Patent Description:
Over the years materials have been developed, consisting of both reinforced cement-based composite materials and ceramic materials in order to implement finished manufactured items having complex shapes and morphologies, to be used in both the steel working and the electric industrial fields. Said materials must have at the same time improved properties of mechanical resistance, high refractoriness, excellent machinability and forming, excellent resistance to thermal shocks and exceptional efficiency in resisting arc in the presence of high and very high voltages.

The reinforced cement-based composite materials have shown strong limits in the event of electric applications such as arc-chutes in extreme electric operation conditions (high/very high powers): the manufactured items implemented with said materials collapse, whereas ceramic materials do not ensure those properties of machinability and forming which allow them to be used for implementing finished manufactured items with a very high quality standard of dimensional reproducibility, in compliance with tolerances, and accuracy of surfaces.

<CIT> discloses a "refractory heating element" and a "refractory insulating material" made out of a composition consisting of corundum, beta-alumina, silica sol and fibers, said composition normalized to <NUM>% and calculated on <NUM> parts by weight of the total of Na<NUM>O, Al<NUM>O<NUM> and SiO<NUM>, said composition comprises in wt%: <NUM>% Al<NUM>O<NUM> , <NUM>% beta-alumina, providing <NUM>% Na<NUM>O and <NUM>% silica sol, providing <NUM>% SiO<NUM>. Moreover it is explained the difference among β- β'- and β"-aluminas: β-alumina is represented with general formula R<NUM>O·11Al<NUM>O<NUM> (R is an alkali metal) to a broad sense, and points out at it the sodium β-alumina whose alkali metal R is Na to in a narrow sense. Although an idealization study composition is Na<NUM>O· 11Al<NUM>O<NUM>, generally Na<NUM>O is included excessively. Furthermore, it is known that the β'- alumina by which a chemical composition is represented with Na<NUM>O· 7Al<NUM>O<NUM>, the β"- alumina by which the chemical composition is represented with Na<NUM>O· <NUM>-6Al<NUM>O<NUM>, etc. exist.

Mineral Data Publishing <NUM> - <NUM> dated October <NUM>, <NUM> concerning "Data Sheet: Diaoyudaoite - NaAl<NUM>O<NUM>" discloses the chemical composition of Chinese Diaoyudaoite, that, as listed, may comprises apart from the oxides of Al, Na and Si also the oxides of K, Mg, Ca, Cr and Sr.

<CIT> discloses a new refractory material with improved mechanical and thermal resistance properties to be used as internal coatings in blast furnaces for the steel industry, said material having greater resistance to abrasion and a lower tendency to accumulate layers of "calamine", but electrical insulation properties are not disclosed or pursued.

The solution provided by <CIT> is a refractory material comprising ZrO<NUM>, SiO<NUM>, Al<NUM>O<NUM> at least one alkali metal oxide with a crystallographic composition of Corundum, Zirconia and Vitreous phase, but not beta-alumina, wherein the SiO<NUM> is present in the vitreous phase as amorphous silica, not as quartz.

Accordingly, there existed a strong need to develop new composite materials which were able to overcome simultaneously all the specific limits of the above-mentioned materials, as a valid alternative to both reinforced cement-based composites, characterised by limits of applicability in the electric industrial fields in certain conditions of use at high and very high voltages, and ceramic materials, having a very complex and hardly cost-effective production cycle and also being particularly unsuitable to produce manufactured items with a particularly complex shape and morphology.

Continuing research in the present technical field, the applicant has surprisingly and unexpectedly implemented a new refractory composite material based on corundum, quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, as well as a refractory body or manufactured item or a body or manufactured item made of thermal or electric insulator based on said refractory composite material.

Accordingly, the objects of the present invention are:.

It is also disclosed a process for preparing the refractory composite material based on corundum, quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, as well as a process for preparing a refractory body or manufactured item or a body or manufactured item made of thermal or electric insulator based on said refractory composite material.

The refractory composite material resulting from the present invention therefore comprises corundum, quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite.

It is also disclosed the slurry to obtain the refractory composite material based on corundum, quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, the slurry comprising:.

It is also disclosed the use of the refractory composite material according to the present invention, such as: plates for arc-chutes, particularly arc-chutes for high-voltage contactors, insulating plates resisting to high temperatures and high voltages, finned insulators, resistor supports, as well as coatings for high-temperature furnaces and heat-exchanger pipes of the bodies or manufactured items made of electric/thermal refractory or insulating material as described above; further objects of the present invention and also objects of the present invention are plates for arc-chutes, particularly arc-chutes for high-voltage contactors, insulating plates resisting to high temperatures and high voltages, finned insulators, resistor supports, as well as coatings for high-temperature furnaces and heat-exchanger pipes of the bodies or manufactured items made of electric/thermal refractory or insulating material as described above, all comprising the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, according to the present invention.

An object of the present invention is accordingly a refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and NaAl<NUM>O<NUM>, diaoyudaoite syn beta NaAl<NUM>O<NUM> ½ (Na<NUM>O·11Al<NUM>O<NUM>), having a hexagonal structure with: <NUM>,<NUM> - b: <NUM>,<NUM> - c: <NUM>,<NUM> - α: <NUM>,<NUM> - β: <NUM>,<NUM> - γ: <NUM>.

The refractory composite material which is the object of the present invention was submitted to qualitative and quantitative SEM/EDS analysis and diffractometric analysis in order to identify its chemical and morphological composition by identifying the prevailing chemical components and the main crystalline phases characterising it.

As to the diffractometric analysis, the refractory composite material which is the object of the present invention was analysed under the following conditions:.

In particular, the refractory composite material which is the object of the present invention comprises, as the main crystalline forms, Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, whose crystalline phases are identified in the diffractogram shown in <FIG>, wherein comparing the graph of the obtained diffractogram, hence the relevant most significant peaks, to which the signals corresponding to the standard values of the angle <NUM> of the three main identified crystallographic phases are overlapped: the diffractogram peaks corresponding to the standards of Al<NUM>O<NUM> in the form of corundum (vertical strokes marked by the square symbol), the diffractogram peaks corresponding to the standards of SiO<NUM> in the form of quartz (vertical strokes marked by the diamond-shaped symbol), the diffractogram peaks corresponding to the standards of the mixed sodium and aluminium oxide (vertical strokes marked by the round symbol). The following three main crystalline phases are identified by said diffractogram:.

As to the qualitative morphologic and quantitative SEM/EDS analysis, the refractory composite material which is the object of the present invention was analysed under the following conditions:
Instrumental conditions: SEM microscope model JEOL JSM-5910LV.

Accomplished measurements: SEM images with different magnifications of the cross section of the composite material which is the object of the present invention, shown in <FIG>; EDS analysis on the milled sample.

From the morphological standpoint, the samples of the refractory composite material according to the present invention show several macroscopic closed porosities ranging between <NUM> and <NUM>. Aside from said macroporosities, the material has a well dense and even look.

The size of the crystalline grains ranges between <NUM> and <NUM>.

The refractory composite material which is the object of the present invention has a particle size distribution preferably not exceeding <NUM> micrometres, more preferably ranging between <NUM> and <NUM> micrometres.

The refractory composite material which is the object of the present invention was milled and the powder was homogenised in an alumina mortar; the powder was then pressed so as to accomplish the microanalytical chemical analysis thereon. The EDS analysis was then accomplished by collecting five EDS spectra, with the results shown in the following tables.

The analysis does not allow elements with a lower weight than nitrogen to be quantified; oxygen is instead stoichiometrically recalculated on the other identified elements, hence expressing them in the form of oxides.

The quantitative diffractometric analysis allowed the crystallographic composition to be identified as % by weight of the individual identified crystalline phases and of the amorphous silica phase, as shown in the following table <NUM>:.

The quantitative diffractographic analysis also shows that the weight ratio between α and β Al<NUM>O<NUM> ranges from <NUM>-<NUM>% of α Al<NUM>O<NUM> and <NUM>-<NUM>% of β Al<NUM>O<NUM>.

In particular, the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, according to the present invention, comprises: Al, Si, Na and oxygen, which in the form of Al<NUM>O<NUM>, SiO<NUM>, Na<NUM>O oxides are present in such amounts that: the overall amount of Al<NUM>O<NUM> is no less than <NUM> wt%, preferably no less than <NUM> wt%, the overall amount of SiO<NUM> in the form of quartz is no less than <NUM> wt%, preferably no less than <NUM> wt%, the overall amount of Na<NUM>O is no less than <NUM> wt%, preferably no less than <NUM> wt%, said percentages being calculated on <NUM> parts by weight of the combination of said oxides.

More particularly, the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, according to the present invention, comprises: Al, Si, Na, K, Ca and oxygen, which in the form of Al<NUM>O<NUM>, SiO<NUM>, Na<NUM>O, K<NUM>O and CaO oxides are present in such amounts that: the overall amount of Al<NUM>O<NUM> is no less than <NUM> wt%, the overall amount of SiO<NUM> in the form of quartz is no less than <NUM> wt%, the overall amount of Na<NUM>O is no less than <NUM> wt%, the overall amount of K<NUM>O is no less than <NUM> wt% and the overall amount of CaO is no less than <NUM> wt%, said percentages being calculated on <NUM> parts by weight of the combination of said oxides.

Even more particularly, the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, according to the present invention, comprises: Al, Si, Na, K, Ca and oxygen, which in the form of Al<NUM>O<NUM>, SiO<NUM>, Na<NUM>O, K<NUM>O and CaO oxides are present in such amounts that: the overall amount of Al<NUM>O<NUM> ranges between <NUM> and <NUM> wt%, the overall amount of SiO<NUM> in the form of quartz ranges between <NUM> and <NUM> wt%, the overall amount of Na<NUM>O - K<NUM>O - CaO ranges between <NUM> and <NUM> wt%, said percentages being calculated on <NUM> parts by weight of the combination of said oxides.

The peculiarity of the refractory composite material which is the object of the present invention is that the overall amount of SiO<NUM> in the form of quartz, as defined above, is no less than <NUM> wt% relative to the combination of said overall amount of SiO<NUM> in the form of quartz with the overall amounts of Al<NUM>O<NUM> and Na<NUM>O, the combination of said oxides present in the refractory composite material which is the object of the present invention being <NUM> parts by weight. The same applies to the refractory composite material according to the present invention also comprising K and Ca in the form of oxides, so that the overall amount of SiO<NUM> in the form of quartz, as defined above, is expressed as % by weight relative to the combination of said overall amount of SiO<NUM> in the form of quartz with the overall amounts of Al<NUM>O<NUM>, Na<NUM>O, K<NUM>O and CaO, the combination of said oxides present in the refractory composite material which is the object of the present invention being <NUM> parts by weight.

The refractory composite material which is the object of the present invention has also been characterised from the chemical-physical standpoint in that measures have performed thereon to characterise the relevant properties of actual and theoretical density, open porosity and absorbed water, as well as to characterise the relevant properties of thermal conductivity and specific heat.

Characterisation of the properties of actual and theoretical density, open porosity and absorbed water of samples of the refractory composite material which is the object of the present invention.

The samples were dried in an oven at <NUM> for <NUM> hours and their dry weight (ms) was determined; the samples were immersed in distilled water and heated to boiling temperature for <NUM> hour. The samples were then cooled in distilled water. The immersed weight (mw) and the wet weight (mh) were determined. The theoretical and actual density, the open porosity and the absorbed water were calculated with the following formulas: <MAT> <MAT> <MAT> <MAT>.

The refractory composite material which is the object of the present invention has a density ranging between <NUM> and <NUM>/cm<NUM> with an average value of <NUM>/cm<NUM>, a porosity ranging between <NUM> and <NUM>% by volume with an average value of <NUM>%, a theoretical density ranging between <NUM> and <NUM>/cm<NUM> with an average value of <NUM>/cm<NUM>, measured according to standard UNI <NUM>: determining actual and theoretical density, open porosity and absorbed water.

Characterisation of the properties of thermal conductivity and specific heat of samples of the refractory composite material which is the object of the present invention.

The refractory composite material which is the object of the present invention has an average specific heat of <NUM> J/(g·°K) and an average thermal conductivity of <NUM> W/(m·°K), measured at <NUM>, and an average specific heat of <NUM> J/(g·°K) and an average thermal conductivity of <NUM> W/(m·°K), measured at <NUM> according to the methods ASTM E-<NUM> and DIN EN <NUM>.

Optionally, the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, according to the present invention, can also comprise glass fibres based on silica and zircon which, when present, preferably have an average length ranging between <NUM>-<NUM>, more preferably between <NUM>-<NUM>, even more preferably <NUM>.

When present, the glass fibres based on silica and zircon according to the present invention are fibres having a density ranging between <NUM> and <NUM>/cm<NUM>, and even more particularly the glass fibres based on silica and zircon are present in an amount by weight ranging between <NUM> and <NUM> per cm<NUM>, more preferably between <NUM> and <NUM> per cm<NUM> of the refractory composite material according to the present invention, more particularly the glass fibres based on silica and zircon are present in a number ranging between <NUM> and <NUM> per cm<NUM>, more preferably between <NUM> and <NUM> per cm<NUM> of the refractory composite material according to the present invention.

Glass fibres based on silica and zircon are to be understood as glass fibres which are well known in the art, consisting of glass fibres made alkali resisting by the presence of Zr in the form of zirconia or zirconium dioxide, in general consisting of glass fibres comprising a % of zirconia ranging between <NUM>-<NUM>%, preferably <NUM>-<NUM>%, even more preferably <NUM>-<NUM>% by weight of the glass fibres.

In said glass fibres the component silica SiO<NUM> is prevailing, since it may range between <NUM>% and <NUM>%, between <NUM>% and <NUM>% by weight of the fibres, and a minimum part of flux components, such as mainly calcium and sodium oxides, are present therein in a maximum amount of <NUM>% by weight of the fibres.

Expressing said percentages as ratios between SiO<NUM> and zircon, namely zirconium silicate Zr(SiO<NUM>), said percentages can be expressed as: ZrSiO<NUM> <NUM>-<NUM>% and SiO<NUM> <NUM>-<NUM>%, ZrSiO<NUM> <NUM>-<NUM>% and SiO<NUM> <NUM>-<NUM>%, ZrSiO<NUM> <NUM>-<NUM>% and SiO<NUM> <NUM>-<NUM>%, in all cases the remaining % comprising flux components.

In a particularly preferred form of embodiment of the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, said refractory composite material is obtained through a process comprising the following steps:.

It is also disclosed a process for preparing the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, which is the object of the present invention, said process comprising the steps a), b) and c) as described above.

In a preferred form of embodiment of the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, which is the object of the present invention, in step a) of mixing in water:.

the remaining part being water, said percentages being calculated on <NUM> parts by weight of the resulting slurry.

Melted alumina according to the present invention is to be understood as melted alumina selected from pure alumina with a pureness degree of no less than <NUM>% and a particle size distribution of no more than <NUM>, preferably with a particle size distribution of <NUM>-120µ (micrometres), or pure alumina with a pureness degree of no less than <NUM>% and a particle size distribution ranging between 10µ and 60µ (micrometres), or mixtures thereof.

Tabular alumina according to the present invention is to be understood as tabular alumina selected from pure alumina with a pureness degree of no less than <NUM>% and a particle size distribution of no more than <NUM>, preferably with a particle size distribution of no more than <NUM>, or mixtures thereof.

Reactive alumina according to the present invention is to be understood as reactive alumina with a pureness degree of <NUM>%, with unimodal and bimodal particle size distribution curves, having the size of elementary crystals <NUM>/2µ (micrometre).

The use of these aluminas, mainly of the reactive aluminas, is aimed at improving the rheological features of the slurry and provides the manufactured item with a high structural evenness.

Micosilica (MS) according to the present invention is to be understood as a material based on silica with a pureness degree of more than <NUM>% in SiO<NUM>, with particle size ranging from <NUM> to 1µ (or micrometres), hence submicronic (nanometric). The particles are spheroidal and provide good flowability to the slurry. In gel-bonding systems, microsilica cross-links, contributing to provide toughness to the manufactured item upon stripping.

Microsilica <NUM> very reactive during sintering, improving the "quality" of the "ceramic" bond at reduced temperatures.

Microsilica is a material which is easily retrieved on the market.

Colloidal silica according to the present invention is to be understood as colloidal silica comprising an aqueous dispersion, from <NUM> to <NUM> % by weight of water, of "silica nuclei or sols", silicon spheroidal amorphous particles, having a nanometric size, cross-linking in the gel-bonding systems, similarly to microsilica.

Inorganic dispersant additive based on SiO<NUM> and Al<NUM>O<NUM> according to the present invention is to be understood as a material comprising <NUM>/<NUM>% by weight of SiO<NUM> and <NUM>/<NUM>% by weight of Al<NUM>O<NUM>, in the form of powder, suitable to be used in systems based on microsilica or colloidal silica. This is a material which can be easily retrieved on the market. Said additive reduces the percentage of water in the slurry and improves the rheological properties of the system, providing more flowability to the slurry in the moulds.

Inorganic accelerating additive based on SiO<NUM> and Al<NUM>O<NUM> according to the present invention is to be understood as a material comprising <NUM>/<NUM>% by weight of SiO<NUM> and <NUM>/<NUM>% by weight of Al<NUM>O<NUM>, in the form of powder, suitable to be used in systems based on microsilica or colloidal silica. This is a material which can be easily found on the market.

It controls the SET-TIME of the slurry, reduces the time of processing of the slurry and stripping of the manufactured item, increases the resistance of the "green", that is the manufactured item being formed before sintering thereof, improves the refractory properties of the resulting composite material.

Inorganic toughening additive based on SiO<NUM> and Al<NUM>O<NUM> according to the present invention is to be understood as a material comprising <NUM>/<NUM>% by weight of SiO<NUM> and <NUM>/<NUM>% by weight of Al<NUM>O<NUM>, in the form of powder, suitable to be used in systems based on microsilica or colloidal silica. It is a material which can be easily retrieved on the market. It improves the toughness of the "green" manufactured item, improves the rheological properties of the slurry, controls the time of processing of the slurry and the time of stripping of the manufactured item.

Flux oxides according to the present invention are to be understood as one or more oxides selected from: Na<NUM>O , K<NUM>O , CaO , MgO , Al<NUM>O<NUM>, B<NUM>O<NUM>, SiO<NUM>, their mixture being preferred, in the form of micronized powder. Said flux oxides are easily retrieved on the market. Said mixture of flux oxides is highly akin to microsilica, improves the formation of stable bonds in the composite at low temperatures, reduces the porosity of the end structure, significantly improves the mechanical resistance of the manufactured item.

Organic dispersant additive according to the present invention is to be understood as a material comprising polymer material based on polyethylene glycol.

This is a material which is easily retrievable on the market. This additive, suitable for gel-bonding systems of composites based on microsilica, allows the amount of water to be used in the slurry to be strongly reduced and helps improve the processing rheological conditions.

Organic de-airing additive comprising alkoxylated fatty alcohols and polysiloxanes on inorganic medium according to the present invention is to be understood as a material of antifoaming powder based on alkoxylated fatty alcohols and polysiloxanes on inorganic medium. This is a material which is easily retrievable on the market. This additive dispersed in the slurry strongly acts as antifoam, positively reducing the porosities of the refractory composite material being built.

In further, even more preferred, forms of embodiment of the refractory composite material according to the present invention, as well as the relevant process of preparation as described above, in step a) of mixing in water, the weight ratios between melted and/or tabular alumina and reactive alumina, with different degrees of pureness and particle size distribution, in the slurry preferably range between <NUM>:<NUM> and <NUM>:<NUM>.

When both melted alumina and tabular alumina are present at the same time, the weight ratio between them ranges between <NUM>:<NUM> and <NUM>:<NUM>, preferably amounts to <NUM>:<NUM>, having the same particle size.

When both microsilica (MS) and colloidal silica are present at the same time, the weight ratio between them ranges between <NUM>:<NUM> and <NUM>:<NUM>, although use of microsilica alone is preferred.

When present, glass fibres based on silica and zircon according to the present invention are added in the process for preparing the refractory composite material which is the object of the present invention in step a) of mixing in water, preferably in the amount of <NUM>% by weight, more preferably in the amount of <NUM>% by weight out of <NUM> parts in weight of the resulting slurry.

It is also disclosed the slurry to obtain the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, the slurry comprising:.

In particular, the slurry to obtain the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, is a slurry wherein:.

In further, even more preferred, forms of embodiment of the refractory composite material according to the present invention, as well as the relevant process of preparation as described above, in step b) of casting, the mould or receptacle is in continuous or discontinuous vibration, preferably at constant frequency, with the purpose to favour the even distribution of the components and to avoid sedimentations and agglomerations and, when present, to allow glass fibres based on silica and zircon to be distributed evenly and in an anisotropic manner, with partial alignment of the same, as well as air bubbles to be removed.

The moulds or receptacles of step b) are of metal, ceramic, wood/cellulose, plastics, in particular silicone, or combinations thereof.

In further, even more preferred, forms of embodiment of the refractory composite material according to the present invention, as well as the relevant process of preparation as described above, in step c) crosslinking is accomplished through following treatments of: drying at ambient temperature <NUM>-<NUM> for <NUM> hours; following heat treatment with average gradient of <NUM>±<NUM> for densification and the formation of stable bonds at <NUM> - <NUM> in a moderately oxidizing environment.

The refractory composite material which is the object of the present invention combines improved properties of excellent resistance to high voltage, high resistance to thermal shocks, accompanied by good mechanical resistance, as well as the absence of shrinking during "gel-bonding" setting or hardening and high chemical inertness.

This allows moulds with high tolerance and exceptional accuracy to be used and accordingly manufactured items made of said refractory composite material to be obtained, with high quality standard of dimensional reproducibility, in compliance with tolerances, and accuracy of surfaces.

The following are also disclosed accordingly to the present invention:.

In particular, in the process for preparing a manufactured item of the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, optionally strengthened with glass fibres based on silica and zircon according to the present invention, as described above, the mould is a silicone mould. When present, glass fibres based on silica and zircon according to the present invention are added in the process for preparing a manufactured item of refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, which is the object of the present invention, in step a) of mixing in water, preferably in the amount of <NUM>% by weight, more preferably in the amount of <NUM>% by weight out of <NUM> parts in weight of the resulting slurry.

The refractory composite material as well as the manufactures items made of said material feature improved properties of electric and thermal resistance and insulation and improved properties of mechanical resistance compared to prior art materials, both ceramic materials and composite materials based on cement, particularly highly aluminous cements.

Accordingly, it is disclosed the use of the refractory composite material according to the present invention as refractory and/or insulating material of an electric and/or thermal type.

In particular, the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, which is the object of the present invention, as well as the manufactured items made of said material feature, in the absence of glass fibres based on silica and zircon: show a mechanical flexural strength (measured according to standard CEI EN <NUM> METHOD A) of more than <NUM> ±<NUM> MPa, an arc resistance (measured according to standard ASTM D495) of more than <NUM> seconds, a dielectric strength (measured according to standard IEC <NUM>) of more than <NUM> kV/mm.

The refractory composite material and the manufactured items consisting of said material, which are the object of the present invention, show blatant advantages of super-arc resistance, high resistance to thermal shocks, good mechanical resistance combined with an improved predisposition to machinability, the latter precisely on account of the structural features of the refractory composite material and as a consequence of the process for preparing the same and for preparing the manufactured items consisting of said refractory composite material. Indeed, along with the achievement of the above improved electric, thermal and mechanical properties for the material and for the manufactured items consisting thereof, a higher ease of implementation of manufactured items of said material is achieved, with a very high quality standard of dimensional reproducibility in compliance with tolerances and with an exceptional accuracy of the surface of the obtainable manufactured item; all this is due to the lack of shrinking of the refractory composite material according to the present invention during setting or hardening of the contents of the moulds or receptacles, no chemical aggression with regard to the moulds, immediate demoulding of the composite, and therefore of the manufactured item consisting thereof, after setting, the possibility to use silicone moulds.

The refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, features: high thermal resistance and refractoriness, suitable to stand the thermal shocks which are specific of both steel working and electric applications such as arc-chutes in extreme electric operation conditions (high/very high powers), at the same time being an excellent alternative to both ceramic materials and cement-based composite materials, particularly highly aluminous cements. The refractory composite material which is the object of the present invention has simultaneously the following properties of:
excellent mechanical resistance (comparable to ceramic materials), high refractoriness, excellent resistance to thermal shocks, exceptional resistance to electric arc as well as such a machinability as to allow finished or semifinished products to be implemented through forming processes based on the use of silicone moulds.

Compared with the other prior art materials such as reinforced cement-based composite materials or ceramic materials, the refractory composite material based on Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O· 11Al<NUM>O<NUM>, particularly sodium aluminate Na<NUM>O 11Al<NUM>O<NUM>β or Diaoyudaoite, which is the object of the present invention, allows the following further technical advantages to be achieved:.

The following non limiting examples describe forms of embodiment of the invention.

In a universal planetary mixer <NUM> of demineralised water are poured, to which the following are added, always under stirring, in succession:.

Once the mixing is accomplished, the smooth fluid composite is poured in a vibrating silicone mould.

The mould and the composite are left to rest at ambient temperature for at least <NUM> hours.

The demoulded manufactured item is introduced in an oven and conditioned at <NUM> with ramp rates of <NUM>/hour, permanence at <NUM> for <NUM> hours, to which natural cooling up to ambient temperature follows.

<NUM> specimens having the following sizes: length <NUM>, width <NUM> and thickness <NUM> were prepared according to the procedure for preparing the composite which is the object of the present invention described in example <NUM>.

As regards mechanical flexural strength measured according to standard CEI EN <NUM> METHOD A), the mechanical characterisation on the strips as described above provided an average value on the <NUM> specimens of more than <NUM> ±<NUM> MPa.

Two samples of the composite which is the object of the present invention were prepared, according to the procedure for preparing said material, for the characterisation of each of the electric properties.

The tests of electric characterisation performed at the department of electric engineering of the University of Genoa on the plates as described above provided the following results:.

On two square plates having the following sizes: side <NUM>, thickness <NUM>, in accordance with example <NUM>, arc resistance was measured according to the standard ASTM D495. The samples, conditioned over <NUM> hours in a normalised atmosphere at a temperature of <NUM> ± <NUM> with a relative humidity of <NUM>%, exhibited an average time of resistance to high-voltage and low-current electric arc of more than <NUM> sec (after the test, the plates were still intact), considering that the arc resistance tests were performed using tungsten electrodes in compliance with the standard ASTM D495.

On two samples of the refractory composite material according to the present invention, in accordance with example <NUM>, the dielectric strength measured according to standard IEC <NUM> provided average values of more than <NUM> kV/mm.

In particular, in order to perform the tests of dielectric strength, <NUM> measurements were made for each samples, using hemispherical brass electrodes having a radius of curvature of <NUM> and submitting the samples to an industrial-frequency sinusoidal voltage (<NUM>); the amplitude of the sinusoidal voltage rose from <NUM> to the breakdown voltage at a growth rate of 1kV/s. The tests were performed at a temperature of <NUM>±<NUM>, immersing the sample in a silicone oil bath (Rhodorsil H604V50).

For each sample, <NUM> measurements of the breakdown voltage: VBD [kV] were performed. The dielectric strength EBD is obtained considering the nominal value of the thickness of the <NUM>-mm sample referred to as s in the following table of dielectric strength:.

Claim 1:
Refractory composite material including Al<NUM>O<NUM> in the form of corundum, SiO<NUM> in the form of quartz and sodium aluminate having the formula NaAl<NUM>O<NUM> or Na<NUM>O. 11Al<NUM>O<NUM> comprising: Al, Si, Na and oxygen, which in the form of Al<NUM>O<NUM>, SiO<NUM> and Na<NUM>O oxides are present in such amounts that: the overall amount of Al<NUM>O<NUM> is no less than <NUM> wt%, preferably no less than <NUM> wt%, the overall amount of SiO<NUM> in the form of quartz is no less than <NUM> wt%, preferably no less than <NUM> wt%, the overall amount of Na<NUM>O is no less than <NUM> wt%, preferably no less than <NUM> wt%, said percentages being calculated on <NUM> parts by weight of the combination of said oxides.