CERAMICS AND PREPARATION THEREOF

Technical Field

The present invention applies to the field of electrically conductive ceramics, and incorporates technologies relating to nanocrystalline materials, more specifically, cellulose nanoparticles, and sintering methods for densification and improving the mechanical properties of ceramics.

Background

The class of engineering ceramics, i.e. alumina, zirconia, silicon nitride, hydroxyapatite, etc., possess interesting mechanical and/or bioactive properties. This class of ceramics is electrically non-conducting. However, electrical conductivity is sometimes desired for certain applications like as resistors, electrodes or heating elements, or for machining when completely dense after sintering process.

Complex shapes are either very costly to manufacture with conventional hard machining technologies that use diamond tools or even inaccessible as monolithic components especially if features such as sharp and complex inner contours or blind holes with high aspect ratio are involved.

An alternative machining process is electro-discharge machining (EDM), which enables manufacturing of customized ceramic components of high accuracy and complexity irrespective of the mechanical properties of the materials. The requirement which has to be fulfilled is an appropriate electrical conductivity (> 1 S/m) of the otherwise insulating ceramic material. This can be achieved by incorporation of an electro- conductive phase into the ceramic matrix. However, the appropriate amount of the electro-conductive phase should be homogeneously introduced into the ceramic matrix to provide the necessary conductivity, while not deteriorating the engineering ceramics' excellent mechanical and/or bioactive properties.

The introduction of an electro-conductive phase into ceramic matrices is usually achieved by incorporation of various covalently bonded nitride- and carbide-based ceramics, e.g., TiN, TiC, WC, or metals, e.g., Ag, Al, etc. The problem in providing appropriate electrical conductivity is to introduce a sufficiently high amount of the electro-conductive phase to attain a conducting percolation network throughout the ceramic matrix. Such high amounts, often as high as 25 vol%, might directly affect and deteriorate the ceramic's inherent excellent mechanical and/or bioactive properties. They are difficult to homogeneously distribute in the ceramic matrix. Thus,

inhomogeneities are introduced in terms of flaws and agglomerates, which directly affect and deteriorate the mechanical properties and material reliability. It is well known that the percolation threshold, i.e. the concentration of the conducting phase that onsets conductivity of the material, is grain size-dependent. Thus, the smaller the grains of the conducting phase the lower the percolation threshold, but fine powders are more difficult to process and to shape into a ceramic composite with homogeneously distributed conductive phase due their increased tendency towards agglomeration. Moreover, the resulting composites are prone to grain growth, agglomeration and segregation of the conductive phase at high sintering temperatures (1300-1800°C). This requires higher sintering temperatures for sufficient densification, which ultimately leads again to an increase of the percolation threshold despite of the use of fine, nanocrystalline particles.

An alternative and effective way to produce dense, bulk electro-conductive engineering ceramics is based on the reinforcement of a ceramic matrix by incorporation of carbon nanofillers (CN), e.g., carbon nanofibers, carbon nanotubes, graphene, fullerenes, carbon black, etc. In order that a composite material becomes electrically conductive, the nanofillers have to form a percolating network to ensure a conducting path through the matrix.

Fibrous, homogeneously dispersed CNs can theoretically form such networks at low filler content in the range of 0.1 -1 vol%. However, due to agglomeration and dispersion problems related with their hydrophobicity and high cohesiveness the actual filler content of carbon nanotubes or graphene-based materials is considerably higher, typically 2-10 vol%. The electrical conductivity of such high filler content

nanocomposite ceramics is in the range of 100-800 S/m. (US6875374 B1 ; US8168291 B2; Shin, J.-H. & Hong, S.-H. Microstructure and mechanical properties of single wall carbon nanotube reinforced yttria stabilized zircona ceramics. Mater. Sci. Eng. A 556, 382-387 (2012); Hvizdos, P., Puchy, V., Duszova, A., Dusza, J. & Balazsi, C.

Tribological and electrical properties of ceramic matrix composites with carbon nanotubes. Ceram. Int. 38, 5669-5676 (2012); Ahmad, K., Pan, W. & Wu, H. High performance alumina based graphene nanocomposites with novel electrical and dielectric properties. RSC Adv. 5, 33607-33614 (2015)).

However, high filler content can compromise the mechanical properties of the material, which implies the necessity of an optimum filler content to satisfy both the electrical and the mechanical properties.

Therefore, methods for obtaining homogeneous and finely dispersed CN/ceramics composites are currently being investigated. These include the aqueous colloidal processing of homogeneous dispersions of CN using surfactants, organic solvents mixtures, polymer electrolytes, or the introduction of functional groups by acid treatment. (US2013/0184143 A1 ; US7807092; Hirota K, Takaura Y, Kato M, Miyamoto Y. Fabrication of carbon nanofiber -dispersed AI2O3 composites by pulsed electric- current pressure sintering and their mechanical and electrical properties. J Mater Sci 2007, 42:4792-800; Fan J, Zhao D, Wu M, Xu Z, Song J. Preparation and

microstructure of multi-wall carbon nanotubes-toughened AI2O3 composite. J Am Ceram Soc, 89, 750-753 (2006); Estili M, Kawasaki A, Sakamoto H, Mekuchi Y, Kuno M, Tsukada T. The homogeneous dispersion of surfactantless, slightly disordered, crystalline, multiwalled carbon nanotubes in oalurmina ceramics for structural reinforcement. Acta Mater 2008, 56:4070-4079). However, such methods have problems at industrial scale due to increased complexity, increased number of preparative steps, processing time, and manufacturing costs (Shirai et al. Effects of the alumina matrix on the carbonization process of polymer in the gel-casted green body. J Eur Ceram Soc 2013, 33:201 -206). The complex surface active agents or organic solvent mixtures used for optimal mixing may be hazardous and pollutant and, importantly, may cause pores or fractures upon burn-out and sintering.

Summarizing, the incorporation of CNs into ceramic matrices faces three major problems: i) Inhomogeneous distribution of the CN caused by mixing problems in the aqueous ceramic slurry due to their hydrophobic nature. As a consequence, the mechanical and electro-conductive properties are sub-optimal and therefore, requiring larger quantities of CN to reach optimal properties. Therefore, homogeneous dispersions are useful not only to obtain homogeneous, sintered ceramic nanocomposites but also to obtain consolidated nanocomposite green bodies that display increased rigidity, which allows their green machining. The mixing problems may be overcome with extensive shear force mixing but this often results in damage of the CN including breaking, tearing, or ripping, which shortens the carbon fibers or graphene sheets. ii) Most CN, i.e., carbon nanofibers, carbon nanotubes, carbon black, lack

sustainability as they are produced from petroleum-derived, precursor-like synthetic polymers (e.g. polyacrylonitrile), mesophase pitches or acetylene and other

hydrocarbon gases etc. iii) Besides being a complex and expensive process, toxicity concerns are related with the synthesis of CNs such as carbon nanotubes. They are produced via catalytic chemical vapor deposition, which requires the use of catalyst based on transition-metal nanoparticles (e.g. Fe, Ni, Co). Such nanoparticles arouse toxicity and environmental concerns. Additionally, fine particulate carbons such as carbon black have been shown to have a severe environmental and health impact including increased human morbidity and premature mortality.

General Notes

A number of patents and publications are cited herein in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Each of these references is incorporated herein by reference in its entirety into the present disclosure, to the same extent as if each individual reference was specifically and individually indicated to be incorporated by reference.

Throughout this specification, including the claims which follow, unless the context requires otherwise, the word "comprise" and variations such as "comprises" and "comprising," will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

Ranges may be expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another

embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent "about," it will be understood that the particular value forms another embodiment.

This disclosure includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.

Summary of the Invention

The present invention relates to ceramic articles and precursor products, along with methods of preparing those.

In a first aspect, the present invention provides a method of preparing a ceramic article, the method comprising mixing a ceramic powder and a nano-filler in water to form an aqueous suspension; drying said aqueous suspension to obtain a composite powder; forming said composite powder into a green body; and sintering said green body to form said ceramic article, wherein said nano-filler is an insoluble cellulose material. In a second aspect, the present invention provides a ceramic article prepared by the method of the first aspect.

In a third aspect, the present invention provides method of preparing a composite powder, the method comprising mixing a ceramic powder and a nano-filler in water to form an aqueous suspension; and drying said aqueous suspension to obtain said composite powder, wherein the nano-filler is an insoluble cellulose material.

In a fourth aspect, the present invention provides a composite powder comprising a mixture of a ceramic powder and an insoluble cellulose material. The composite powder may be prepared by the method of the third aspect.

In a fifth aspect, the present invention provides a method of preparing a green body, the method comprising mixing a ceramic powder and a nano-filler in water to form an aqueous suspension; drying said aqueous suspension to obtain a composite powder; and forming said composite powder into a green body, wherein said nano-filler is an insoluble cellulose material.

In a sixth aspect, the present invention provides a green body prepared by the method of the fifth aspect.

In a seventh aspect, the present invention provides an alumina ceramic article, wherein the alumina grain size is 50 to 600 nm, for example 100 to 500 nm, for example 200 to 400 nm. It may be prepared by one of the methods set out herein.

In an eighth aspect, the present invention provides a zirconia ceramic article, wherein the zirconia grain size is 25 to 400 nm, for example 50 to 300 nm , for example 100 to 200 nm. It may be prepared by one of the methods set out herein. The present nano-filler is insoluble, and suitably also hydrophilic. In certain

embodiments of the invention, it may be for example cellulose fibers, cellulose nanofibers (CNF) or microcrystalline cellulose (MCC).

In some embodiments, the ceramic powder is alumina or zirconia. Of course, it will be recognised that the invention can be applied with other known ceramic materials, as known in the art, for example mullite, silicon nitride, hydroxyapatite, silica, cordierite, silicon carbide, titanium carbide and so on.

In the sintering described herein, suitably sintering is performed in the absence of oxygen. In particular, sintering may be performed in an inert atmosphere. For example, in some embodiments sintering is performed in a vacuum furnace. In some embodiments, sintering is performed by step spark plasma sintering. Where sintering is performed by step spark plasma sintering, suitable conditions may be, for example, applied heating rate 20-300°C/min; final sintering temperature 1250- 1800°C; SPS dwell time 3-30 min; and applied pressure 30-200 MPa.

In some embodiments, the concentration of the insoluble cellulose material in the aqueous suspension is 0.1 to 10 wt%. Furthermore, suitably the concentration of the ceramic powder in the aqueous suspension is 5 to 35 vol%.

The present invention may also include active homogenisation of the aqueous suspension. That is, steps may be taken to increase or improve the homogeneity of the mix of cellulose material and ceramic particles. For example, after mixing to form the aqueous suspension and before drying the aqueous suspension there may a step of homogenising the aqueous suspension.

Alternatively or additionally, a dispersant such as citric acid may be added to or present in the aqueous solution. Of course, many such dispersants are known and can be suitably chosen, for example polyacrylic acids, or other commercially available dispersants, such as Dolapix CE64, etc.

The dispersant may be added to the water before either the ceramic powder or the nano-filler is added.

In certain embodiments of the present invention, drying is performed by freeze drying. In other words, it may be that the present drying is a lyophilisation. Of course, as explained below, it is envisaged that some methods of forming a green body or ceramic article do not include this drying as a necessary step.

According to the present invention a cellulose material such as cellulose fibers or cellulose nanofibers (CNF) are used (i) for the preparation of cellulose nanofiber- reinforced engineering ceramic green bodies and, after a sintering step, (ii) for the preparation of electro-conductive, dense ceramics. Importantly, these electro- conducting ceramics can retain the good mechanical properties of the ceramic matrix. In addition, mechanical properties like strength, hardness and fracture toughness, may be even further improved due to a grain refinement effect and due to the reinforcement of the ex-cellulose carbon nanofibers. These fibers may not only serve for

reinforcement but also for functionalization of non-conducting, engineering ceramics, e.g., alumina, zirconia, silicon nitride, hydroxyapatite, etc., replacing conventional carbon nanofillers (carbon nanotubes, carbon fibers, graphene, carbon black). Unlike these carbon nanofillers the present (for example) highly hydrophilic cellulose nanofibers are easily dispersed in aqueous ceramic powder slurries resulting in homogeneous colloidal dispersions that are not so prone to particle segregation or de- mixing phenomena. Such homogeneous dispersions are useful not only to obtain homogeneous, sintered ceramic nanocomposites, but also to obtain consolidated green bodies that display increased rigidity, which enables their green machining.

It will be appreciated that, herein, for shorthand a naming scheme of "xCM" is sometimes used, where "x" represents the wt% of CNF used and "M" represents the ceramic powder - for example, "A" for alumina AI2O3 or "Z" for zirconia ZrC>2. Brief Description of the Figures

The invention will now be described, without limitation, in the following discussion of embodiments of the invention and with reference to the accompanying Figures, in which:

Figure 1 shows magnified SEM microscopy images of the CNF/ZrC>2 (a) and

CNF/AI2O3 (b) composite powders prepared in Example 1 .

Figure 2 shows a machined CNF/ZrC>2 green body prepared by the procedure of Example 2.

Figure 3 shows SEM micrographs and Energy Dispersive X-ray mapping images of fracture surfaces of the composite materials after sintering by the procedure of Example 3. In particular, Figure 3a illustrates the structure of pure AI2O3, for reference. Figure 3b illustrates the structure of SPS sintered 2wt%CNF/Al2C>3 of the type prepared by the present Examples. Figure 3c illustrates carbon mapping of SPS sintered 2wt%CNF/Al2C>3. The inset SEM micrograph shows the mapped area.

Figure 4 shows the grain size distribution of a diluted aqueous CNF-zirconia powder suspension before and after 2-h attrition milling.

Figure 5 shows the microstructure of green CNF nanocomposites. In particular it shows SEM micrographs of freeze-dried CNF-ceramics powders: Figure 5a: 2CZ; Figure 5b: 3CA.

Figure 6 shows further the microstructure of sintered CNF nanocomposites. In particular, Figure 6a shows a photograph of SPS sintered 3CZ showing EDM erosion pits. Figure 6b shows SEM images of the fracture surface of SPS sintered 2CZ, and Figure 6c the same for 2CA, indicating ex-cellulose carbon fiber inclusions (white arrows). Figure 6d shows a photograph of SPS sintered 2CZ. Figures 6e and 6f show EDX carbon mapping of 2CZ (Figure 6e) and 2CA (Figure 6f).

Figure 7 shows analysis for characterization of the grain boundary phase in sintered ceramic nanocomposites. In particular, it shows DF and BF TEM (Figure 7a) and HRTEM (Figure 7b) images of SPS sintered 2CZ. Figure 7c shows EELS mapping across a grain boundary of 2CZ using the carbon and the zirconium signal, respectively. Figures 7d and 7e show HRTEM images showing graphitic structures in 2CZ. Figure 7f shows Raman spectra of 2CZ and 2CA. Artificial oxygen and nitrogen fluorescence bands are marked with an asterisk. Figure 7g shows core level C 1 s XPS spectra of 2CZ and 2CA.

Figure 8 shows a TEM micrograph of 2CZ and EELS maps across of a grain boundary showing the oxygen, zirconium, and carbon signals. The pixel brightness relates with the signal intensity of the corresponding element.

Detailed Description

The Nano-Filler and its Behaviour

The present nano-filler is insoluble, and suitably also hydrophilic. By insoluble, it is meant that in 100 ml water less than 3 g, for example less than 2 g, for example less than 1 g, for example less than 0.5 g, for example less than 0.25 g, for example less than 0.1 g, for example less than 0.05 g, for example less than 0.01 g, for example less than 0.005 g, for example less than 0.001 g of the material is dissolved.

In certain embodiments of the invention, it may be for example cellulose fibers, cellulose nanofibers (CNF) or microcrystalline cellulose (MCC). In some embodiments, the nano-filler is cellulose nanofibers. Such materials are in general well known, as are methods for their production or synthesis.

For example, the cellulose material may be a nanostructured cellulose, cellulose nanofibers or microfibrillated cellulose, nanocrystalline cellulose, microcrystalline cellulose.

Cellulose nanofibers are a material of nanosized cellulose fibers with a high aspect ratio, that is, high length to width ratio. The fibers may have a width of 1 to 20 nm, for example 2 to 10 nm, for example 3 to 6 nm, for example about 4 nm. The fibers may have a length of 0.5 to 10 μηη, for example 0.7 to 5 μηη, for example 0.9 to 3 μηη, for example 1 to 2 μηη. The fibers may have an aspect ratio (length to width ratio, length/width) of more than about 50, for example more than about 100, for example more than about 15, for example more than about 200, for example more than about 300, for example more than about 400, for example more than about 500, for example more than about 1000. The fibers may have an aspect ratio of 10 to 10000, for example 50 to 5000, for example 100 to 1000, for example 200 to 700.

Microcrystalline cellulose is also a well-known material. It may have, for example, a D50 particle size as generally known in the art. The particle size is not particularly limited, but it may be for example a D50 of 1 to 50 μηη, for example 2 to 40 μηη, for example 5 to 20 μηι.

The cellulose based materials used as nano-fillers in the present invention are insoluble and suitably hydrophilic. This means that they can better produce a homogenous dispersion in the aqueous suspension with the ceramic particles. They can remain homogeneously dispersed in the product ceramic articles, offering advantages in electro-conductivity and processing.

The employed cellulose materials, such as cellulose nanofibers (CNF), are

environmentally friendly. They may be, for example, produced by chemo-mechanical defibrillation of wood, tunicate or flax or cotton fibers, which yields 4 nm thick, semi- crystalline and semi-flexible fibers of up to 1 -2 μηη length (Isogai et al. TEMPO-oxidized cellulose nanofibers. Nanoscale 201 1 , 3, 71 -85; JP2008001728A;

WO2009069641A1 ).

It is thought that because of surface hydroxyl and carboxylate groups CNF are highly hydrophilic and maintain excellent colloidal stability in aqueous media. Therefore cellulose materials which include such hydroxyl and carboxylate groups may be particularly suitable in the present invention.

Contrarily to the mixing problems of hydrophobic CNs in aqueous ceramic slurries caused by their agglomeration due to van der Waals forces and intense entangling, the here present invention may suitably enable attaining homogeneous dispersions of a cellulose material such as CNF or MCC in aqueous ceramic slurries during colloidal processing. The homogeneous distribution may improve the mechanical reinforcement of ceramic green bodies and, thus, the cellulose materials described herein may be said to be acting as a superior binding agent.

The homogeneous distribution in the pre-sintered green body may also suitably lead to a resulting homogeneous distribution of pyrolyzed ex-cellulose carbon nanofibers in the ceramic nanocomposite after sintering.

That is, in the present green bodies the cellulose material is present. On sintering, or pyrolysis if performed, it is thought that the cellulose material is converted to a distributed network, or matrix, of carbon chains and rings which has some fibrous properties but also some graphite-like behavior. These "ex-cellulose carbon nanofibers" can provide the desired electro-conductivity in the sintered ceramic article. In other words, it is thought that a polymeric network with electron conductivity is formed. The Ceramic Powder

The ceramic powder used in the present invention may be any one or mixture of those known in the art for producing ceramic articles. Many such materials are known, such as alumina, zirconia, silica, silicon nitride, hydroxyapatite, TiN, TiC, WC etc. Of course it is contemplated that mixtures of ceramic powders as considered suitable in the art of ceramics may be used on the present invention.

Suitable powders and methods for making them will be apparent to the reader. In the present invention, the ceramic powder may be, for example, alumina (AI2O3) or zirconia (Zr02). It will be recognised that amounts of known dopants, such as yttrium oxide, may also be present, for example in an amount of 1 to 5 mol%, for example about 3 mol%. The ceramic powder or powders maybe, for example, aggregated crystallites. They may have an average (mean) size of, for example, 10 to 1000 nm, for example 50 to 500 nm, for example 100 to 300 nm.

The ceramic powder or powders may have a specific surface area of, for example, 1 to 50 m ² /g, for example 2 to 30 m ² /g, for example 5 to 20 m ² /g, for example 7 to 14 m ² /g. The specific surface area may be measured by, for example the BET adsorption method.

The Mixing Step and the Aqueous Solution

In certain aspects of the present invention there is a mixing step which includes mixing a ceramic powder and a nano-filler in water to form an aqueous suspension. The aqueous suspension may suitably be homogeneous, in other words, may contain a homogeneous dispersion of the ceramic powder and the nano-filler. The suspensions and dispersions of one or more components in an aqueous solution may also be referred to as slurries or suspensions.

The present aqueous solution is, of course, water based. The present invention means that additional stabilizing additives or alternative organic solvent mixtures are not needed, as is the case in the prior art where other carbon nano-fillers are used. It will be recognised that the order of mixing is not particularly limited. For example, the ceramic powder may be added to the water, followed later by the nano-filler. The nano-filler may be added to the water, followed by the ceramic powder. The nano-filler and ceramic powder may be added to the water concurrently. The nano-filler and ceramic powder may be mixed together, and then the mixture added to the water (or the water added to the mixture).

In some embodiments, the ceramic powder is added to water or an aqueous solution to form a ceramic powder slurry. Then, the nano-filler is added, for example as a gel. Then, the slurry is mixed.

The amount of the ceramic powder added to the water or aqueous solution (that is, the amount present in the aqueous solution) may be sufficient to give, for example, a solid loading or concentration of 5 to 35 vol%, for example, 15 to 35 vol%, or for example 5 to 20 vol%. The solid loading or concentration of the ceramic powder in the aqueous solution may be more than about 5 vol%, for example more than about 7 vol%, for example more than about 10 vol%, for example more than about 12 vol%, for example more than about 15 vol%. The solid loading or concentration of the ceramic powder in the aqueous solution may be less than about 50 vol%, for example less than about 40 vol%, for example less than about 35 vol%, for example less than about 30 vol%, for example less than about 25 vol%, for example less than about 20 vol%.

The amount of the nano-filler added to the water or aqueous solution (that is, the amount present in the aqueous solution) may be for example 0.05 to 10 wt%, for example 0.1 to 10 wt%, for example 0.1 to 5 wt%, for example 1 to 4 wt%, for example 1 .5 to 3.5 wt%, for example 2 to 3 wt%, for example about 2 wt%. The nano-filler may be added in the form of particles, or a powder, or a gel, for example. In some embodiments, adding the nano-filler in the form of a gel is preferred. For example, when the nano-filler is cellulose nano-fibers this form of addition is useful. The gel may be water based.

In some embodiments, adding the nano-filler in the form of particles is preferred. For example, when the nano-filler is microcrystalline cellulose this form of addition is useful. It will also be appreciated that other components may be added, at any stage of or time during this mixing process. For example, a dispersant may be added. It may be added before the ceramic powder, before the nano-filler, or before both. It may be added after the ceramic powder, after the nano-filler, or after both.

Suitably, the dispersant is added before the ceramic powder and before the nano-filler has been added to the water. Dispersants are known in the art. A suitable dispersant is, for example, citric acid, or polyacrylic acids, or other commercially available dispersants, such as Dolapix CE64, etc.

The dispersant may be added in an amount required to assist dispersion of the ceramic powder and/or nano-filler in the water (in the aqueous suspension). For example, the amount may be based on the surface area of the powders added to the water.

For example, the dispersant may be added in an amount of 0.4 to 8 mg per square meter of powder surface area.

Another possibility, additionally or alternatively, is that the pH of the solution is adjusted to help stabilize the aqueous suspension. For example, an acid or base may be added to adjust the pH. It may be added before the ceramic powder, before the nano-filler, or before both. It may be added after the ceramic powder, after the nano-filler, or after both.

During mixing, or after, the aqueous suspension or dispersion of the ceramic powder and/or the nano-filler in water may be subject to one or more homogenisation treatments.

For example, a suspension containing water and either the ceramic powder or the nano-filler may be subject to homogenisation. The aqueous suspension containing both the ceramic powder and the nano-filler may be subject to homogenisation. The homogenisation may include a treatment to better disperse or mix the components present during the homogenisation. For example, to better mix the ceramic powder or nano-filler into the water, or to better mix both the ceramic powder and the nano-filler in the water. The dispersion(s) may be homogenised by, for example, mechanical means such as high speed mixing (for example using an Ultra-Turrax machine).

For example, one or more homogenisations may be carried out using ball milling, for example using 3 mm zirconia or Y-TZP balls for a period of time such as 0.5 to 48 hours, for example 0.5 to 24 hours, for example 0.5 to 10 hours, for example 0.75 to 5 hours; or 10 to 36 hours, for example 18 to 30 hours, for example about 24 hours. Other homogenisation techniques includes attrition milling, for example.

Many other suitable methods of homogenisation, milling and so on are well known in the art.

Before, during, or after, mixing, the pH of the aqueous solution or dispersion may be adjusted, for example when citric acid used as dispersant. Suitably the pH may be adjusted to, for example, 7-9, or 5 to 8, or 5 to 7. This may be achieved by known methods, such as by adding an acidic or basic component such as ammonium hydroxide solution.

During, or after, mixing, there may also be further dilution of the aqueous suspension or dispersion of the ceramic powder or nano-filler in water. For example, this may be done to reduce the effective content or concentration of the ceramic powder, the nano- filler or both. It may assist with dispersion and processing. For example, the ceramic powder content may be reduce from 15 to 25 vol% solid loading or concentration to 5 to 20 vol% solid loading or concentration.

For example, dilution may reduce the solid loading or concentration of the ceramic powder or the nano-filler in the aqueous suspension or dispersion of the ceramic powder or nano-filler in water by 10 to 90%, for example 15 to 75%, for example 20 to 60%, for example 25 to 50%. The Drying Step and the Composite Powder

Certain methods of the present invention include a drying step, in which the aqueous solution containing the ceramic powder and the nano-filler is dryed. This drying may suitably be done by freeze drying. In other words, the aqueous suspension may be subjected to lyophilisation. This process is well known in the art. It may involve, for example, rapid freezing with liquid nitrogen followed by sublimation of water. Drying and in particular freeze drying is considered suitable because it is thought to guarantee or at least improve the nanodispersion of the nano-filler within the resultant composite powder, thereby giving rise to the advantageous properties of the present green body and ceramic articles. Freeze drying in particular is thought to help avoid the aggregation of the nano-filler and to help ensure the nanodispersed distribution of the nano-filler in the composite powder. This nanodispered state of the nano-filler is an important aspect for the high mechanical properties and high electrical conductivity described herein. Other drying processes are also known in the art and may be suitable in the present invention.

However, in comparison to those methods, freeze drying can help avoid potential solid phase separation of the ceramic powder (such as particles) and nano-filler (such as cellulose nanofibers), which can occur under some forming or consolidation steps such as centrifugation, slip casting, solvent evaporation and so on discussed in more detail below.

In those cases where the nano-filler is fibrous, it might form aggregates and loose its nanofibrous characteristics leading to possible deterioration of the mechanical and electroconductive properties.

The drying process creates a composite powder containing the ceramic powder and the nano-filler. The mixture of the ceramic powder and nano-filler in the composite powder may preferably be substantially homogeneous. The Forming Step and the Green Body

Certain methods of the present invention include a forming step, in which the composite powder is consolidated, formed, shaped or otherwise manipulated to create a green (green ceramic) body made of the composite powder.

It will be appreciated that many suitable techniques exist for this. For example, the composite powder may be pressed, in a die for example, or otherwise cast or shaped with a mold or die. It may even be that the aqueous suspension is dried in the desired form or shape to directly create the green body. The aqueous suspension or composite powder may be poured into a die, without further pressing or forming steps.

The method used may be 'dry' or 'wet. For example, 'dry' techniques include the above described drying, such as freeze drying, and for example subsequent dry pressing. 'Wet' techniques include wet, colloidal consolidation techniques such as, for example, slip casting, centrifuge casting, pressure filtration and/or gel casting of the aqueous suspension.

Indeed, where such wet techniques are used the above described drying stage may not be necessary, or may only be a partial drying. This leads to methods as described herein which do not include the described drying step. Those are also envisaged as part of the present invention.

In such embodiments, then, the present invention might be framed as providing a method of preparing a ceramic article, the method comprising mixing a ceramic powder and a nano-filler in water to form an aqueous suspension; forming a green body from said aqueous suspension; and sintering said green body to form said ceramic article, wherein said nano-filler is an insoluble cellulose material. Here, the forming may involve drying, shaping, casting and so on as described above. Equivalent methods for forming a green body (comprising mixing a ceramic powder and a nano-filler in water to form an aqueous suspension; and forming a green body from said aqueous

suspension) can be envisaged.

Dry-shaping routes are particularly suitable in the present invention. For example, the composite powder may be subjected to one or more of uniaxial dry pressing, cold isostatic pressing and so on. For example, uniaxial dry pressing may be conducted at 50 to 100 MPa. Cold isostatic pressing may be conducted at, for example, 150 to 500 MPa. By consolidating a suitable amount of the composite powders described herein, a green body can be prepared. The green bodies formed of, that is, composed of or consisting of, the present composite powder and therefore containing the ceramic powder and the nano-filler, have useful properties. In particular, the present green bodies may have sufficiently high strength to allow machining of the green body. Machining can typically be performed if the bending strength of the mill block (that is, here, the green body) is above 8 MPa, for example within a range of 15 to 50 MPa. If machining is performed outside this, the obtained structure is often not machined adequately and may show chipping.

The present green bodies have such a strength, that is, a flexural strength of above 8 MPa, for example 15 to 50 MPa.

The present green bodies may therefore be machined without the need for an additional process to increase their strength, for example an additional, energetically demanding and time consuming pre-sintering step ensuring slight necking of the packed particles.

Of course, it will be appreciated that the present methods may include a step or steps of machining the formed green body.

The green bodies may also suitably have a high green relative density. For example, the density of the mixed ceramic powder and nano-filler, the composite powder, may be 30% or more, for example 35% or more, for example 40% or more, for example 45% or more, for example 50% or more.

Treatment of the Green Body

It will be appreciated that before sintering further procedures may be conducted on the green body, in addition to or instead of the machining mentioned above. For example, there may be some pre-sintering dehydration treatment, and/or preliminary pyrolysis, to further enhance the product properties.

For example, a controlled pre-sintering pyrolysis at 200 to 600°C, for example 350 to 450°C, for example about 400°C, may be performed. Such a pyrolysis step may act to stabilise the fiber-like morphology of cellulose fibers used in the present invention, for example. It may help convert the present nano-filler into a series of carbon-rich fibers before the harsher conditions of sintering are applied. The fiber-like morphology may be better preserved in the sintered product if a pyrolysis is performed first.

The pyrolysis may be performed in the absence of oxygen. For example it may be performed in an inert atmosphere, such as under an inert gas such as nitrogen or argon, or under vacuum. This can assist during the pyrolysis to make sure that pyrolytic transformation of the nano-filler (cellulose material, such as nanofibers) into conducting carbonaceous materials that remain intimately associated with the ceramic grains in the resulting matrix is achieved. That is, inert conditions are preferred to improve conversion of the polysaccharide carbon source (the present nano-filler) into 'graphite-like carbon', that is, the conductive carbonaceous polymeric network dispersed or distributed in the ceramic article.

Alternatively, or additionally, or even during this there may also be performed a dehydration step at 100 to 300°C, for example 150 to 270°C, for example 200 to 250°C.

Such treatments may be particularly favourable where the ceramic powder is, for example, zirconia.

The pyrolysis product itself may be useful and an aspect of the invention. It may include the percolated carbonaceous network described herein.

Therefore in some methods of the present invention a pyrolysis step is carried out without a sintering step. Of course, in other methods both pyrolysis and sintering are performed. The Sintering Step

Certain methods of the present invention include a sintering step, in which the green body comprising the present composite powder is sintered. This forms a (sintered, or fired) ceramic article.

The sintering may be carried out by methods well known in the art.

Suitably, sintering is performed in the absence of oxygen. For example it may be performed in an inert atmosphere, such as under an inert gas such as nitrogen or argon, or under vacuum. If sintered in the presence of graphite under described conditions, the sintering atmosphere becomes slightly reductive.

This can assist during the sintering to make sure that pyrolytic transformation of the nano-filler (cellulose material, such as nanofibers) into conducting carbonaceous materials that remain intimately associated with the ceramic grains in the resulting matrix is achieved.

That is, inert conditions are preferred to improve conversion of the polysaccharide carbon source (the present nano-filler) into 'graphite-like carbon', that is, the conductive carbonaceous polymeric network dispersed or distributed in the ceramic article. Sintering may be performed, for example, in a vacuum (high vacuum) furnace, or by (step) spark plasma sintering (SPS, using an SPS furnce). SPS is also known in the art as field assisted sintering, FAST or pulsed electric current sintering, PECS.

The sintering regime can be suitably selected by the skilled worker.

Where SPS is used, the applied heating rate may be, for example, 20-300°C/min. The final sintering temperature may be, for example, 1250-1800°C. The SPS dwell time may be, for example, 3-30 minutes. The applied pressure may be, for example, 30- 200 MPa. Where sintering is performed in a vacuum furnace, it may be, for example, a high vacuum graphite furnace. The heating rate may be, for example, 2-60°C/min. The final sintering temperature may be, for example, 1250-1800°C. The dwell time may be, for example, 30-240 min.

The Ceramic Article

The ceramic articles formed by the present processes are useful in a variety of technical fields, as discussed herein. As a result of using a nano-filler as described herein, a more homogeneous dispersion or distribution of carbonaceous material is present in the sintered ceramic article. As explained herein, this can provide a number of advantages for example to electro- conductivity and mechanical properties in the sintered ceramic article. Such ceramic articles are therefore a subject of the present invention.

As recited above, the homogeneous distribution of ceramic raw material and nano-filler in the pre-sintered green body may suitably lead to a resulting homogeneous distribution of pyrolyzed ex-cellulose carbon nanofibers in the ceramic nanocomposite after sintering.

That is, in the present green bodies the cellulose material is present. On sintering or pyrolysis, if performed, it is thought that the cellulose material is converted to a distributed network, or matrix, of carbon chains and rings which may have some fibrous properties but also some graphite-like structure and behavior (in particular electrical conductivity). The percolated carbonaceous network or matrix formed may suitably have structural similarity to graphite, that is, include graphitic sheets and/or organized carbon rings. These sheets or rings may be linked by carbon chains, and may provide electron mobility to permit high electrical conductivity. These "ex-cellulose carbon nanofibers" can provide the desired electro-conductivity in the sintered ceramic article. In other words, it is thought that a polymeric network with electron conductivity is formed. The percolating conductive network is formed easily in the present invention. The network or matrix is distributed in the ceramic article, suitably with significant homogeneity. In other words, the network or matrix is homogeneous in the ceramic article. The present invention is directed to ceramic articles which include the above described post-sintering/pyrolysis network or matrix formed from the cellulose nano-filler.

Ceramic articles having this type of distributed carbonaceous network are

advantageous, as explained herein. For example, the good mechanical properties may be ensured by the fibrous morphology of the ex-cellulose carbon nanofiber reflecting the fibrous morphology of the original cellulose nanofibers.

The glucosidic rings of the cellulose material assist with forming a crystalline pre- organized structure, which allows for a higher degree of graphitic organization of carbon atoms.

This organized network of conductive carbonaceous fibers is thought to be further beneficial in certain embodiments.

For example, good bonding between ex-cellulose carbon nanofibers and the ceramic matrix interface is maintained, which originates in interfacial compatibility and strong adhesion. The nanoscale distribution and strong adhesion of ex-cellulose carbon nanofibers with the ceramic matrix may leads to a hindering of grain growth and, consequently, to microstructural grain refinement with direct implications on the improvement of the mechanical properties like strength, hardness and fracture toughness of the nanocomposite material. The present invention further promotes the use of sustainable nano-fillers derived from re-growing, natural cellulose nanofibers and can ease the toxicity issue related with carbon black or with the synthesis of nanocarbon materials such as carbon nanotubes by using environmentally benign cellulose fibers.

The cellulose material, after sintering/pyrolysis, has a marked effect on the

microstructure of the ceramic article. In particular, in some embodiments the present ceramic articles have a reduced grain size as compared with previously known ceramic articles.

The carbon phase (percolated network discussed above) is thought to be present between the ceramic grains. In the present invention the percolated conductive network is thought to be formed. In prior art methods, as discussed herein, this is not always possible without high loading and sacrifice of other properties.

For example, the present invention enables production of alumina based ceramic articles with a grain size of for example below about 500 nm, for example below about 450 nm, for example below about 400 nm, for example 350 nm, for example below about 300 nm, for example below about 250 nm, for example below about 200 nm, for example below about 150 nm. The alumina ceramics may also have a grain size above about 50 nm, for example above about 100 nm, for example above about 200 nm. The grain size may be, for example, 50 to 400 nm, for example 200 to 400 nm or for example 75 to 250 nm, for example 100 to 150 nm.

For example, the present invention enables production of zirconia based ceramic articles with a grain size of for example below about 300 nm, for example below about 250 nm, for example below about 200 nm, for example below about 150 nm. The zirconia ceramics may also have a grain size above about 25 nm, for example above about 50 nm, for example above about 75 nm, for example above about 100 nm.

Such reduced grain size can increase the mechanical strength and other properties of the ceramic articles.

The present ceramic articles may have this interesting grain sizes because of the present of the homogeneously percolated carbonaceous network at lowered amounts discussed herein. In the present invention a nano-filler is used which forms such a continuous matrix, while before sintering/pyrolysis it does not have such a structure. It may be, for example, fibrous. The presently formed network may demonstrate both fiber-like behaviour and graphite-like behaviour, manifested in enhanced mechanical properties like material toughness as well as enhanced electrical conductivity. Specific examples of these procedures, and other investigations performed by the present inventors, will now be explained.

Combinations

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The same is true for features described in the context of products and methods.

Any one or more of the aspects of the present invention may be combined with any one or more of the other aspects of the present invention. Similarly, any one or more of the features and optional features of any of the aspects may be applied to any one of the other aspects. Thus, the discussion herein of optional and preferred features may apply to some or all of the aspects. Furthermore, optional and preferred features associated with a method or use may also apply to a product and vice versa.

The options, features, preferences and so on mentioned herein apply both

independently and in any combination, except where such a combination is expressly prohibited or clearly impermissible. Examples

Example 1 : Procedure for preparation of CNF-zirconia (CNF/ZrC>2) and CNF-alumina (CNF/AI2O3) ceramic/composite powders: Zirconia and alumina suspensions (15-35 vol.% solid loading) were prepared by using commercially available tetragonal polycrystalline zirconia powder doped with 3 mol.% of yttrium oxide (Y-TZP) and oalurmina powder. The powders consisted of aggregated crystallites of 100-300 nm in average size and exhibited a specific surface area of 7-14 m ² /g. The powders were dispersed in water, where 0.4-8 mg of citric acid per square meter of powder surface area was dissolved and used as a dispersant. The final pH of 7-9 was adjusted by using ammonium hydroxide solution.

Homogenization of the suspension was performed by 24-hour ball milling in a plastic container using Y-TZP balls (φ = 3 mm). After homogenization the suspension was diluted with deionized water to 5-20 vol.% solid loading to which 0.1-5 wt.% of CNF was added. CNF is used in the form of a gel (1 wt%) and was prepared according to the procedure described in JP2008001728A and WO2009069641A1 . The suspension was homogenized by using Ultra-turrax for 2-15 minutes or attrition milling for 1-4 hours using 3-mm zirconia balls. The as-prepared suspensions were rapidly frozen using liquid nitrogen, followed by freeze-drying to yield CNF/ZrC>2 and CNF/AI2O3 composite powders (Figure 1 ). Example 2: Procedure for preparation of CNF/ZrC>2 or CNF/AI2O3 green bodies:

The CNF/ZrC>2 or CNF/AI2O3 composite powders prepared as described in Example 1 was poured into a steel die (20 mm diameter) and was uni-axially dry-pressed at 50- 100 MPa, followed by cold isostatic pressing (CIP) at 150-500 MPa. High green relative densities were measured exceeding 54 % and 60 % for CNF/ZrC>2 and

CNF/AI2O3, respectively.

Mechanical testing of the CNF/ZrC>2 and CNF/AI2O3 green bodies with a universal testing machine showed flexural strength exceeding 17 and 15 MPa, respectively.

The as-obtained green body pellet possessed sufficiently high strength to allow green machining. It was machined with a widia drill to obtain a cylindrical shape (Figure 2, illustrating the CNF/ZrC>2 green body). Machining can typically be performed if the bending strength of the mill block is within a range of 8 to 50 MPa. If machining is performed outside these ranges, the obtained structure is often not machined adequately and may show chipping.

For comparison, the commercially available ZrC>2 powder analogous to the one prepared in Example 1 , but granulated using polymeric binders (2-3 wt.%), was pressed and fractured. It exhibited an average strength of 7.99±0.70 MPa. Therefore, such green bodies would require an additional, energetically demanding and time consuming pre-sintering step ensuring slight necking of the packed particles in order to be machined.

Example 3: Procedure for preparation of CNF/ZrC>2 and CNF/AI2O3 electro-conductive ceramics with SPS:

The CNF/ZrC>2 or CNF/AI2O3 composite powder prepared as described in Example 1 was poured into a graphite die (20 mm diameter), which was inserted in a Spark Plasma Sintering (SPS) furnace (also known as field assisted sintering, FAST or pulsed electric current sintering, PECS).

The following sintering regime was used: the applied heating rate was 20-300°C/min, the final sintering temperature was 1250-1800°C, the SPS dwell time was 3-30 min, the applied pressure was 30-200 MPa.

Full densification (>97% of theoretical density) was attained for both composite materials.

CNF has a clear and significant refinement effect on the microstructural evolution after SPS sintering, as shown by SEM micrographs of fracture surfaces (Figure 3).

The grain size of reference pure ZrC>2 was relatively fine, i.e., less than 200 nm, while for reference AI2O3 the microstructure was much coarser, i.e., exceeding 3 μηη, due to the fact that no MgO (as dopant) was used.

The uncontrolled, exaggerated grain growth observed especially in the case of alumina is generally undesired as it compromises the mechanical properties such as hardness and strength.

However, with 2wt% of added CNF the grain growth was greatly suppressed and microstructure refined to -150 nm and to a remarkable -220 nm for CNF/ZrC>2 or CNF/AI2O3, respectively. This is an important and advantageous finding in view of controlling the mechanical properties of the ex-cellulose carbon nanofiber reinforced ceramic.

The conditions during SPS sintering appear to drive the conversion of cellulose into graphite-like carbon, which is more conductive than amorphous carbon. The Raman spectra of ex-cellulose nanofiber-like carbon embedded in the ceramics are similar to the one of graphitic carbon nanofibers. Cellulose is generally considered as non- graphitizable carbon source, since its conventional pyrolysis does not yield graphitic or ordered carbon but amorphous carbon phases.

For example, the electric conductivity of SPS-sintered CNF/ZrC>2 is 480 S/m, which is in the range of carbon nanotubes (CNT) or graphene containing zirconia, having 800 S/m at 1 wt% CNT or 800 S/m at 5 vol% graphene in the best cases. On the other hand, most examples in literature on CNT- or graphene-reinforced ceramics report conductivity values in the range below 100 S/m, and only when the amount of carbon nanomaterial was exaggerated (i.e. >15 vol%) the conductivity was beyond 1000 S/m. However, at this high carbon content the mechanical properties of the ceramic are generally deteriorated.

The good mechanical properties are ensured by the fibrous morphology of the ex- cellulose carbon nanofiber reflecting the fibrous morphology of the original cellulose nanofibers. Strong, fibrous additives like inorganic whiskers, carbon fibers and nanotubes are effective reinforcement agents. The mechanical properties observed here (flexural strength, hardness, toughness) of CNF/ceramics are superior to the ones, where, for an example, sugar (sucrose) was used as additive.

Specifically, 2 wt% CNF/ZrC>2 has a flexural strength and hardness exceeding 900 MPa and 1 1.89 GPa, respectively, as opposed to 520 MPa and 10.47 for 2 wt% sugar/Zr0 ₂ . This result clearly demonstrates the effect of adding a fibrous polysaccharide as compared to adding a non-fibrous additive like a disaccharide as the precursor of pyrolytic carbon.

The reason is related to a crystalline pre-organization of the glucosidic rings in the cellulose nanofibers, which allows for a higher degree of organization of carbon atoms as compared to monomeric glucose or polymer chains. This hypothesis of beneficial pre-organization for carbon chains leading to graphite-like behaviour is also manifested by a 8-fold increased electrical conductivity of 2 wt% CNF/ZrC>2 as compared to 2 wt% sugar/ZrC>2, supported by the fact that ordered graphite is more conductive than disordered carbon.

Example 4: Procedure for preparation of ex-cellulose carbon nanofiber-reinforced CNF/ZrC>2 and CNF/AI2O3 electro-conductive ceramics with pressureless, vacuum furnace sintering:

Comparable results to those obtained in Example 3 were obtained when CNF-ZrC>2 and/or CNF/AI2O3 green bodies were prepared according to the description from Example 2. The as-obtained green body pellets were then sintered in a high vacuum graphite furnace. The heating rates employed were 2-60°C/min, the final sintering temperature was set to 1250-1800°C, while dwell time was 30-240 min.

Example 5: Using microcrystalline cellulose instead of CNF:

Alternatively, it is possible to substitute CNF with microcrystalline cellulose (MCC) by adopting the methods described in the above Examples.

Example 6: Further demonstration of the present invention:

CNF-containing alumina (AI2O3) and zirconia (ZrC>2) powders are prepared by dispersing 1 - 4 wt% of CNF in a stabilized suspension (pH 9) of the respective ceramic particles (30 wt% solid loading), followed by attrition milling. The aqueous

CNF/ceramic powder slurries are homogeneous and colloidally stable without particle segregation or de-mixing phenomena (Figure 4) attributed to the hydrophilicity of CNF. Freeze-dried 3 wt% CNF/AI2O3 (3CA) and 2 wt% CNF/Zr0 ₂ (2CZ) composite powders maintain a uniform distribution of non-aggregated, individual cellulose nanofibers (Figures 5a, 5b).

Cold isostatic pressing (CIP) at 800 MPa of these powders renders highly compacted green bodies (56 % theoretical density (TD)) displaying very high flexural strength of 15 and 17 MPa for 3CA and 2CZ, respectively. These values are orders of magnitude higher than for consolidated green bodies containing a polymeric binder like polyvinyl alcohol. This demonstrates the mechanical reinforcement by CNF. The CNF composite green bodies allow for green machining, a convenient pre- sintering shaping technique, since they exceed the required minimum strength of 8 MPa. As explained herein, green machining cannot generally be applied to green bodies due to inadequate mechanical properties. Spark plasma sintering (SPS) at 1250°C of the green bodies yields dense

nanocomposite ceramics (98 %TD) with high mechanical and electrical properties. The electric conductivity of 2CZ is as high as 480 S nr ¹ , which is five-fold higher than most reported zirconia nanocomposites containing carbon nanofillers such as carbon nanotubes or graphene as the conducting phase. The electric conductivity of 3CA is 360 S rrr ¹ .

These values suffice the threshold for electro-discharge machining. Figure 6a shows the erosion pits on the surface of 3CZ produced by die sink EDM. The volumetric material removal rate (VMRR) is 0.05 mm ³ min ^"1 and the die sink rate is 3.8 μηη min ^"1 .

Even though both values are lower than for zirconia reinforced with 30-50% of titanium carbides and nitrides (Put et al., British Ceramic Transactions 2001 Vol. 100 No. 5), it should be noted that 3CZ contains as little as 0.6 wt% of carbon. The Vickers hardness H _v of 2CZ and 2CA is 12 and 19 GPa, respectively, which is on par with 3% yttria doped zirconia (3YZ) and 3 GPa higher than for pure alumina.

The fracture surfaces of the CNF nanocomposites show nanoscopic, fibrous particles between the ceramic grains (Figures 6b, 6c), which can be identified as pyrolyzed cellulose nanofibers as the sintered bodies turn black after SPS (Figure 6d).

EDX (energy dispersive X-ray spectroscopy) mapping shows uniformly distributed carbon in the samples (Figures 6e, 6f), which is a consequence of the homogeneous dispersion of CNF in the starting ceramic slurry and within the pre-sintered green body matrix. Chemical analysis of sintered 2CA and 2CZ reveals a similar carbon content of 0.45 and 0.55 wt% in both nanocomposites, which is in the range of values (1 .3 and 1 .1 wt%, respectively) obtained from integration of wide scan XPS spectra. The sintered CNF/AI2O3 nanocomposite does not show grain size growth (cf. Figure 6c), a typical and undesired phenomenon during sintering of ceramic nanopowders.

The initial average crystallite size of the alumina powder is 100-150 nm, which persists in the sintered 2CA but increases by more than one order of magnitude in sintered pure alumina. It is important to control grain size growth during sintering as increased grains deteriorate mechanical properties like toughness and hardness.

The presence of homogeneously distributed ex-cellulose carbon fibers at the grain boundaries of the ceramic particles (here alumina) is thought to hamper uncontrolled grain growth and results in microstructural refinement, which likely explains the significant increase in hardness of 2CA.

A controlled pyrolysis step at 400 °C including prolonged dehydration at 230 °C of the green body prior to SPS enhanced significantly the toughness of 2CZ from 3.90 Pa-m ^1/2 to 5.06 Pa-m ^1/2 .

It is understood that the toughness of composite materials increases with the presence of fibers. Therefore, it can be assumed that the pyrolyzed sample contains better quality fibers than the directly sintered homologues.

Careful pyrolysis including controlled dehydration of cellulose fibers has been shown to preserve the original fiber morphology during conversion into graphite-like, carbon-rich fibers. In the present case, the pyrolysis step improved the cellulose fiber conversion prior to SPS.

The microstructure of the sintered ceramic nanocomposites, exemplarily for 2CZ, was further characterized by a combination of transmission electron microscopy (TEM) and analytical spectroscopic methods. Low-magnification TEM images (Figure 7a) show that most ZrC>2 grains are surrounded by an amorphous layer (contrast in bright field (BF) and dark field (DF) mode). High-resolution (HR) TEM imaging (Figure 7b) clearly demonstrates the amorphous structure of the 1.5-2 nm thick layer while quantitative electron energy loss spectroscopy (EELS) mapping across this grain boundary (Figure 7c) shows that the layer mainly consists of carbon. Quantification of the composition suggests that 87% is carbon and 13 % is zirconium, while oxygen is not found in this boundary layer. However, there is also evidence for ordered carbon phases at triple points and between grains showing the typical layered structure of graphitic carbon (Figures 8d, 8e). This phase appears to be similar to turbostratic carbon of small graphitic sheets with the absence of 3-dimensional organization.

Raman spectra of 2CZ and 2CA show strong G bands indicative of sp ² hybridized carbon together with the 2D band at 2700 cm ^"1 that is a strong indication for graphitic carbon structures (Figure 7f). The intensity of the 2D band is in both materials lower than the intensity of the G band, which is attributed to the presence of interlayer ττ- interactions.

This confirms the HRTEM observation of few-layer graphite.

The chemical composition of the carbon phase was further analyzed by X-ray photoelectron spectroscopy (XPS) collecting core level C 1 s spectra from 2CZ and 2CA, respectively (Figure 7g). Both spectra are similar showing a symmetric band at 285 eV. This band can be attributed to aliphatic carbon groups C-C/C-H, while C-0 groups with a band at 289 eV is absent. This result is in agreement with EELS data that also indicates the absence of oxygen in the carbon phase (Figure 8). Therefore, it can be assumed that cellulose is largely reduced to pure carbon. Also other work has shown that carbohydrates can be reduced under inert sinter conditions. Despite their high electric conductivity 2CZ and 3CA show thermal conductivity values (2.5 and 18.4 W/mK, respectively) only slightly higher than for pure 3YZ and alumina. Usually, thermal conduction in solids scales with electric conductivity, which would suggest high thermal conduction in the CNF nanocomposites. However, the nanometric dimensions of the carbon network in the nanocomposite ceramics are thought to give rise to high thermal interface resistance similar to carbon nanotube composites. High electrical conductivity at simultaneously low thermal conductivity can be an interesting behavior for certain technological applications and is therefore a further advantage of the present invention. In the described Examples the following chemicals were used:

Citric acid (Sigma Aldrich), ammonium hydroxide solution (Alfa Aesar), zirconia (TZ-3Y, Tosoh) and alumina (TM-DAR, Taimei) powders. Cellulose nanofibers (CNF) were prepared by chemo-mechanical treatment of eucalyptus pulp.