Ceramic material for radome, radome and process for the production thereof

Abstract

A ceramic material for radome is illustrated comprising: -about 80-95% (wt %) of Si.sub.3N.sub.4; about 5-15% (wt %) of magnesium aluminosilicates including 2.5-12.5% (wt %) of Si0.sub.2, 0.5-3% (wt %) of MgO and 2-6% (wt %) of Al.sub.2O.sub.3; and having a density not lower than 2.5 g/cm.sup.3 and a dielectric constant not exceeding 6.5. A process for producing a radome is also illustrated.

Claims

1. Process for producing a radome comprising the following steps: a. forming a homogeneous mixture of about 80-95% (wt %) of Si.sub.3N.sub.4 powder and about 5-15% (wt %) of magnesium aluminosilicates powder including 2.5-12.5% (wt %) of SiO.sub.2, 0.5-3% (wt %) of MgO and 2-6% (wt %) of Al.sub.2O.sub.3; b. adding at least one organic binder to the mixture; c. atomising the mixture; d. subjecting the mixture to isostatic pressing at ambient temperature in a special mould so as to form a green semi-finished product; e. mechanically machining the green semi-finished product so as to substantially confer it a final shape; f. subjecting the shaped green semi-finished product to a thermal cycle; and g. sintering the green semi-finished product so as to obtain the finished product.

2. Process for producing a radome according to claim 1, wherein the step a of forming a homogeneous mixture comprises the following two sub-steps: a. mixing Si.sub.3N.sub.4 with SiO.sub.2 so as to form a pre-mixture; and a. mixing the pre-mixture with MgO and with Al.sub.2O.sub.3.

3. Process for producing a radome according to claim 1, wherein the step f of subjecting the semi-finished product to a thermal cycle comprises the following sub-steps: f. raising the temperature 8 C./hr up to reaching a temperature of 300 C.-390 C.; and f. leaving the semi-finished product at temperature for 3-6 hours.

4. Process for producing a radome according to claim 1, wherein the step f of subjecting the semi-finished product to a thermal cycle is actuated in a furnace which comprises a specific support and/or a system for conveying gases such to ensure the organic binder to exit from the semi-finished product.

5. Process for producing a radome according to claim 1, wherein the step g of sintering is carried out on a support made of the same material as the semi-finished product.

6. Process for producing a radome according to claim 1, wherein said mixture comprises: about 90-94% (wt %) of Si.sub.3N.sub.4; and about 6-10% (wt %) of magnesium aluminosilicates including 3.2-5.2% (wt %) of SiO.sub.2, 0.7-2% (wt %) of MgO and 2.1-4% (wt %) of Al.sub.2O.sub.3.

Description

(1) For a better understanding of the invention and in order to observe the advantages thereof, following is a description of some exemplifying and non-limiting embodiments of the ceramic material for radome and the process for producing a radome of the invention, with reference to the attached figures, wherein:

(2) FIG. 1 schematically shows the steps of the process for producing a radome according to the present invention; and

(3) FIG. 2 shows an example of an article made using the material of the invention, during some specific production steps.

(4) The ceramic material for radome according to the present invention is a material based on silicon nitride, it actually comprises about 80-95% (wt %) of Si.sub.3N.sub.4.

(5) Preferably about 15-35% (wt %) of Si.sub.3N.sub.4 is -Si.sub.3N.sub.4. It was actually discovered that a specific controlled percentage of such phase allows obtaining a low dielectric constant and thus improving the dielectric. capacities of the material, i.e. its capacities to be transparent to the electromagnetic waves.

(6) In particular, the material of the invention is optimal for radome, i.e. for structures adapted to protect antennae, thus. the expression good dielectric properties is used to indicate the capacity of the material to be transparent to the energy emitted and received by an antenna.

(7) The material of the invention further comprises about 5-5% (wt %) of magnesium aluminosilicates including 2.5-12.5% (wt %) of SiO.sub.2, 0.5-3% (wt %) of MgO and 2-6% (wt %) of Al.sub.2O.sub.3.

(8) Preferably the material comprises 90-94% (wt %) of Si.sub.3N.sub.4; and 6-10% (wt %) of magnesium aluminosilicates including 3.2-5% (wt %) of SiO.sub.2, 0.7-2% (wt %) of MgO and 2.1-4% (wt %) of Al.sub.2O.sub.3.

(9) A particularly preferred composition of the material according to the present invention comprises 90% (wt %) of Si.sub.3N.sub.4, 5.1% (wt %) of SiO.sub.2, 1.4% (wt %) of MgO and 3.5% (wt %) of Al.sub.2O.sub.3.

(10) Particularly desired results were obtained using such composition.

(11) According to the present invention, the ceramic material has a density not lower than 2.5 g/cm.sup.3 and preferably comprised between 2.5 and 2.9 g/cm.sup.3.

(12) Such characteristic, together with the composition, is fundamental for defining the mechanical resistance properties of the material and thus for obtaining a product suitable for aerospace applications.

(13) Furthermore, the dielectric constant of the material does not exceed 6.5, in particular comprised between 5.7 and 6.4, at ambient temperature and at high temperatures in the X, Ku and Ka bands.

(14) According to a further aspect, the invention relates to an article comprising such material and, in particular, radome comprising such material. Preferably it is a radome for missile applications and, generally aerospace applications, hut it could also be applied in different contexts, for example in nautical applications.

(15) Following is a description of a process for producing a radome according to the present. invention.

(16) The process of the invention provides a first step a of forming a homogeneous mixture of about 80-95% (wt %) of Si.sub.3N.sub.4 powder and about 5-15% (wt %) of magnesium aluminosilicates powder including 2.5-12.5% (wt %) of SiO.sub.2, 0.5-3% (wt %) of MgO and 2-6% (wt %) of Al.sub.2O.sub.3.

(17) Preferably the material comprises 90-94% (wt %) of Si.sub.3N.sub.4; and 6-10%. (wt %) of magnesium aluminosilicates including 3.2-5% (wt %) of SiO.sub.2, 0.7-2% (wt %) of MgO and 2.1-4% (wt %) of Al.sub.2O.sub.3.

(18) A particularly preferred composition of the material according to the present invention comprises 90% (wt %) of Si.sub.3N.sub.4, 5.1% (wt %) of SiO.sub.2, 1.4% (wt %) of MgO and 3.5% (wt %) of Al.sub.2O.sub.3.

(19) Such step may be carried out by mixing all the components together or through two subsequent sub-steps, i.e. a first sub-step a which provides for the homogeneous mixing of Si.sub.3N.sub.4 with SiO.sub.2 so as to form a ore-mixture; and then. a second sub-face of a which provides for mixing the pre-mixture with MgO and with Al.sub.2O.sub.3.

(20) In both cases, the mixing preferably occurs with addition of water, in that it aids the subsequent atomization step. Alternatively, ethanol or any other solvent of the known type could be used.

(21) The mixing is such to guarantee homogeneity and intimate contact between the powders.

(22) Such step is preferably carried out in special mills or roller mixers.

(23) There follows a step b of adding at least one organic hinder to the mixture.

(24) Such binder is of the known type and it may for example be polyethylene glycol.

(25) It is adapted to aid the mixing facilitating an intimate bond between the powder particles.

(26) Subsequently the mixture is atomised in a known manner, preferably by means of an atomiser provided with a cyclone, according to a step c.

(27) At the end of such step, the mixture is in a homogeneous and stable amorphous dispersion form.

(28) Subsequently the mixture is subjected to isostatic pressing at ambient temperature in special mould (step d). Such pressing is carried out at a pressure comprised between 1500 and 1800 bars.

(29) The mould has a shape compatible to the shape of the product intended to be obtained and is preferably of the elastomeric type.

(30) In the specific application it is cylindrically-shaped and it is provided with a cylindrical core concentric thereto so as to confer the hollow cylindrical shape to the mixture, as shown in FIG. 2.

(31) After having introduced the mixture into the cylinder, the core is introduced, then the mould is sealed and the mixture is subjected to isostatic pressure.

(32) The obtained product is a green semi-finished product.

(33) At this point a step e is carried out of mechanical machining the green semi-finished product so as to confer the semi-finished product the desired shape, which substantially coincides with the final shape of the product, shown in FIG. 2.

(34) As previously mentioned, such stepbeing carried out on a green semi-finished product, and thus on a more malleable productallows obtaining considerable advantages in terms of process and features of the finished product.

(35) Thus, at the end of the mechanical machining, the semi-finished product, though green, substantially has its final shape.

(36) According to a preferred embodiment, such shape is conoidal or ogive-shaped.

(37) Subsequently a step f is carried out of subjecting the shaped green semi-finished product to a thermal cycle adapted to eliminate the organic binder.

(38) It is to optimised according to the specific composition used and the dimensions of the semi-finished product and it is preferably actuated in atmosphere.

(39) According to preferred embodiments, the thermal cycle comprises the sub-steps of gradually raising the temperature, particular the temperature is raised of 8 C./h up to reaching a temperature of 300 C.-390 C. (step f), and leaving the semi-finished product at the reached temperature for 3-6 hours (step f).

(40) Preferably the step f is carried out in a furnace which provides a specific support or base and/or a system for conveying gases such to guarantee the organic binder to exit from the piece gradually and uniformly.

(41) In other words, the organic binder evaporated from the material may remain trapped within the cavity of the article causing deformations or breakage.

(42) In order to prevent this, the furnace is provided with a grid or a base provided with openings so as to allow the passage of such binder therethrough.

(43) Alternatively or additionally, the furnace may be provided with a suitable support which allows positioning the piece with the opening, and thus the concavity, facing upwards.

(44) Alternatively or additionally, further systems for conveying gases may be provided which force the motion gases into the desired direction.

(45) According to preferred embodiments of the process of the invention, follows a step h of applying an anti-oxidant on the surface of the finished product.

(46) Such application is preferably made by spraying.

(47) The green semi-finished product, is then subjected to the step g of sintering so as to obtain the finished product.

(48) Such step is preferably carried out at temperature of 1500 C.-1650 C. and/or with liquid phase in inert atmosphere, preferably in nitrogen.

(49) The sintering thermal cycle is optimised so as to obtain a specific microstructure.

(50) It was observed that the sintering kinetic is strongly affected by the specific initial composition. of the material, in addition to the temperature.

(51) Preferably, the step g of sintering is carried. out on a support made of the same material as the semi-finished product.

EXAMPLE

(52) 90% (wt %) of Si.sub.3N.sub.4, 5.1% (wt %) of SiO.sub.2, 1.4% (wt %) of MgO and 3.5% (wt %) of Al.sub.2O.sub.3 were mixed in a blade mill together with water and polyethylene glycol so as to obtain a slurry.

(53) The mixture was atomised and then subjected to isostatic pressing at ambient temperature in a cylindrical mould at a pressure of 1500 bar.

(54) The semi-finished product, thus obtained was machined externally by means of numerical control machining, as to confer it an ogive shape.

(55) Subsequently, it was subjected to the following thermal cycle: raising the temperature of 8 C./h up to reaching a temperature of 300 C.-390 C.; maintaining in temperature for 3-6 hours.

(56) The product was sintered at temperature up to 1550 C. for a time of about two hours so as to obtain a finished product.

(57) The finished product was subjected to standard verification tests attaining the following results:

(58) TABLE-US-00001 Young's Modulus GPa 220 Poisson's Coefficient - 0.26 Dielectric constant - 6 Resistance to bending (@ 21 C.) MPa 349 Fracture toughness MPa .Math. m.sup.1/2 3.69 Coefficient of thermal dilation 10.sup.6 K.sup.1 3.42 (@ 25 1300 C.) Density g/cm.sup.3 2.7

(59) Wherein, for Young's Modulus and Poisson's Coefficient, the measures were carried out through the method of bending resonance frequency on test pieces measuring 80108 mm according to guidelines of the EN 843-2 standard.

(60) The measurement of the Resistance to Bending was carried out according to the guidelines of the EN 843-1 standard bending in 4 points on bars with bevelled edges (measuring 252.52 mm) using a Zwick Z050 universal machine with a 0.5 mm/min speed of the beam, a distance of 10 mm for the upper blades and 20 mm for the lower ones. The test was carried out on 5 test pieces.

(61) The measurement of the Fracture toughness was carried out using the Chevron Notched beam method in bending according to the guidelines of the FprEN 14425-3 standard. The bending test was carried out using a Zwick Z050 universal machine, with a 0.02 mm/min speed of the beam. The test was carried out on three test pieces (measuring 252.52 mm) previously notched using a blade having a 0.1 mm thickness.

(62) Regarding the coefficient of thermal dilation, the thermal dilation tests were carried out using a Netsch DIL E 402 dilatometer on a 252.52 mm test piece up to 1450 C. in an argon flow with 5 C./min heating speed.

(63) The measurements of dielectric constant were carried out with a waveguide method filled with dielectric.

(64) The density measurements were carried out on sintered samples, geometrically, using Archimede's method according to the ASTM 0373 standard.

CONCLUSIONS

(65) The obtained results show that the particular composition used and the combination of the specific steps of the process allow obtaining a material with good mechanical characteristics, good heat resistance and good dielectric characteristics.

(66) In the description above and in the subsequent claims, all the numerical quantities indicating amounts, parameters, percentages, and so on shall be deemed preceded under any circumstances by the term about unless indicated otherwise. Furthermore, all numerical quantity ranges include all possible combinations of the maximum and minimum numerical values and all possible intermediate ranges, besides those specifically indicated in the text.

(67) The ceramic material, radome and production process according to the present invention shall be subjectedby a man skilled in the art with the aim of meeting contingent and specific needsto further modifications and variants, all falling within the scope of protection of the present invention.