Refractory powder comprising coated mullite grains

Abstract

A powder is disclosed having a coarse fraction representing more than 60% and less than 85% of the powder, as a weight percentage on the basis of the oxides, and that is constituted of particles having a size greater than or equal to 50 m, referred to as coarse particles, the powder comprising at least 5% of coated grains having a size greater than or equal to 50 m, as a weight percentage on the basis of the oxides of the powder, and a fine fraction, forming the balance to 100% as a weight percentage on the basis of the oxides, constituted of particles having a size of less than 50 m, referred to as matrix particles. The powder can be applied in combustion chambers in which the temperature may reach 1400 C.

Claims

1. A powder comprising: a coarse fraction representing more than 60% and less than 85% of the powder, as percentage by weight based on the oxides, and consisting of particles exhibiting a size of greater than or equal to 50 m, referred to as coarse particles, the powder comprising at least 5% of coated grains exhibiting a size of greater than or equal to 50 m, as percentage by weight based on the oxides of the powder, and a fine fraction, forming the remainder to 100% as percentage by weight based on the oxides, consisting of particles exhibiting a size of less than 50 m, referred to as matrix particles, said coated grains each comprising: a base grain comprising more than 30% by weight of mullite; a coating extending over more than 50% of the surface of said base grain and consisting, for more than 90% of its weight, of a coating material chosen from: alumina; CaSiO.sub.3; lanthanide, Ta, Ti, Sc, Y, Mg , Mo aluminates and their mixtures; lanthanide, Sc, Y, Zr , Hf silicates and their mixtures; oxides of one or more elements chosen from Ti, Sc, Y, Yb, Mg, Ce, Mo, Zr and Hf; lanthanide and/or Y and/or Sc phosphates; and their mixtures, the powder comprising more than 20% of particles exhibiting a size greater than 150 m, as percentage by weight based on the oxides of the powder, the powder exhibiting a content of alumina of greater than 70%, as percentage by weight based on the oxides.

2. The powder as claimed in claim 1, in which said coating extends over more than 70% of the surface of said base grain.

3. The powder as claimed in claim 2, in which said coating extends over more than 90% of the surface of said base grain.

4. The powder as claimed in claim 1, in which the surface porosity of the coating is less than 25%.

5. The powder as claimed in claim 4, in which the surface porosity of the coating is less than 3%.

6. The powder as claimed in claim 1, in which the base grain comprises more than 50% of mullite, as a percentage by weight based on the oxides.

7. The powder as claimed in claim 6, in which the base grain comprises more than 80% of mullite, as a percentage by weight based on the oxides.

8. The powder as claimed in claim 1, in which the coating is composed of: an external layer made of said coating material, and a transition layer which extends between the base grain and said external layer and which comprises: silica and/or alumina, and at least one of the constituents of the coating material.

9. The powder as claimed in claim 1, in which the coating material is chosen from: Al.sub.2O.sub.3; GdAlO.sub.3, Gd.sub.4Al.sub.2O.sub.9, Gd.sub.3Al.sub.5O.sub.12, AlTaO.sub.4, Al.sub.2TiO.sub.5, Y.sub.3Al.sub.5O.sub.12, YAlO.sub.3, Y.sub.4Al.sub.2O.sub.9, Yb.sub.4Al.sub.2O.sub.9, YbAlO.sub.3, Yb.sub.3Al.sub.5O.sub.12, MgAl.sub.2O.sub.4 and their mixtures; Gd.sub.2SiO.sub.5, Gd.sub.2Si.sub.2O.sub.7, Sc.sub.2Si.sub.2O.sub.7, Sc.sub.2SiO.sub.5, Y.sub.2Si.sub.2O.sub.7, Y.sub.2SiO.sub.5, Yb.sub.2Si.sub.2O.sub.7, Yb.sub.2SiO.sub.5, ZrSiO.sub.4, HfSiO.sub.4, (Zr,Hf)SiO.sub.4, CaSiO.sub.3; ZrO.sub.2, HfO.sub.2, HfTiO.sub.4, ScTaO.sub.4, Sc.sub.4Hf.sub.3O.sub.12, Sc.sub.4Zr.sub.3O.sub.12, a partially stabilized and/or stabilized zirconia, a partially stabilized and/or stabilized hafnone, and their mixtures; LaPO.sub.4, YPO.sub.4 and their mixtures; and their mixtures.

10. The powder as claimed in claim 9, in which the coating material is alumina.

11. The powder as claimed in claim 1, containing more than 40% of said coated grains with a size of greater than or equal to 50 m, as a percentage by weight based on the oxides of the powder.

12. The powder as claimed in claim 11, in which the coarse fraction comprises, for more than 95% of its weight, particles composed, for more than 95% of their weight, of alumina and/or zirconia and/or magnesia and/or silica, and said coated grains, and in which the fine fraction consists of particles composed of alumina and/or zirconia and/or magnesia and/or silica.

13. The powder as claimed in claim 1, in which the fine fraction comprises less than 90% of silica, as percentage by weight based on the oxides of the fine fraction.

14. The powder as claimed in claim 1, exhibiting a content of alumina Al.sub.2O.sub.3 of greater than 60% and a content of silica SiO.sub.2 of less than 30%, as percentage by weight based on the oxides.

15. The powder as claimed in claim 1, exhibiting a content of alumina Al.sub.2O.sub.3 of greater than 85% and a content of silica SiO.sub.2 of less than 10%, as percentage by weight based on the oxides.

16. The powder as claimed in claim 1, in which the thickness of the coating is less than 300 m.

17. The powder of claim 1 exhibiting a content of greater than 80%, as percentage by weight based on the oxides.

18. The powder of claim 1, wherein the coarse fraction comprises particles composed, for more than 95% of their weight, of alumina and/or magnesia and/or silica.

19. The power of claim 1, wherein the coarse fraction comprises, for more than 95% of its weight: particles composed, for more than 95% of their weight, of alumina and/or magnesia and/or silica, or mixtures of such particles, and coated grains having base grains made of mullite / zirconia and/or mullite.

20. The powder of claim 1, wherein the fine fraction comprises particles composed of alumina and/or magnesia and/or silica.

21. The powder of claim 20, wherein the fine fraction consists of particles composed of alumina and/or magnesia and/or silica.

22. The powder of claim 1, wherein the coarse fraction comprises alumina coarse particles.

23. A device chosen from a combustion chamber of a gas turbine and a reformer, said device comprising a sintered refractory product manufactured from a powder as claimed in claim 1, in particular in the form of a protective part.

24. A sintered product manufactured according to a process comprising the following successive stages: A) preparation of a starting charge by addition of water to a powder comprising: a coarse fraction representing more than 60% and less than 85% of the powder, as percentage by weight based on the oxides, and consisting of particles exhibiting a size of greater than or equal to 50 m, referred to as coarse particles, the powder comprising at least 5% of coated grains exhibiting a size of greater than or equal to 50 m, as percentage by weight based on the oxides of the powder, and a fine fraction, forming the remainder to 100% as percentage by weight based on the oxides, consisting of particles exhibiting a size of less than 50 m, referred to as matrix particles, said coated grains each comprising: a base grain comprising more than 30% by weight of mullite; a coating extending over more than 50% of the surface of said base grain and consisting, for more than 90% of its weight, of a coating material chosen from: alumina; CaSiO.sub.3; lanthanide, Ta, Ti, Sc, Y, Mg , Mo aluminates and their mixtures; lanthanide, Sc, Y, Zr , Hf silicates and their mixtures; oxides of one or more elements chosen from Ti, Sc, Y, Yb, Mg, Ce, Mo, Zr and Hf; lanthanide and/or Y and/or Sc phosphates; and their mixtures, B) shaping said starting charge so as to form a preform; C) sintering said preform, the product exhibiting the appearance of a block or of a layer, said block or layer having a thickness greater than 3 mm.

25. A method comprising: subjecting a sintered refractory product manufactured from a powder as claimed in claim 1 to gases laden with water vapor exhibiting a temperature of greater than 1000 C.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a photograph obtained using a scanning electron microscope (SEM). It represents, in cross section, a portion of a coated grain according to the invention, in the case in point a cross section of a grain of example G2 described below.

(2) FIGS. 2 and 3 represent, after having carried out the water vapor corrosion test, a polished cross section of the refractory products R1 and R3, respectively, an alumina coating having been deposited, prior to the test, on the external surface of the product R1, outside the invention.

DETAILED DESCRIPTION

(3) The manufacture of coated grains according to the invention can be carried out according to any technique known to a person skilled in the art. Preferably, it results from a process comprising the following stages: a) preparation of a slip comprising particles of coating material in suspension; b) dipping base grains in said slip and then extraction of the grains thus wetted; c) optionally, drying the wetted grains; d) optionally, heat treatment.

(4) In stage a), the term slip denotes a suspension of particles made of coating material or of precursors of said coating materials in a liquid, generally water, with or without additives, such as dispersing agents, deflocculating agents, polymers, and the like.

(5) Preferably, the slip comprises a temporary binding agent, that is to say a binding agent which is removed during the drying and/or sintering. The slip can be manufactured in a container, according to techniques known to a person skilled in the art, by mixing and homogenizing the particles and the desired amount of liquid. The particles can exhibit a median size of between 0.1 and 50 m. The slip typically comprises more than 60% of dry matter.

(6) According to the invention, the coating material is chosen from: alumina; lanthanide, Ta, Ti, Sc, Y, Mg or Mo aluminates and their mixtures; lanthanide, Sc, Y, Zr or Hf silicates and their mixtures; oxides of one or more elements chosen from Ti, Sc, Y, Yb, Mg, Ce, Mo, Zr and Hf; lanthanide and/or Y and/or Sc phosphates; and their mixtures.

(7) The mixtures of the various materials mentioned above within a coating material can be in the form of a defined compound, of a solid solution or of a more or less intimate mixture of the materials (for example dispersoid).

(8) The coating material preferably exhibits a thermal expansion coefficient of between 4.Math.10.sup.6 C..sup.1 and 10.Math.10.sup.6 C..sup.1, preferably of less than or equal to 9.Math.10.sup.6 C..sup.1. More preferably, the coating material is chosen so that its thermal expansion coefficient, at ambient temperature, is greater than that of the material of the base grain. At high temperature, the coating layer is thus compressed, which limits the risks of cracks, indeed even of detachment of the coating.

(9) The coating material preferably exhibits a melting point which is greater than or equal to the temperature of use of the sintered refractory product in which the coated grains are intended to be incorporated and/or greater than the sintering temperature used to manufacture said refractory product.

(10) In stage b), the base grains are temporarily immersed in the slip. Optionally, mixing can improve the wetting of the base grains by the slip. The wetted grains are coated grains according to the invention.

(11) In stage c), after having been dipped in the slip, the wetted grains can optionally be dried. The coating is then advantageously solid.

(12) Any conventional drying process can be used.

(13) In stage d), the wetted and optionally dried grains can optionally be subjected to a heat treatment. Preferably, the wetted grains are dried and are subjected to a heat treatment. In one embodiment, these operations are carried out in just one processing stage.

(14) The heat treatment is adapted so as to adjust the surface porosity of the coating to the targeted application. In particular, the coated grains can be subjected to a firing at a temperature of between 1200 C. and 1700 C., for a period of time of 5 hours.

(15) The surface porosity, which can be measured as described below, can be in particular less than 25%, preferably less than 20%, preferably less than 15%, preferably less than 10%, preferably less than 5%, preferably less than 3%, indeed even less than 2%. The inventors consider that a surface porosity of less than 3% can only be obtained by subjecting the coated grain to a heat treatment. The surface porosity of a grain which has not been subjected to the heat treatment is typically greater than 20%.

(16) The use of a heat treatment advantageously makes it possible to obtain a coating which exhibits neither cracks nor flakes. It also results in the adhesion of the coating being considerably improved.

(17) The inventors have demonstrated, by microprobe, with regard to the coated grains which have been subjected to a heat treatment, the presence, in the coating, at the interface with the base grain, of a transition layer enriched in silica, this silica originating from the base grain. For example, in a grain of electrocast mullite covered with an alumina coating, a heat treatment at 1420 C. resulted in a thin transition layer between the mullite of the base grain and the alumina of the coating, said transition layer comprising alumina silicate.

(18) The heat treatment can also result from a sintering of a preform in order to manufacture a refractory product according to the invention, as explained below. Such a sintering also results in the appearance of a transition layer.

(19) In one embodiment, the coated grains according to the invention, for example manufactured according to stages a) to d) above, are intended for the manufacture of a sintered refractory product. It can then be used to prepare a ready-for-use powder according to the invention.

(20) To this end, coated grains according to the invention exhibiting a size of greater than or equal to 50 m are preferably selected, in order for them to be able to form at least a portion of the coarse fraction. Coarse particles, in particular made of a ceramic material chosen from alumina, zirconia, magnesia, silica and their mixtures, can supplement the coated grains according to the invention to form the coarse fraction.

(21) Matrix particles are also selected. The latter are intended, after sintering, to form the matrix of the refractory material.

(22) Finally, preferably, a shaping additive is added, so that the amount of shaping additive represents less than 6% of the weight obtained. The shaping additive is preferably chosen from the group consisting of: clays; plasticizers, such as polyethylene glycol (PEG) or polyvinyl alcohol (PVA); cements, preferably comprising more than 70% of alumina; hydratable aluminas, such as boehmite; binders, including temporary organic binders, such as resins, lignosulfonates, carboxymethylcellulose or dextrin; deflocculating agents, such as alkali metal polyphosphates, alkali metal polyacrylates, or polycarboxylates; and the mixtures of these products.

(23) Preferably, the shaping additive is chosen from the group consisting of cements, deflocculating agents, clays, lignosulfonates, PVA and their mixtures.

(24) The powder can then be packaged, for example in bags, in particular of big bag type, or in kegs. The preparation of the starting charge for manufacturing the preform is thereby facilitated.

(25) Starting from this powder, a refractory product according to the invention can be manufactured according to a process comprising the above stages A) to C).

(26) In stage A), a starting charge is prepared by addition of water to the powder.

(27) The amount of water is adjusted in stage B). For example, in the case of shaping by cold pressing, addition of an amount of water of between 1.5% and 4%, as percentage by weight based on the powder, without taking into account the shaping additives, is preferred. In the case of shaping which involves a hydraulic bond, addition of an amount of water of between 3 and 9%, as percentage by weight based on the powder, without taking into account the shaping additives, is preferred.

(28) The starting charge is mixed in order to obtain a homogeneous mass capable of being shaped in stage B), for example by pouring into a mold or projecting onto a wall.

(29) In stage B), any known method can be envisaged.

(30) In stage C), the sintering conditions and in particular the sintering temperature depend on the composition of the powder. Generally, a sintering temperature of greater than 1400 C., indeed even of greater than 1450 C., and/or of less than 1900 C., indeed even of less than 1750 C., is highly suitable.

(31) In the refractory product obtained, the coating remains visible over a cross section of the refractory product, even when the matrix is the same in nature as the coating. This is because it is possible to differentiate, by observations by scanning electron microscopy (SEM) or using a microprobe, the coating of the matrix, in particular by the fact that the coating exhibits a lower porosity than the porosity of the matrix, which generates a gangue around the base grain visible, inter alia, using these methods.

(32) In particular, FIG. 1 represents a coated grain 10 comprising a base grain 12 of mullite and a coating 14 of alumina spread out on the surface of the base grain 12. The matrix is referenced 16.

EXAMPLES

(33) Coated grains G2 and G3 according to the invention were manufactured according to a process comprising stages a) to d) described above. The grains G1 were not coated.

(34) More particularly, a slip was prepared by suspending an alumina powder, with a purity of greater than 99% and with a median size of 10 m, in a solution of polyvinyl alcohol (PVA).

(35) Grains of electrocast mullite exhibiting sizes of between 50 m and 2 mm were poured into and mixed in the slip, and then extracted from said slip. The mullite grains, wetted with slip, were subsequently dried at 110 C. for 24 hours and then heat treated at a temperature T for six hours.

(36) Analyses by microprobe made it possible to observe, in the coated grains G2 and G3, the presence of a transition layer between the base grain made of mullite and the alumina forming the upper layer of the coating. This transition layer comprises silica in the form of an alumina silicate.

(37) Observation of the coated grains G2 and G3 showed that, after sintering, the entire surface of the base grains is covered with an alumina-based coating.

(38) The coated grains G1, G2 and G3 were subsequently used to produce powders P1, P2 and P3 respectively. Corn dextrin and water-soluble oil were used as shaping additives.

(39) The powders P1, P2 and P3 were subsequently used to manufacture sintered refractory products R1, R2 and R3 respectively. To this end, they were mixed with 2.6% of water, as percentages by weight based on the powder, so as to foal a starting charge. The starting charge was pressed in the form of bars with dimensions of 1122320 mm.sup.3 with a single-acting mechanical press, under a pressure of 470 kg/cm.sup.2. These bars were subsequently sintered at a temperature of 1650 C., maintained for six hours, the rate of temperature rise being 100 C./h. After the temperature stationary phase, the temperature was reduced, the rate of temperature fall being 100 C./h, down to 800 C., the fall in temperature subsequently being free down to ambient temperature.

(40) The surface porosity of a coating of a coated grain is the percentage of the surface of a cross section of this coating occupied by the pores. For the examples, it was evaluated in the following way:

(41) Preparation of the Sample

(42) At least 200 grains are randomly withdrawn. They are subsequently coated in an epoxy resin and polished using sandpaper of decreasing particle size P80 to P1200. The final polishing is carried out on felt with an aqueous diamond suspension with a median particle size of 3 m.

(43) Taking of Photographs

(44) Photographs are subsequently taken using a scanning electron microscope (SEM), preferably in a mode using backscattered electrons (BSE mode) in order to obtain a very good contrast between the different phases. Each photograph exhibits a minimum of 1280960 pixels, without taking into account the scale bar. The magnification used is such that the width of the image is between 20 times and 400 times the mean size of the pores. A first photograph can be taken by virtue of a visual estimation of the mean size of the pores.

(45) Prior to analyzing the surface porosity, it is necessary to make sure that the porosities are discernible to the naked eye on the photograph and that the porosities as well as the continuous background appear darker than the material observed.

(46) Image Analysis of the Surface Porosity

(47) The image is analyzed using the JMicrovision v1.2.7 2002-2008 software, available on http://www.jmicrovision.com. Open the image in Jmicrovision Select the heading object extraction Select Color or gray intensity threshold In the open window, choose the heading Area and mark, by virtue of Area editor and the polygonal tool, the coating region to be analyzed on the grain. Rename this region coating and select it as analytical region. Select the heading Fill object and select this option In the heading Threshold, in the tab Red, move the indexes until the pores are completely filled with red color. In the heading Assign class, add a class named Porosity and select it. Press Process: the pores are then surrounded in red. Go into the Data Viewer window and select the Classes mode and press Compute result from Area The surface porosity is indicated in the Coating (image surface %) column 200 pores have to be analyzed and the same measurement is repeated on ten different photographs. The mean of these analyses gives the surface porosity of the coating.

(48) The thickness of the coating of a grain is the mean thickness of this coating. For the examples, it was evaluated in the following way:

(49) The preparation of the sample is the same as for the analysis of surface porosity.

(50) Taking of Photographs

(51) Photographs are taken using an optical microscope. Each photograph exhibits a minimum of 2080*1544 pixels, without taking into account the scale bar. The magnification used is such that the width of the image is between 20 times and 100 times the mean thickness of the coating. A first photograph can be taken by virtue of a visual estimation of the thickness of the coating.

(52) Prior to analyzing the thickness of the coating, it is necessary to make sure that the coating is discernible to the naked eye on the photograph.

(53) Image Analysis of the Coating Thickness

(54) The image is analyzed using the JMicrovision v1.2.7 2002-2008 software, available on http://www.jmicrovision.com. Open the image in Jmicrovision Select the tool straight in the tool bar and carry out a measurement of thickness at 10 points of the coating on the periphery of each grain equidistant from one another. This operation is repeated on 10 grains taken randomly from the grains initially withdrawn. Go into the Data Viewer window and select in Data 1D measurement. A mean then has to be taken with regard to the sample of data of the Line length column.

(55) The measurements of the bulk density and of the open porosity of the refractory products were carried out by hydrostatic measurements according to standard NF EN 993-1 on samples with dimensions of 1252525 mm.sup.3.

(56) The modulus of rupture (MOR) of the refractory products was measured according to the standard NF EN 993-6.

(57) A water vapor corrosion test was carried out on the refractory products according to the following method:

(58) This test is carried out in a tubular furnace, the tube of which is made of pure AL99 alumina. The samples are 114*23*20 mm.sup.3 bars positioned on two superimposed pure alumina sheets. The furnace is brought to 1400 C. and then maintained at this temperature for 320 hours. Injection of an aqueous sodium hydroxide solution comprising 0.15 g/l of Na is carried out at the inlet of the furnace and discharge of the water vapor is provided by condensation with a cooler in the rear part of the furnace.

(59) The following parameters are evaluated before and after passing through the furnace: the modulus of rupture (MOR) of the sample, the variation in modulus of rupture (MOR %).

(60) The variation in modulus of rupture MOR % is defined as being:
100.Math.(MOR before testMOR after test)/(MOR before test).

(61) The inventors consider that a decrease of 5% in the variation of modulus of rupture MOR % with respect to the refractory product R1 represents a significant improvement.

(62) The characteristics of the coated grains, of the powders and of the refractory products, and also the results obtained, are summarized in the following table 1:

(63) TABLE-US-00001 TABLE 1 G1 (*) G2 G3 Coated grains Nature of the base grain electrocast electrocast electrocast mullite mullite mullite Nature of the coating material no coating alumina alumina Temperature T of the heat 1420 C. 1630 C. treatment ( C.) Properties Mean thickness of the coating 50 50 (m) Surface porosity of the coating 19 9 (%) Powders P1 (*) P2 P3 Amount of coated grains with a 34.5 45 45 size greater than or equal to 50 m (%) Amount of the other coarse 34.5 25 22 particles (%) Nature of the other coarse alumina alumina alumina particles Amount of matrix particles (%) 29.5 30 30 Nature of the matrix particles alumina alumina alumina Corn dextrin 1 1 1 Water-soluble oil 0.5 0.5 0.5 Refractory products R1 (*) R2 R3 Composition by weight of the refactory product % Al.sub.2O.sub.3 90.7 90.7 90.7 % SiO.sub.2 9.1 9.1 9.1 % Others 0.2 0.2 0.2 Properties Bulk density (g/cm.sup.3) 2.88 2.94 2.93 Open porosity (%) 19.5 16.9 17.5 MOR at 20 C. before corrosion 9.6 11.6 12.7 test (MPa) MOR at 20 C. after corrosion 7.9 10.1 11.9 test (MPa) MOR % 18% 12% 7% (*) Comparative example

(64) As is shown in table 1, the refractory products R2 and R3 according to the invention exhibit a smaller variation in modulus of rupture MOR % than that of the refractory product R1 which does not comprise alumina-coated mullite grains. The improvement is respectively 33% (100.Math.(1812)/18) and 61% (100.Math.(187)/18).

(65) These refractory products additionally exhibit a greater modulus of rupture MOR than that of the refractory product R1, before and after the corrosion test. The mechanical strength during use is advantageously improved thereby.

(66) Finally, a comparison between the refractory products R2 and R3 shows the advantage of a high-temperature heat treatment, in particular at a temperature greater than 1500 C.

(67) Furthermore, a water vapor corrosion test was carried out under the same conditions as those described above, on the refractory product R1, after having deposited, at its external surface, an alumina coating with a thickness equal to 400 m identical to that used to coat the mullite grains G3.

(68) Photographs were taken with an optical microscope on polished sections of this coated product R1 and on the refractory product R3 (FIGS. 2 and 3, respectively). A comparison of these photographs makes it possible to demonstrate a better resistance of the refractory product R3 according to the invention. This is because the depth down to which the mullite grains were attacked by the water vapor is 4.5 mm for the refractory product R1 coated with an alumina layer and less than 1 mm for the product R3.

(69) These measures thus show a better behavior in the water vapor corrosion test of a refractory product according to the invention comprising mullite grains, said grains exhibiting an alumina coating, with respect to a refractory product comprising mullite grains, said refractory product exhibiting an alumina coating deposited on its external surface.

(70) As is now clearly apparent, the invention thus provides novel materials suited to the highly specific environment of combustion chambers.

(71) Of course, the invention is not limited to the embodiments provided by way of examples.

(72) In particular, it is possible to manufacture grains according to the invention using a technique of hot projection of particles which advantageously makes it possible to form the coating and to sinter it in a single stage.