CA3082943A1 - Process for the production of ceramic bodies consisting of nanofibers of silicon nitride and ceramic bodies thus obtained - Google Patents

Abstract

The invention concerns a process for the production of ceramic bodies consisting of a bundle of nanofibers of silicon nitride and the ceramic bodies obtained by means of said process.

Description

PROCESS FOR THE PRODUCTION OF CERAMIC BODIES CONSISTING OF
NANOFIBERS OF SILICON NITRIDE AND CERAMIC BODIES THUS OBTAINED
********************
FIELD OF THE INVENTION
The present invention refers to a process for the production of ceramic bodies consisting of bundles of nanofibers of silicon nitride and the ceramic bodies thus obtained.
BACKGROUND
Nanofibers of silicon nitride or silicon carbide are used in the production of composite materials, in particular metal matrix composites, known in the sector as MMC, and ceramic matrix composites, known in the sector as CMC.
In these composites, the nanofibers constitute the discontinuous phase embedded in the continuous phase (matrix), and are used to modify the physical chemical and mechanical characteristics of the latter.
In particular, in the case of the MMC, the nanofibers give the composite a high tensile strength, but also abrasion resistance, reduced friction coefficients and improved thermal conductivity.
These composites are used for example for the production of cutting tools, or in particular applications like car parts (for example brake discs) or bicycle frames, due to the reduced weight of the parts produced with these materials compared to the same parts made of metal.
In the case of the CMC, the nanofibers give the composite, in particular, high breaking strength and toughness, abrasion resistance, thermal conductivity and resistance to thermal shocks. CMC are used in particular to produce heat-resistant parts (tiles for space shuttle heat shields, combustion chambers and turbine blades, components of burners or ducts for hot gases), or in turn for the production of brake discs.
Ceramic nanofibers are produced using various techniques, including for example methods deriving from the sol-gel technique, plasma deposition, chemical vapour deposition (CVD), methods that comprise sublimation of the compound from which the nanofibers are formed and subsequent condensation thereof in a cold area, pyrolysis of agricultural waste such as rice husks or, in the case of silicon carbide nanofibers, high temperature reduction of silica powders with carbon.
Another method of producing ceramic materials is by the pyrolysis of preceramic polymers; in this regard, see for example the article "Polymer-derived-ceramics: 40 years of research and innovation in advanced ceramics", P.
Colombo .. et al., J. Am. Ceram. Soc. 93 (2010) 1805-1837. The process and the materials obtained are known in the sector as PDC.
The paper "Processing of polysiloxanes-derived porous ceramics: a review", B. V. Manoj Kumar et al., Science and Technology of Advanced Materials, 11, Vo.
11 (2010), 4, pages 1-16, reports various methods of producing PDCs. One of these methods is described in paragraph 2.1 of the paper and referred to as "Replica".
The results of the application of this method are shown in Fig. 2, and consist in the replication of the structure of the starting porous material (that can be a polymeric foam, wood, or other similar materials), with no nanofibers present.
Similar techniques have been used to produce ceramic nanofibers.
The article "Nanostructure of polymer-derived silicon nitride", S.T. Schwab et al., Materials Science and Engineering A204 (1995) 201-204, describes a synthesis of silicon nitride (Si3N4) in nanostructured form starting from perhydropolysilazanes.
The article "Mass production of very thin single-crystal silicon nitride nanobelts", F. Gao et al., Journal of Solid State Chemistry 181 (2008) 211-215 describes the production of Si3N4 nanofibers using polyaluminasilazane as a preceramic precursor and aluminium as a reaction catalyst.
Finally, two papers from C. Vakifahmetoglu et al., "Growth of One-Dimensional Nanostructures in Porous Polymer-Derived Ceramics by Catalyst-Assisted Pyrolysis. Part I: Iron Catalyst" (Journal American Ceramic Society 93 (2010), Vol.
4, 959-968) and "Growth of One-Dimensional Nanostructures in Porous Polymer-Derived Ceramics by Catalyst-Assisted Pyrolysis. Part II: Cobalt Catalyst"
(Journal American Ceramic Society 93 (2010), Vol. 11, 3709-3719), describe the production of nanofibers of SiC/SiOC and/or Si3N4 by milling a polysilsesquioxane to which 1`)/0 b.w. azodicarbonamide as a foaming agent and 3% b.w. of a salt or iron or cobalt .. are added as catalysts for the growth of the nanofibers. The results of these preparations, as reported in the section "Conclusions" of both papers, are porous structures of dense ceramics with the surface of the walls of the pores decorated by

2 nanowires.
All the methods described above generally result in nanofibers deposited both on the starting powders or structures and on the walls of the crucible or furnace, the form of which is not easy to control. Furthermore, part of the powder used as precursors of the nanofibers is not reacted at the end of the process, requiring a further step for purifying the nanofibers from the excess reagents.
The object of the present invention is to provide a process for the production of ceramic bodies consisting of bundles of nanofibers of silicon nitride forming one single body having easily controllable shape and size.
SUMMARY OF THE INVENTION
In a first aspect, the present invention concerns a process for the production of a ceramic body consisting of a bundle of nanofibers of silicon nitride forming a single body, which comprises the following steps:
a) impregnation of an open-cell organic polymer with a precursor of a preceramic polymer containing silicon, oxygen, carbon and hydrogen;
b) cross-linking of said precursor to form the preceramic polymer;
c) pyrolysis of the impregnated organic polymer at a temperature ranging between 1400 and 1500 C in a controlled atmosphere of nitrogen.
In a second aspect, the invention concerns the ceramic bodies, consisting of bundles of nanofibers of silicon nitride, obtained according to the process of the invention.
BRIEF DESCRIPTION OF THE FIGURES
Fig. 1 shows three microphotographs at different enlargements of an open-cell organic polymer.
Fig. 2 shows three photographs of the material at various stages in the process of the invention.
Fig. 3 shows three microphotographs at different enlargements of a ceramic body of the invention consisting of silicon nitride nanofibers.
Fig. 4 shows four microphotographs at high enlargements of the ceramic bodies of Fig. 3, which highlight the silicon nitride nanofibers.
Fig. 5 shows two X-ray diffractograms of two samples of silicon nitride nanofibers which constitute a first type of ceramic body of the invention.

3 Fig. 6 shows two graphs obtained as a result of stress-deformation tests on ceramic bodies of the invention consisting of silicon nitride nanofibers.
Fig. 7 shows in graphic form the result of surface area measurement tests of two ceramic bodies of silicon nitride of the invention.
Fig. 8 shows a graph relative to the dimensional distribution of the pores of two ceramic bodies of silicon nitride of the invention.
Fig. 9 shows an X-ray diffractogram of a sample of silicon nitride nanofibers.
DETAILED DESCRIPTION OF THE INVENTION
In the description and in the claims, the following terms should be understood as specified:
- "organic polymer": in the present text, this term indicates a polymer consisting of a skeleton of carbon atoms and containing hydrogen atoms and optionally oxygen, nitrogen or sulphur atoms;
- "precursor of a preceramic polymer": a polymer or a mixture of polymers containing silicon and at least one silicon-oxygen bond in their repetitive unit. These compounds can already contain in their chain the functional groups necessary for the formation of bonds between adjacent chains (for example, vinyl, epoxy, alkoxy groups, ...), in which case the precursor can consist of these polymers only;
alternatively, when these compounds do not contain in their chain functional groups suitable for cross-linking, it is possible to use a mixture of these polymers and compounds able to react with at least two chains of said polymers containing silicon-oxygen bonds; for convenience of description, the polymers containing silicon-oxygen bonds are also defined simply as "inorganic polymers";
- "preceramic polymer": in the present text, this term indicates a cross-linked polymer containing silicon obtained by reaction of the above-defined precursors;
- "polymeric foam": this term indicates an organic polymer in the open-cell form.
In its first aspect, the invention concerns a process for the production of ceramic bodies consisting of bundles of nanofibers of silicon nitride; in the ceramic bodies of the invention, the nanofibers are entangled but not bonded together, a feature accounting for the elasticity of these bodies.
The first step of the process, a, consists in impregnation of a polymer foam with the precursors of a preceramic polymer containing silicon. The organic polymer

4 can be of any type, on condition that it can be produced in an open-cell form.
Latex can be employed as the organic polymer, but the preferred polymers for use as organic polymers in the present invention are polyurethanes; these polymers, produced in the open-cell form, have a density of approximately 70 kg/m3 in the case of the latex, and varying between approximately 6 and 20 kg/m3 in the case of the polyurethanes.
Fig. 1 shows three microphotographs with medium-low enlargements (35x to 300x) obtained with a scanning electron microscope (SEM) on a sample of an open-cell polyurethane; the microphotographs show the structure of the polymer, which can be described as a set of irregular polyhedrons in which the material is present essentially only on the vertexes and edges of the polyhedrons, while the faces (which would constitute the walls of the cell) are absent; in this way, the porosity of the material is continuous throughout the structure of the polymer, and by means of impregnation it is possible to reach all the points inside said structure.
Organic polymers that have proved suitable for the purposes of the invention are, for example, latex and, above all, polyurethanes. Naturally, organic copolymers or organic block polymers having the above-mentioned characteristics can also be used.
The organic polymer is impregnated with the precursors of the preceramic polymer. These precursors can consist of one single inorganic polymer, A, which has on its structure the functional groups necessary for the reaction with the adjacent chains; alternatively, it is possible to use a mixture of an inorganic polymer, A', which does not contain reactive groups able to react with adjacent chains of the same polymer, and at least one compound, B, able to react with two or more molecules of the inorganic polymer. The precursors can also contain a catalyst to favour the reaction between the chains of polymer A, or between the components A' and B.
It is obviously possible to use mixtures of several inorganic polymers of type A, several inorganic polymers of type A' together with a cross-linking compound B, or mixtures containing both inorganic polymers of type A and inorganic polymers of type A', together with the compound B.
The inorganic polymer A or A' contains at least one Si-0 bond in its repetitive

5

6 PCT/EP2018/082003 unit. The polymer can have a linear, branched or dendritic chain, but must consist of discrete chains, not bound to other similar chains; this characteristic is necessary for the inorganic polymer A or A' to be liquid at room temperature, or to be soluble at room temperature in a suitable solvent, and therefore impregnate the pores of the organic polymer.
The inorganic polymer can be, for example, a polysiloxane, a polysilsesquioxane or a polycarbosiloxane.
The polysiloxanes are linear chain polymers formed of repetitive units having the formula (1):

I
-[-Si-0+- (1) I

in which R1 is an organic radical, generally alkyl or aryl, and R2 can be hydrogen or an organic radical, identical to or different from R1.
The polysilsesquioxanes are oligomers with the general formula (2):

I
-[-Si-(0)1,5+- (2) in which R1 is an organic radical, and can assume various configurations, for example the "cage" configuration (similar to the spatial configuration of adamantane), "ladder" configuration (two parallel linear chains connected to each other at several points), disorderly configurations (defined "random") or others.
Lastly, the polycarbosiloxanes are linear chain polymers formed of repetitive units having the formula (3):

I I
-[-Si-O-Si-CH2]n- (3) I I

in which R1, R2, R3 and R4, identical to or different from one another, are hydrogen or organic radicals, generally alkyl or aryl.
The inventors have observed that the best results, in terms of silicon nitride nanofiber yield, are obtained when in the precursor (or in the mixture of precursors) of the preceramic polymer the atomic ratios Si/0 and Si/C are close to 1; when there is a significant deviation from these ratios, the conversion yields decrease and in the final material carbon residues can be found; these, however, can be easily eliminated by post-treatments at relatively low temperatures, for example approximately 200-300 C, in oxidising atmosphere, for example in air.
The compound A must have reactive sites in its structure (for example, Si-H, Si-OH or Si-CH=CH2 groups) which can react with the compound B in step b of the process of the invention.
The compound B is a compound having at least two (preferably more) functional groups able to react with the reactive sites of the inorganic polymer. The compound B is therefore able to bind with two or more different adjacent chains or molecules of the inorganic polymer. An example of cross-linking agent is the compound tetramethyl-tetravinyl-cyclotetrasiloxane (abbreviated TMTVS), having the following formula:
/
0 s, Ni, rc!
"Si TMTVS too, when used, is transformed at the end of the process of the invention into Si3N4, so when planning a production run that involves this compound it is necessary to take into account its amount as a precursor of the desired phase.
The mixture of precursors of the preceramic polymer can further contain an accelerator (catalyst) of the cross-linking reaction between the compound B
and the inorganic polymer. Known catalysts for the cross-linking of inorganic polymers comprise (according to the reactive groups present on said inorganic polymers):
peroxides, like di-acyl peroxides, di-aralkyl peroxides or di-alkyl peroxides, in

7 particular when the inorganic polymer has vinyl reactive groups; titanates, like titanium tetrabutanolate; metallic octoates like zinc octoate; amines, for example N-(3-(trimethoxysilyl)propyl)ethylendiamine; or the Karstedt catalyst, a compound having the formula given below, when the inorganic polymer contains vinyl groups and the cross-linking agent contains Si-H groups.
Me Me Me Me N I Me St Si¨Me Me ¨Si ---e"% ,* Si ¨Me 0/ NOr S.. \o ,=6 II
Me ¨Si j/ __ Si ¨Me /
Me Me Lastly, when said precursor or mixture are solid, or have a viscosity too high to allow effective impregnation of the pores of the organic foam, the precursor or the mixture of precursors of the preceramic polymer can be used in solution or in a mixture with a solvent that acts as a viscosity regulator; the precursor or the mixture of precursors of the preceramic polymer, or their solutions or mixtures with solvents must be liquid, and preferably have a viscosity ranging from 1 to 500 cP at room temperature. If solvents are used, acetone, cyclohexane, n-hexane, tetrahydrofuran, toluene, xylene or mixtures thereof are suitable, for example. The solvent must be chosen so that it does not dissolve the organic polymer foam during the impregnation phase with the precursors of the preceramic polymer; the inventors have observed, however, that a solvent able to swell the structure of the organic polymer is compatible with the invention.
The weight ratio between the precursor and the foam allows controlling the bulk density of the final ceramic body; the lower the starting weight ratio precursor/foam, the lower the bulk density of the final ceramic body.
Operating with a weight ratio precursor:foam 1:1, the inventors have been able to obtain ceramic bodies of bulk density as low as 15 kg/m3, lower than the density of the starting polymeric foam. By "bulk density" is meant the weight of the body divided by the volume defined by the outer contour of the same.
The impregnation of the organic polymer with the mixture of precursors described above can be accomplished in various ways, for example by immersing the organic polymer in the mixture in an open container and subjecting it to compression-decompression cycles, for example with a piston, or having it

8 impregnated completely with said mixture and then causing it to pass between two rollers to eliminate the excess; or by forcing the mixture to pass through the organic polymer (for example arranging it so as to completely occupy the section of a duct through which the mixture is made to flow, in this case preferably retaining in position the organic foam with a retaining element, like a wire mesh downstream thereof); or other similar methods.
Figure 2(a) shows a photograph of a sample of an open cell polyurethane impregnated with precursors of the preceramic polymer.
In step b of the process of the invention, the precursor or the mixture of precursors (or solutions thereof or mixtures with solvents) described so far are made to cross-link. From the microscope observations of the material obtained at the end of the process, it can be seen that the cross-linking occurs both on the surface and inside the organic polymer: the precursor is able to dissolve at least partly inside the organic polymer, swelling it. The result of this step is formation of the preceramic polymer. This operation is carried out by simply bringing the system to the reaction temperature between the polymer A and the cross-linking agent, if necessary taking account of the presence of the catalyst. Since the next step, c, involves heating of the system to much higher temperatures than the cross-linking temperature, the separation between steps b and c can be effective or only formal. In fact, it is possible for step b to be carried out as a distinct well-defined step of the process;
this can be useful for example when it is desirable to subject the organic polymer impregnated with cross-linked inorganic polymer to intermediate process controls (for example, to verify the uniformity of the impregnation); the interruption of the process after a first cross-linking step can also be exploited to repeat the impregnation process, in order to increase the quantity of preceramic polymer and therefore the density of the final material. Alternatively, step b can be carried out as part of step c, as an initial phase of a continuous heating ramp which leads to pyrolysis conditions of the material.
If step b is performed as a separate distinct step from step c, the cross-linking operation is carried out with a heat treatment that entails a temperature slope from room temperature to cross-linking temperature at a rate of between 1 and 20 C/min, stay at the cross-linking temperature for a time between 5 and 120 minutes, and

9 natural or forced cooling of the system. If the mixture of precursors of the preceramic polymer does not contain a catalyst, the cross-linking temperatures vary generally between 120 and 250 C; if a catalyst is used, these temperatures vary generally between 60 and 180 C.
Lastly, the last step of the process, c, consists in heat treatment of the organic polymer impregnated with preceramic polymer (already cross-linked or not, depending on the adoption of one or the other of the two above-mentioned alternatives) at a temperature between 1400 and 1500 C in a nitrogen controlled atmosphere. The controlled atmosphere is maintained preferably in conditions of controlled flow of nitrogen. The preferred range of temperatures for carrying out this step is between 1480 and 1500 C, more preferably between 1485 and 1495 C, and the duration of this step is preferably between 2 and 8 hours.
The heating ramp has typically a heating rate between 5 and 10 C/min;
preferably, during this heating ramp, T is kept constant for about 10-15 min to 600 C, to assure complete pyrolysis of the starting polymeric foam.
Whether step b has been carried out as a separate step (bringing the system obtained at the end of step a to the cross-linking temperature and then back to a lower temperature), or whether step b is carried out as part of step c, in this step the temperature is brought from the initial temperature (which can be room temperature, the temperature at which the cross-linking took place, or an intermediate temperature) to the pyrolysis temperature at a rate of between 1 and 20 C/min, the pyrolysis temperature is maintained for a time ranging from 60 to 240 minutes, and the system is naturally or preferably forcibly cooled to room temperature.
In a second aspect, the invention concerns the ceramic bodies obtained by means of the process of the invention.
These ceramic bodies consist of bundles of ceramic nanofibers entangled into one another so as to form structures having their own shape and mechanical strength.
The pyrolysis step (step c) is carried out in a nitrogen atmosphere, leading to the formation of silicon nitride, Si3N4.
The bodies made of Si3N4 have the consistency of felt, that is, are flexible and compressible; these bodies can be used to produce, among other things, filters for the purification of liquids or to filter gases or vapours containing suspended powders (for example for application as filters for diesel engine emissions).
Figures 2(b) and 2(c) show two photographs of ceramic bodies of the invention, obtained by means of pyrolysis treatments conducted at 1200 C and 1485 C
respectively in nitrogen; the sample in Fig. 2(b) is still an oxycarbide, an intermediate phase formed in the process, while the sample shown in Fig. 2(c) has composition Si3N4.
Figures 3(a), 3(b) and 3(c) show microphotographs at low enlargements (between 10x and 100x) of a section of the sample of figure 2(c); these SEM
microphotographs (in particular the one in figure 3(c)) show how the silicon nitride has grown, uniformly occupying all the pores of the original structure of the organic polymer.
The figures 4(a), 4(b), 4(c) and 4(d) show the same sample as figure 3, at higher enlargements (between 1.000x and 50.000x); these SEM microphotographs show that the nanofibers of Si3N4 grow in the shape of a band, with a highly flattened rectangular section having width of approximately 300 nm and thickness of approximately 50 nm.
Fig. 5 shows two X-ray diffractograms of two samples of silicon nitride nanofibers obtained according to the process of the invention. These diffractograms confirm that the material is made solely of silicon nitride, in particular the a-Si3N4 phase, by means of assignment of the peaks to the peaks of this phase known in the literature. The conditions for preparation of the samples and obtaining the diffractograms are described in greater detail in the examples.
The invention will be further illustrated by the following examples. In the experimental tests conducted, the following instruments and conditions were adopted:
- XRD Italstructures IPD3000 diffractometer provided with a source consisting of a Cu anode (radiation Ka of copper, A -,--, 1,540 A) or a Co anode (radiation Ka of cobalt, A -,--: 1,789 A), a multilayer monochromator to suppress the radiation Ki3 of the source and fixed opening of 100 pm. The samples were positioned in reflection geometry with fixed angle with respect to the incident beam and the spectra were collected by means of an Inel CPS120 detector over an interval +1200 20;
- MTS 810 (MTS Systems Corporation, USA) compression resistance measurement system with force transducer of 1 kN (precision 0.1"Yo on scale end); the tests were performed on cubic samples with 1 cm sides using a deformation speed of 1 mm/min;
- the specific surface, the volume of the pores and the distribution of the pore size of the samples were measured by means of physisorption of N2 at -196 C (77 K) with a Micromeritics ASAP 2010 instrument. The samples were first degassed at 150 C. The specific surface was calculated in the relative pressure interval (p/po) between 0.06 and 0.30, applying the Brunauer-Emmett-Teller (BET) multipoint method. The distribution of the wall pore size was evaluated by the Barrett-Joyner-Halenda (BJH) method from the isotherm desorption branch. The total volume of the single-point pores (TPV) was calculated at P/Po = 0.995;
- the surface morphology of the starting organic polymer was examined using a scanning electron microscope (SEM, Jeol-JSM-5500), metallizing the samples with Au or Pd;
- the field emission SEM (FE-SEM) images of the fracture surfaces of the ceramic samples were acquired with Zeiss supra 60 equipment (Carl Zeiss NTS GmbH, Germany) operating in high vacuum at 2.00 kV and after depositing on the samples (by sputtering) a thin film of Pt/Pd;
- the optical images of the ceramic samples were acquired with a stereomicroscope (ZEISS, Stemi 2000-C).

Production of a silicon nitride-based ceramic body.
A parallelepiped was prepared measuring approximately 4 x 4 x 0.6 cm, of weight 0,29 g, of an ester-based polyurethane sold by the company Articoli Resine Espanse s.r.l. of Rosate (MI) under code no. PPI 90; this polymer has completely open cells with diameter ranging from 440 to 520 pm and density of approximately 0.03 g/cm3.
The mixture of precursors of the preceramic polymer was prepared separately, consisting of:

- 0.23 g of polymethylhydrosiloxane (PMHS) inorganic polymer with average molecular weight 1900 Da, sold by Alfa Aesar under catalogue no. L14561 (CAS
Reg. No. 63148-57-2);
- 0.06 g of tetramethyl-tetravinyl-cyclotetrasiloxane (TMTVS) cross-linking .. agent, Alfa Aesar, catalogue no. L16645 (CAS Reg. No. 2554-06-5);
- 0.67 pL of Karstedt catalyst as 2% solution by weight of Pt in xylene, Sigma Aldrich, catalogue no. 479519 (CAS Reg. No. 68478-92-2);
- 2.55 mL of acetone as solvent.
The mixture thus obtained was poured onto the polyurethane, and caused to be absorbed by the same, subjecting it to a series of compression and expansion cycles, until obtaining a uniform appearance of the impregnated polyurethane.
The impregnated polyurethane was kept at room temperature for 24 hours to allow evaporation of the acetone, placed in a tubular alumina oven (Lindberg Blue, Thermo Fisher Scientific, Waltham, MA, USA) through which nitrogen was flushed for 5 hours to remove the air; lastly, under a nitrogen flow of 300 cc/min, the impregnated polyurethane was subjected to thermal treatment with a heating slope of 5 C/min up to 1485 C, kept at this temperature for 2 hours and naturally cooled to room T.
The sample thus obtained is called Sample 1; the sample has a bulk density (weight/volume determined by the three lateral dimensions of the ceramic body) of 0.025 g/cm3 and sufficient consistency for low intensity handling and machining.
On this sample FE-SEM images at different enlargements were acquired, following the above procedures; the four microphotographs shown in Fig. 4 were obtained, showing that the sample is formed of a thick weave of nanofibers with rectangular section (nanobelts) with average width of 300 nm and thickness of nm, uniform throughout the growth direction of the nanofibers, and smooth surfaces.
The sample was then subjected to a XRD test as described above (X-ray source: copper anode); the result is shown in Fig. 5(a), and the comparison between the position of the peaks in the diffractogram with the data in the literature confirms that the nanofibers consist of the a-Si3N4 phase.
The sample is elastic and flexible, and can be wrapped around a metallic rod of 3 cm diameter without breaking, recovering its initial shape (again without breaking) when released.

Production of a silicon nitride-based ceramic body.
The procedure of Example 1 was repeated, the only difference being that the quantities of PMHS and TMTVS were increased to 0.67 g and 0.17 g respectively.
The sample thus obtained is called Sample 2; the sample has a bulk density of 0.09 g/cm3 and sufficient consistency for low intensity handling and machining.
An FE-SEM investigation on this sample showed that it consists of the same thick weave of flat nanofibers (nanobelts) as Sample 1.
Also this sample was subjected to a XRD test following the same procedures as Example 1; the result is shown in Fig. 5(b), and the test confirms that also Sample 2 consists of nanofibers of a-Si3N4.

In this example some mechanical properties of the ceramic bodies of the invention consisting of silicon nitride are measured.
Samples 1 and 2 were subjected to compression resistance and elastic modulus measurements. The diagrams obtained in the two tests are shown in Figure 6.
The compression measurement on denser Sample 2 results in a force-deformation diagram that shows three distinct regions: an almost linear increase of the force up to 15% deformation, a plateau up to approximately 60%
deformation, and a rapid increment in the force above 60% deformation, due to compacting of the sample. The compression resistance is defined as the maximum force prior to beginning of the compacting (therefore the force at 60% deformation), and was equal to 150 kPa; the elastic modulus is estimated by the slope of the first part of the curve (deformation lower than 15%) and was approximately 920 kPa.
The force-deformation curve obtained in the measurement on Sample 1, less dense, is more continuous and the three above-mentioned areas are not easily distinguishable; from the curve, however, compression resistance and elastic modulus values of 25 kPa and 145 kPa were respectively estimated.

In this example the morphological characteristics of Samples 1 and 2 are evaluated.
The specific surface, the total volume and distribution of the pore size of Samples 1 and 2 are measured with the Micromeritics ASAP 2010 instrument and according to the methods described above.
The isotherms of absorption and desorption of N2 are shown in Fig. 7; the trend of the curves, which are between type II and type IV of the IUPAC definition, suggests that the samples are essentially solid macroporous, with some mesopores in the case of Sample 2 which is denser.
From the data of the isotherms, the graph of dimensional distribution of the pores is obtained, shown in Fig. 8, likewise the data of the surface area and total volume of the pores, shown in Table 1.
Table 1 Sample Surface area (m2/g) Total pore volume (cc/g) 1 11 0.03 2 46 0.17 Production of a silicon nitride-based ceramic body.
The procedure of Example 1 was repeated, but using 0.29 g of inorganic polymer only, without the addition of a cross-linking agent; the inorganic polymer used is Polyramic SPR-036, produced and sold by the company Starfire Systems, Inc., of Schenectady (New York, USA).
The material obtained was subjected to a XRD test following the procedures of Example 1, the only difference being that in this case a cobalt anode was used as the X-ray source; the result is shown in Fig. 9, and the comparison between the position of the peaks in the diffractogram with the literature data confirms that the entire sample consists of the a-Si3N4 phase.
The sample has the same elastic and flexibility properties of Sample 1, and can be wrapped without breaking around a metallic rod of 3 cm diameter.
This example demonstrates that in the process of the invention it is not necessary a catalyst, e.g. a platinum-based catalyst.

Claims (17)

1. A process for the production of a ceramic body consisting of a bundle of nanofibers of silicon nitride forming a single body, comprising the following steps:
a) impregnation of an open-cell organic polymer with a precursor of a pre-ceramic polymer containing silicon, oxygen, carbon and hydrogen;
b) crosslinking said precursor to form the pre-ceramic polymer;
c) pyrolysis of the impregnated organic polymer at a temperature between 1400 and 1500 °C in a controlled atmosphere of nitrogen.

2. The process according to claim 1, wherein the open-cell organic polymer employed in step a is selected between latex and polyurethane.

3. The process according to any one of claims 1 or 2, wherein the precursor of the pre-ceramic polymer used in step a comprises at least one inorganic polymer, A, which has on its structure the functional groups necessary for the reaction with the adjacent polymer chains.

4. The process according to any one of claims 1 or 2, wherein the precursor of the pre-ceramic polymer employed in step a is a blend comprising at least one inorganic polymer A', not containing reactive groups capable of reacting with adjacent chains of the same polymer, and at least one compound, B, capable of reacting with two or more inorganic polymer A' molecules.

5. The process according to claim 4 wherein said compound B is tetramethyl tetravinyl-cyclotetrasiloxane (TMTVS), having the following formula:

6. The process according to any one of claims 3 to 5, wherein said inorganic polymers A and A' are selected among polysiloxanes, polysilsesquioxanes and polycarbosiloxanes and have a structure with linear, branched or dendritic discrete chains.

7. The process according to any one of claims 3 to 6, wherein in said inorganic polymers A and A' the Si/O and Si/C atomic ratios are close to 1.

8. The process according to any one of claims 3 to 7, wherein the precursor of the pre-ceramic polymer employed in step a further comprises a catalyst to favor the reaction between chains of polymer A or between components A' and B, selected among peroxides, titanates, metallic octoates, amines and the Karstedt catalyst, a compound having the following formula:

9. The process according to any one of the preceding claims, wherein the precursor of the pre-ceramic polymer employed in passage a has a viscosity between 1 and 500 cP at room temperature.

10. The process according to claim 9 wherein, when at room temperature polymers A and A' are not liquid or have a viscosity greater than 500 cP, they are used in the form of a solution having a viscosity between 1 and 500 cP at room temperature, in a solvent selected among acetone, cyclohexane, n-hexane, tetrahydrofuran, toluene, xylene and mixtures thereof.

11. The process according to any one of the preceding claims, wherein the impregnation of step a is accomplished by immersing the organic polymer in said precursor in an open container and subjecting the organic polymer to compression-decompression cycles; or by completely impregnating the organic polymer of said precursor and then passing it between two rollers to eliminate the excess precursor; or forcing the precursor to pass through the organic polymer.

12. The process according to any one of the preceding claims, wherein step b is carried out as a separate step distinct from step c, with a heating from ambient temperature to the crosslinking temperature at a rate of between 1 and 20 °C/min, stay at the crosslinking temperature for a time between 5 and minutes, and natural or forced cooling of the system, and wherein said crosslinking temperature is between 120 and 250 °C in case said precursor does not contain a catalyst, and is between 60 and 180 °C in case said precursor contains a catalyst.

13. The process according to claim 12, wherein the organic polymer impregnated of inorganic crosslinked polymer is subjected to a further impregnation passage with a precursor of a preceramic polymer containing silicon, oxygen, carbon and hydrogen.

14. The process according to any one of the preceding claims, wherein step c is carried out at a heating rate of between 1 and 20 °C/min, stay at the pyrolysis temperature for a time ranging from 60 to 240 minutes, and natural or preferably forced cooling of the system to room temperature.

15. The process according to any one of the preceding claims, wherein step c is carried out at a temperature between 1480 and 1500 °C.

16. The process according to claim 15, wherein said temperature is between and 1495 °C.

17. A ceramic body consisting of a bundle of nanofibers of silicon nitride, Si3N4, forming a single body, obtained by the process of any one of claims 1 to 16.