate system. The process includes the preparation of an intermediate product in the hydroxy-hydrogel form which is in a reactive state. The process involves incorporation of nucleating materials such as zirconia in the composition that not only increases the product quality but also reduces the overall sintering temperature. The coefficient of thermal expansion of the material ranges from -48.0 to + 27.8 X 10-7/°C measured in the temperature range from room temerature to 800°C. The sintered product may find application in the area of electronic devices, heat engine components, spark plug or as hot plate tops, domestic cooking ware and such other engineering and technological applications where zero to a moderate thermal expansion coefficients and higher mechanical strength are required. Accordingly the present invention provides a process of making low thermal expansion lithium aluminosilicate ceramics which comprises adding concentrated nitric acid drop-wise to a mixture of 4 to 18% lithium salt in 45 to 53% water by weight so that entire lithium salt goes into solution to obtain lithium nitrate solution, adding this solution to a mixture consisting of 36 to 43% aluminium salt solution and 2 to 10% zirconium salt solution to obtain a mix solution, adding into this mix solution reactive silica in the range of 6 to 10%, stirring for a period of 0.5 to 1 hour to obtain a suspension, pouring the suspension so obtained into ammonia solution under stirring, maintaining a pH in the range of 4 to 7, allowing the resultant mixture to stand for a period in the range of 24 to 36 hours at room temperature, drying to obtain the solid material formed, calcining the solid so obtained at a temperature in the range of 500 to 800°C, grinding the calcined material by known methods, preparing green shapes by known methods, sintering at a temperature in the range 1100 to 1350°C for a period in the range of 2 to 4 hours. In an embodiment of the present invention, the lithium salt is such as lithium nitrate, lithium carbonate, except lithium sulfate. In another embodiment of the present invention, the aluminium salt is such as aluminium nitrate, aluminium chloride, except aluminium sulfate. In yet another embodiment of the present invention, the reactive silica is such as precipitated silica, fumed silica. In still another embodiment of the present invention, the zirconium salt is such as zirconium oxy-chloride, zirconium nitrate. In still yet another embodiment of the present invention, the intermediate calcination step is eliminated, if porous sintered body is desired. The novelty of the process of making low thermal expansion lithium aluminosilicate ceramics, of the present invention is the development of sintered compacts in the lithium aluminosilicate system having lower thermal expansion, with precise control over the composition of reactants to make the desired product at a lower sintering temperature. The novelty of the process of the present invention has been achieved by the non-obvious inventive step of incorporating nucleating materials such as zirconia in the composition that not only increases the product quality but also reduces the overall sintering temperature. The following examples are given by of illustration of the process of the present invention in actual practice and therefore should not be construed to limit the scope of the present invention. Example 1 31.89 gm of lithium carbonate was mixed with 253 gm of aluminium nitrate and 2gm of zirconium oxy chloride and 316 cc of water was added into it. Dilute nitric acid was added in to the mixture till the effervescence was ceased. 52.73gm of fumed silica was added into the solution and stirred for 30 mins. The entire mixture so produced was poured into 500 ml of liquor ammonia. The mixture was kept for 24 hours and the solid mass was collected. The solid mass was calcined at 500°C. The calcined material was ground by pot milling for 2 hours. Green shapes were fabricated from this powder by isostatic pressing under 200 Mpa. The green product was sintered at 900°C for 2 hours. The coefficient of thermal expansion was -17.2 to + 15.4X 10"7/°C when measured from room temperature to 300°C. Example 2 29.29 gm of lithium carbonate was mixed with 191 gm of aluminium chloride and 4gm of zirconium oxy chloride and 239 cc of water was added into it. Concentrated nitric acid was added in to the mixture till the effervescence was ceased. 47.68 gm of fumed silica was added into the solution and stirred for 60 mins. The entire mixture so produced was poured into 500ml of liquor ammonia. The mixture was kept for 24 hours and the solid mass was collected. The solid mass was calcined at 600°C. The calcined material was ground by pot milling for 4 hours. Green shapes were fabricated from this powder by isostatic pressing under 200 Mpa. The green product was sintered at 1200°C for 4 hours. The coefficient of thermal expansion was -48.0 to - 3.35X 10"7/°C when measured from room temperature to 500°C. Example 3 87.24 gm of lithium carbonate was mixed with 266 gm of aluminium nitrate and 6gm of zirconium oxy chloride and 333 cc of water was added into it. Concentrated nitric acid was added in to the mixture till the effervescence was ceased. 53.21gm of fumed silica was added into the solution and stirred for 50 mins. The entire mixture so produced was poured into 700 ml of liquor ammonia. The mixture was kept for 40 hours and the solid mass was collected. The solid mass was calcined at 700°C. The calcined material was ground by pot milling for 2.5 hours. Green shapes were fabricated from this powder by isostatic pressing under 150 Mpa. The green product was sintered at 1250°C for 3 hours. The coefficient of thermal expansion is -26.2 to + 5.25X 10"7/°C when measured from room temperature to 500°C and the same is -26.2 to + 10.80X 10"7/°C when measured up-to 800°C. Example 4 78.84 gm of lithium nitrate was mixed with 155 gm of aluminium chloride and 8gm of zirconium nitrate and 193 cc of water was added into it. Concentrated nitric acid was added in to the mixture till the effervescence was ceased. 57.71gm of fumed silica was added into the solution and stirred for 40 mins. The entire mixture so produced was poured into 700 ml of liquor ammonia. The mixture was kept for 30 hours and the solid mass was collected. The solid mass was calcined at 750°C. The calcined material was ground by pot milling for 3 hours. Green shapes were fabricated from this powder by isostatic pressing under 200 Mpa. The green product was sintered at 1300°C for 4 hours. The coefficient of thermal expansion is -31.4 to + 4.20X 10"7/°C when measured from room temperature to 500°C and the same is -31.4 to + 12.90X 10"7/°C when measured up-to 800°C. Example 5 21.59 gm of lithium carbonate was mixed with 141 gm of aluminium chloride and 10gm of zirconium oxy chloride and 176 cc of water was added into it. Concentrated nitric acid was added in to the mixture till the effervescence was ceased. 61.43gm of fumed silica was added into the solution and stirred for 45 mins. The entire mixture so produced was poured into 800 ml of liquor ammonia. The mixture was kept for 35 hours and the solid mass was collected. The solid mass was calcined at 600°C. The calcined material was ground by pot milling for 3 hours. Green shapes were fabricated from this powder by isostatic pressing under 200 Mpa. The green product was sintered at 1350°C for 2 hours. The coefficient of thermal expansion is -17.2 to + 27.8 X 10"7/°C when measured from room temperature to 600°C. In the examples as given herein above for a process for preparing low thermal expansion ceramics in the lithium aluminmosilicate system, the coefficient of thermal expansion of the material ranges from -48.0 to + 27.8 X 10-7/°C measured in the temperature range from room temperature to 800°C. The advantages of the process of the present invention for making low thermal expansion lithium aluminosilicate ceramics: 1. Low thermal expansion ceramics may be manufactured. 2. Sintering temperature is lowered. 3. Precision control over the composition in the precursor is possible. We claim: 1. A process of making low thermal expansion lithium aluminosilicate ceramics which comprises; adding concentrated nitric acid drop-wise to a mixture of 4 to 18% lithium salt in 45 to 53% water by weight so that entire lithium salt goes into solution to obtain lithium nitrate solution, adding this solution to a mixture consisting of 36 to 43% aluminium salt solution and 2 to 10% zirconium salt solution to obtain a mix solution, adding into this mix solution reactive silica in the range of 6 to 10%, stirring for a period of 0.5 to 1 hour to obtain a suspension, pouring the suspension so obtained into ammonia solution under stirring, maintaining a pH in the range of 4 to 7, allowing the resultant mixture to stand for a period in the range of 24 to 36 hours at room temperature, drying to obtain the solid material formed, calcining the solid so obtained at a temperature in the range of 500 to 800°C, grinding the calcined material by known methods, preparing green shapes by known methods, sintering at a temperature in the range 1100 to 1350°C for a period in the range of 2 to 4 hours. 2. A process as claimed in claim 1, wherein the lithium salt is lithium nitrate, lithium carbonate, except lithium sulfate. 3. A process as claimed in claim 1, wherein the aluminium salt is aluminium nitrate, aluminium chloride, except aluminium sulfate. 4. A process as claimed in claim 1, wherein the reactive silica is precipitated silica, fumed silica. 5. A process as claimed in claim 1, wherein the zirconium salt is zirconium oxychloride, zirconium nitrate. 6. A process of making low thermal expansion lithium aluminosilicate ceramics substantially as herein described with reference to the examples.

Documents:

645-del-2004-Abstract-(26-03-2010).pdf

645-del-2004-abstract.pdf

645-del-2004-Claims-(26-03-2010).pdf

645-del-2004-claims.pdf

645-DEL-2004-Correspondence Others-(05-08-2011).pdf

645-del-2004-Correspondence-Others-(26-03-2010).pdf

645-del-2004-correspondence.pdf

645-del-2004-description.pdf

645-del-2004-Form-3-(26-03-2010).pdf

645-del-2004-form1.pdf

645-del-2004-form2.pdf

645-del-2004-form3.pdf

645-del-2004-form5.pdf