HIGH PERFORMANCE CERAMICS FROM COLD SINTERED NANOSCALE POWDERS

Abstract

The invention relates to a process for making a ceramic body that comprises providing particles of a metal salt precursor material wetted by a liquid medium. The particles are characterized by a grain size of below 600 nm, and the precursor material has a solubility in the liquid medium of at least 10.sup.5 mol/L. A pressure of 100 MPa is applied at a temperature of below 100 C., rendering a material of high theoretical density values previously unattainable at low temperatures. The invention further relates to a calcium carbonate ceramic material of the vaterite isomorph having a density of the material 1.76 g/cm3 and a Modulus of rupture 30 MPa, and to a calcium phosphate ceramic material consisting of the monetite isomorph with 2.5 g/cm3 density and a Modulus of rupture 18 MPa.

Claims

1. A process for making a ceramic body, comprising the steps of a. providing a precursor composition consisting of particles of a precursor material wetted by a liquid medium, wherein i. said precursor material is a metal salt; ii. said particles are characterized by a grain size of below 600 nm, even more particularly below 100 nm, or even at 50 nm or less, and iii. said precursor material has a solubility in said liquid medium of at least 10.sup.5 mol/L; b. applying i. a pressure of 100 MPa, particularly 150 MPa, 200 MPa, 300 MPa, 400 MPa, or even more particularly 500 MPa, ii. at a temperature of 100 C., particularly at a temperature below 80 C., even more particularly below 60 C. or even at room temperature (approx. 25 C.) to said precursor composition, resulting in a product ceramic body.

2. The process of claim 1, wherein said particles are characterized by a grain size of below 100 nm.

3. The process of claim 1, wherein said particles are characterized by a grain size of 50 nm or less.

4. The process of claim 1, wherein the pressure is applied at room temperature.

5. The process of claim 1, wherein said pressure is applied for longer than 300 s, particularly longer than 10 min or even 30 min or more.

6. The process of claim 1, wherein said product is characterized by a density of greater or equal to 64%, particularly 67%, even more particularly 70%, 73%, 78% or 80% of a theoretical maximal density determined for said precursor material.

7. The process of claim 1, wherein said precursor material is a salt of a group 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15 metal and a mineral acid, particularly a carbonate, phosphate, silicate, hydroxide, sulfate, oxide, chloride, fluoride, more particularly a carbonate or a phosphate of a group 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 or 15 metal.

8. The process of claim 1, wherein said precursor material is selected from a salt of any one of magnesium, calcium, strontium, barium, titanium, zirconium or aluminium.

9. The process of claim 1, wherein said precursor material is selected from calcium carbonate, magnesium carbonate, calcium phosphate, magnesium phosphate, calcium sulfate, barium titanate, zirconium oxide, yttrium oxide and zinc oxide.

10. The process of claim 1, wherein said precursor material is selected from a. the vaterite isomorph of calcium carbonate, b. the monetite isomorph of calcium phosphate, c. the hydroxyapatite isomorph of calcium phosphate, and d. the boehmite isomorph of aluminium oxide hydroxide.

11. The process of claim 1, wherein said precursor material is calcium carbonate having a purity 90%, particularly 95%, more particularly 98%, 99% or 99.9%.

12. The process of claim 1, wherein said liquid medium is selected from a. water, b. an aqueous solution of a compound selected from the group consisting of methanol, ethanol, propanol, ethylene glycol, a mineral acid, an organic acid, an inorganic or organic base, and a chelant, particularly wherein the chelant is selected from EDTA, HEDTA, EDDHA, HBED and catecholate, catechol- and pyrogallol-based ligands.

13. The process of claim 1, wherein said precursor material is obtained by a. dissolving a first metal salt in a first solvent, wherein said first metal salt is constituted of a first anion and a first metal cation, yielding a first solution; b. subsequently, mixing said first solution with i. carbon dioxide, or ii. a second solution of a second metal salt in a second solvent, wherein said second metal salt is constituted of a second anion and a second metal cation, and a salt of said second anion and said first metal cation is not completely soluble in said first or second solvent or a mixture of said first and second solvent.

14. The process of claim 1, wherein said precursor material is calcium carbonate and said particles of precursor material are obtained by mixing aqueous solutions of sodium carbonate and calcium chloride.

15. The process of claim 1, wherein said precursor material is calcium carbonate and said particles of precursor material are obtained by streaming gaseous carbon dioxide through an aqueous calcium chloride solution.

16. The process of claim 1, wherein the pressure is applied uniaxially.

17. An industrially produced ceramic material, particularly obtained by a process according to claim 1, characterized by the following parameters: a. the material essentially consists of calcium carbonate of the vaterite isomorph; b. the density of the material exceeds 1.76 g/cm.sup.3 and c. the Modulus of rupture exceeds 30 MPa.

18. An industrially produced ceramic material, particularly obtained by a process according to claim 1, characterized by the following parameters: a. the material essentially consists of calcium phosphate of the monetite isomorph; b. the density of the material exceeds 2.5 g/cm.sup.3 and c. the Modulus of rupture exceeds 18 MPa.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0104] FIG. 1 shows the pressure solution creep mechanisms leading to cold sintering and vaterite nanoparticle synthesis and morphology. a, Schematics of the ionic transport mechanisms around the contact point between particles subjected to an external mechanical load, P. b, Formation of nanovaterite particles from the reaction of sodium carbonate with calcium chloride in an aqueous solution of ethylene glycol. Alternative sources of calcium and carbonate ions are also displayed. c, d, Scanning electron micrographs showing the hierarchical morphology of precipitated nanovaterite particles at different magnifications.

[0105] FIG. 2 shows results of the uniaxial compaction of nanovaterite powder in different continuous phases. a, Schematics of the compaction setup and the stress ramp applied during the experiments. is the externally applied stress, L is the measured net dimensional change and t is the elapsed time. b, Dimensional change as a function of time for powder compacts in water, paraffin oil or in the dry state subjected to a maximum applied stress of 280 MPa. c, d, Densification behaviour and creep response of powder compacts in the presence of different continuous phases under an applied stress of 280 MPa.

[0106] FIG. 3 shows the densification behaviour of nanovaterite compacts and comparison with model geological calcite. a-c, Scanning electron micrographs of vaterite compacts subjected to different external stresses. d, Grain size distribution of compacts obtained at 10, 100 and 500 MPa. e,f, Densification behaviour and creep response of nanovaterite samples measured at different applied stresses. g, Logarithmic dependence of the relative density of vaterite compacts on the applied stress. h, Comparison between the creep response of nanovaterite and model geological calcite. i, Timescales required to increase the relative density of powder compacts by 0.4% as a function of the grain size and applied stress at room temperature.

[0107] FIG. 4 shows the mechanical properties of nanovaterite compacts and comparison to other classes of materials. a, Strength and elastic modulus of nanovaterite specimens as a function of the compact relative density. b, Ashby diagram displaying the specific modulus and specific compressive strength of the nanovaterite compacts in comparison to other man-made and natural materials. Data for the nanovaterite materials are indicated by red, orange and pink circles, which correspond to specimens with 87, 78, and 72% relative density, respectively.

[0108] FIG. 5 shows X-ray diffraction pattern of the powder synthesized according to Example 1. The powder is a mixture of vaterite and calcite. An average vaterite crystallite size of 37 nm was obtained by fitting the Scherrer equation to the three most intense Bragg peaks below 40: (110), (112) and (114). The XRD patterns were recorded on an X'Pert Pro powder diffractometer (PANalytical B. V., Netherlands) operated in reflection mode with Cu K radiation (45 kV, 40 mA).

[0109] FIG. 6 shows thermogravimetric and differential thermal analysis of the nanovaterite powder synthesised according to Example 1. The synthesised powder was slip cast to remove the excess of ethanol and dried at room temperature and pressure overnight. A 4 wt % weight loss above 100 C. can be observed and is probably due to residual solvent desorption. The second weight loss of 42 wt % is associated with the transformation from the carbonate CaCO.sub.3 to the oxide CaO, releasing CO.sub.2. The endothermic peak measured at this temperature supports the occurrence of this reaction. The exothermic peak around 400 C. is tentatively associated with the transformation of vaterite into calcite during heating1. The test was performed at a heating rate of 5 C./min from 35 C. to 900 C. under a 80:20 N.sub.2:O.sub.2 mixture with a TGA-DSC device (STA 449 C, Netzch).

[0110] FIG. 7 shows typical stress-strain curves obtained for cold sintered samples as provided in Example 1 under compression (a) and three-point bending (b). The average relative densities of the samples are written alongside the curves.

[0111] FIG. 8 shows the XRD spectrum of the material produced according to Example 2.

EXAMPLES

Example 1: Synthesis of Nanovaterite Particles

[0112] In this Example, sodium carbonate and calcium chloride are used as sources of CO.sub.3.sup.2 and Ca.sup.2+ ions, respectively. Vaterite nanoparticles are easily formed through simple mixing of these reactants in an aqueous solution of ethylene glycol (FIG. 1b). The resulting carbonate powder exhibits a unique hierarchical structure comprising 37 nm particles that are densely arranged into 0.6 m spherical agglomerates (FIG. 1c, d, FIG. S1). After synthesis, the nanovaterite powder is washed with excess ethanol to remove the ethylene glycol before proceeding with compaction measurements.

[0113] Compaction tests were performed by applying an uniaxial mechanical load onto a vaterite-liquid mixture added to a cylindrical mold at an initial solid-liquid weight ratio of 0.2 (FIG. 2a). The mechanical load applied to the mixture was first increased at a rate of 0.5 mm/min until the target maximum stress was reached. The dimensional change (L) and respective uniaxial deformation () along the loading axis were measured as a function of time while keeping the specimen under the constant target stress, .

[0114] Considering that an aqueous continuous phase is necessary to enable the dissolution of ions during pressure solution creep (FIG. 1a), the inventors first conducted experiments in the presence or absence of water to elucidate the role of cold sintering in the compaction process. For that purpose, vaterite powders mixed with water, paraffin oil or in the dry state were compacted uniaxially at the same target stress of 280 MPa. To quantify the level of densification achieved in the compaction process, the obtained raw deformation data (FIG. 2b) were converted into relative density values, leading to the densification curves shown in FIG. 2c. Compacts containing water reach a striking relative density of 84% at room temperature in a timeframe of only 30 minutes. By contrast, the presence of air or paraffin oil as continuous phase increases the relative density to only 68 and 64%, respectively. As ion dissolution should not occur in the oil or in air, this confirms that pressure solution creep is a key mechanism in the densification process of compacts in water. The deformation behavior of the investigated samples was also evaluated in terms of the strain rate as a function of normalized porosity (FIG. 2d). Although the finite increase in relative density in the absence of water (FIG. 2c) indicates that pure mechanical compaction also occurs in all the investigated specimens, the much higher strain rates achieved in water provides further evidence of the major role of pressure solution creep in enabling strong densification during compaction.

[0115] The evolution of the nano- and microstructure of the powder at different compaction stages was examined using scanning electron micrographs obtained from samples subjected to distinct applied stresses (FIG. 3a-c). At the low applied stress of 10 MPa, the spherical morphology of the nanovaterite agglomerates is still recognizable and the large inter-agglomerate interstices constitute a major fraction of the overall porosity. A higher applied stress of 100 MPa leads to significant densification and deformation of the agglomerates. This effect is further enhanced at 500 MPa, where agglomerates are no longer visible and only residual porosity remains. Interestingly, the densification processes achieved by increasing the applied pressure is not accompanied by the grain coarsening typically observed during sintering of ceramics at high temperatures (FIG. 3d). This is an important advantageous feature of the cold sintering process, since large grain sizes have a deleterious effect on the mechanical strength of brittle materials. Importantly, preliminary aging experiments in water revealed no phase transformation from vaterite to calcite within a timeperiod of two weeks after compaction, suggesting that the constrained environment within the dense compact prevents conversion of the carbonate into its thermodynamically stable phase.

[0116] The dependence of the relative density on the applied stress at room temperature was further evaluated by performing compaction tests over a wide range of pressures (FIG. 3e). A remarkable relative density of 87% is achieved at room temperature at a pressure of 500 MPa. Such a pressure level can be easily applied to compact parts at the tens of centimetre scale using a standard industrial hydraulic press. At the highest applied pressure of 800 MPa, the compact additional deformation is mostly elastic without significant further densification. The relative density of the powder compact was found to exhibit a logarithmic dependence on the applied stress (FIG. 3g), which qualitatively agrees with the behaviour expected for geological and synthetic materials. When the deformation response during the constant loading step is displayed in the form of creep curves (FIG. 3f), we observe that the strain rate of the compacts decreases from an initial value around 10.sup.3 s.sup.1 to a level in the order of 10.sup.5 s.sup.1 as densification proceeds. At any given relative porosity, higher strain rates are observed in compacts subjected to higher stresses, which explains the stronger densification of samples at elevated pressures.

[0117] A comparison of the creep data determined here with values reported in the literature at a low applied stress of 10 MPa reveals that the nanovaterite compact (grain size of 0.6 m) deforms at strain rates many orders of magnitude higher than those obtained for a coarser natural calcite sample (grain size of 94 m, FIG. 3h) (Croizet al., J. Geophys. Res. 115, B11204 (2010)). Since other important parameters like diffusion coefficient or solubility are the same or only change slightly among CaCO.sub.3 polymorphs, this difference must arise from the more than 100 times smaller sizes of the vaterite particles. Strain rates comparable to those measured for the nanovaterite at 10 MPa can only be achieved with geological model calcite by increasing the compaction temperature and pressure to 150 C. and 30 MPa, respectively, and by reducing the grain size to 12 m (Zhang et al., J. Geophys. Res. 115, B09217 (2010)).

[0118] The densification of nanovaterite compacts occurs within remarkably shorter timescales if compared to model geological specimens with larger grain sizes. Experimental timescales obtained for the vaterite specimens produced according to the invention and literature values for calcite samples are shown in FIG. 3i for a wide range of grain sizes and applied stresses. The reported values correspond to the time needed to densify the compact by an arbitrary value of 0.4% at a constant applied stress and are thus indicative measures for the very early stage of the cold sintering process. Strikingly, reducing the grain size and increasing the applied stresses decreases the densification timescales from about 10 days to only a few seconds.

[0119] The compaction stresses and particle sizes covered herein exploit a completely new parameter space compared to earlier investigations on geological model systems (FIG. 3i). This opens the opportunity to produce CO.sub.2-based powder compacts of relative densities comparable to those of state-of-the-art building materials within economically viable timescales and utilizing raw materials that can be used as CO.sub.2 sinks. A major additional benefit of the proposed method is that it proceeds at room temperature, which contrasts to the energy-intensive heat treatments beyond 1000 C. typically employed in the cement and steel-making industries.

[0120] The high relative densities achieved through high-pressure cold sintering of nanopowders translate into surprisingly high mechanical properties for compacts fabricated at room temperature without the addition of binders (FIG. 4a). At a relative density of 87%, the cold sintered compacts exhibit an elastic modulus of 30 GPa combined with flexural and compressive strength of 50 and 225 MPa, respectively. If compared with other materials classes (FIG. 4b), the dense nanovaterite compacts outperform state-of-the-art construction materials like stone and concrete, reaching higher specific strength at comparable or even higher elastic moduli.

[0121] In conclusion, cold sintering of nanoscale carbonates at high pressures enables the fabrication of strong and dense structural materials at room temperature within timescales comparable to those of typical manufacturing processes. This simple up-scalable process can potentially revert today's negative environmental impact of the fast-growing construction sector, by providing a structural material that is sufficiently stiff and strong to replace current CO.sub.2-emitting building resources and by utilizing carbonaceous raw materials that can work as CO.sub.2 sinks at large industrial scales.

Synthesis of Nanovaterite Particles

[0122] The inventors synthesized vaterite nanoparticles following a protocol originally proposed by Parakhonskiy et al. (Angew. Chem. Int. Ed. Engl. 51, 1195-7 (2012)). Briefly, two solutions comprising 20 vol % of distilled water and 80 vol % of ethylene glycol (>99%, Sigma Aldrich) were first prepared. CaCl.sub.2.2H.sub.2O (EMSURE ACS, Reag. Ph Eur, Merck Millipore) was added to one solution, whereas NaCO.sub.3.10H.sub.2O (puriss. 99.5%, Sigma-Aldrich) was dissolved in the other. This resulted in water/ethylene glycol solutions containing 0.33 mol/L of either CaCl.sub.2 or NaCO.sub.3. A typical calcium chloride solution contained 8.00 g of water, 36.06 g of ethylene glycol, and 4.66 g of CaCl.sub.2.2H.sub.2O. For the sodium carbonate solution, 8.00 g of water, 36.06 g of ethylene glycol, and 9.06 g of NaCO.sub.3.10H.sub.2O were used. The two solutions were mixed quickly and kept under agitation at 600 rpm using a magnetic stirrer for 2 h. The precipitated powder was washed two times with ethanol (94% denaturated with Toluene, Alcosuisse) through sequential centrifugation steps and eventually stored in ethanol if not used. All chemicals were used as received without further purification.

[0123] Other methods can potentially be adapted to use carbon dioxide to produce carbonate powder. The differences compared to the route described in the preceding paragraph relate to the powder synthesis.

[0124] According to one alternative route, a solution of distilled water (20 vol %) and ethylene glycol (80 vol %; purity >99%, Sigma Aldrich) is first prepared. CaCl.sub.2.2H.sub.2O (EMSURE ACS, Reag. Ph Eur, Merck Millipore) is added until a concentration of 0.33 mol/L (higher concentrations are possible and raising the concentration usually improves results). CO.sub.2 is bubbled through the suspension while mixing at around 600 rpm until completion of the reaction.

[0125] Alternatively, two solutions comprising 20 vol % of distilled water and 80 vol % of ethylene glycol (>99%, Sigma Aldrich) are first prepared, and CaCl.sub.2.2H.sub.2O is added to one of them. Carbonation is then accelerated by amine-containing compounds, such as aqueous ethanediamine (EDA) and monoethanolamine (MEA), with characteristic absorption abilities toward acidic CO.sub.2 gas. In one suspension, EDA and PEG (with 300 molar mass) are dissolved in equimolar quantity as the desired CO.sub.2 and calcium source.sup.3. In one embodiment, both reagents are present at 0.33 mol/L. The two solutions are mixed quickly and keep under agitation at 600 rpm using a magnetic stirrer for 2 h.

[0126] For both alternatives of the powder synthesis method, the precipitated powder may optionally be subsequently washed two times with ethanol through sequential centrifugation steps and stored in ethanol, or used directly for compaction.

[0127] In a typical example of the present invention, the powder stored in ethanol was slip casted on a gypsum mould to remove the excess of solvent and dried at 100 C. 2 h (the drying step is optional). The desired amount of powder was weighted (typically 0.5 g) and loaded in a 13 mm diameter pressing tool. A 0.9 wt %. NaCl in distilled water solution was added at a weight ratio L/P=0.2, then the pressing tool was closed and loaded in an uniaxial press (200 kN capacity, P/O/Weber, Germany) at various load for at least 2 h. The samples were then removed and dried 2 h at room temperature. A typical sample size obtained were 13 mm diameter by 2.2 mm thick, but larger samples has also been obtained. The best samples were obtained for a pressure of 66 kN on a pressing tool of 13 mm diameter (corresponding to 500 MPa), but pressure and diameter were varied (pressure from 100 MPa to 500 MPa, diameter of the sample from 11 mm to 30 mm) successfully.

Structural Characterization

[0128] Cross-sections of the sample were obtained using a Broad Ion Beam milling instrument (IM4000, Hitachi, Japan) available at ETH Zurich (ScopeM). Milling was performed using an argon gun accelerated under 6 kV while the sample was wobbled at a middle speed (C3) to avoid heating. Electron microscopy images were acquired after deposition of a 5 nm layer of platinum on the nanovaterite powder and on the polished compact surface (LEO1530, Zeiss, Germany).

Creep Tests

[0129] The suspension of nanovaterite particles in ethanol was first slip cast in a gypsum mold to remove the excess of ethanol. The powder obtained was dried at 100 C. for 2 h. To perform the compaction experiments, 0.3 g of nanovaterite powder was added into the cavity of a pressing tool of 11 mm diameter (P/O/Weber, Germany). The desired amount of liquid was added directly on top of the powder. A liquid-to-powder (L/P) weight ratio of 0.2 was used. This ratio was typically obtained by mixing 0.060 g of liquid with 0.300 g of powder. The liquid used for most creep tests consisted of a 0.9 wt % aqueous solution of NaCl (EMSURE, Merck). This NaCl concentration is known to increase the CaCO.sub.3 solubility. Paraffin oil (Sigma-Aldrich) was used in selected experiments to test the effect of the type of liquid on the compaction behavior. To investigate the creep response of the nanovaterite powder the pressing tool was closed and placed in a universal testing machine (Instron 8562, Instron) equipped with a 100 kN load cell. A preload of 200 N (corresponding to 2.1 MPa) was applied on the specimen to ensure a common starting point for the compaction tests. The compaction pressure was then applied at a rate of 0.5 mm/min until the maximum load was reached. This was followed by the application of a constant load that was maintained for time periods ranging from 30 minutes to 1 hour. For each load, a background displacement curve was obtained with an empty pressing tool. This displacement was then subtracted from the actual values measured with the powder to remove the contribution of the tools to the total deformation. To calculate the density of the compacted specimens, the powder masses were measured 24 h after pressing to take into account any loss that may have occurred during the compaction process. The thickness and diameter of the samples were deduced from the displacement of the machine and the diameter of the pressing tool, respectively. Archimedes measurements confirmed that the geometric density measured during the tests were correct within +/5%.

Grain Size Analysis

[0130] The grain size of the cold sintered samples was measured with the freely available software Fiji. Several SEM images were stitched together with a dedicated plug-in.sup.3 to obtain a larger number of grains while keeping a resolution high enough to distinguished the pores and grain boundaries. A simple threshold was used to separate the pores from particles, followed by a 2 pixels median filtering procedure to remove noise in the image. The plug-in Watershed was then used to join the pores together and thus retrace the grain boundaries. The resulting grain boundaries and the original image were overlaid to check the accuracy of the method. The plug-in Analyse Particles was finally used to obtain the grain size distribution.

Strain Rate Modelling

[0131] The calculation of the theoretical strain rate of the vaterite samples during compaction was performed using the established pressure solution creep model outlined by Zhang et al. (J. Geophys. Res. 115, B09217 (2010)).

[0132] Mechanical testing

[0133] Samples for mechanical testing were prepared following the same overall protocol used for the creep tests but upscaled for a larger amount of powder (typically 0.5 g). In this case, compaction was carried out using a 13 mm diameter pressing tool in an uniaxial press (200 kN capacity, P/O/Weber, Germany) at various loads for at least 2 h. After pressing, samples were removed and dried for 2 h at room temperature. A typical sample showed a diameter of 13 mm and was 2.2 mm thick, but larger specimens were also obtained. Such disks were cut with a 300 m wire saw to generate beams of approximately 112.21.8 mm.sup.3 (LengthDepthWidth) and cuboids of approximately 1.71.72 mm.sup.3 for the three point bending and compression tests, respectively. Samples for bending tests were bevelled at the edges and used directly after cutting. All the tests were performed with a Instron 8562 universal testing machine equipped with a 1 kN load cell. A three-point bending setup with a span of 9.4 mm and a constant loading speed of 1 m.s.sup.1 was utilized. The beam deflection was measured using a linear variable differential transducer (LVDT) setup. Compression experiments were also performed at a constant displacement speed of 1 m.s.sup.1. Representative curves for each test are plotted in FIG. 7. At least three specimens were tested for each composition. The reported values are averages and standard deviations thereof.

Example 2: Calcium Phosphate Synthesis Route

[0134] The method used to synthesize the calcium hydrogen phosphate platelets was originally developed by Jha et al., AIMS Materials Science 1 (2014). The following steps characterize the protocol: [0135] 1. Two solutions containing the precursor salts in water at a concentration of 1 mol/l each are prepared: [0136] a. calcium nitrate tetrahydrate (Ca(NO.sub.3).sub.2.4H.sub.2O, Sigma-Aldrich) and [0137] b. ammonium phosphate dibasic ((NH.sub.4).sub.2HPO.sub.4, Sigma-Aldrich). [0138] 2. 120 ml deionized water was added to 120 ml Ca(NO.sub.3).sub.2.4H.sub.2O solution and 100 ml deionized water was added to 100 ml (NH.sub.4).sub.2HPO.sub.4 solution. [0139] 3. The (NH.sub.4).sub.2HPO.sub.4 solution was added drop by drop to the Ca(NO.sub.3).sub.2.4H.sub.2O solution under vigorous stirring (1000 rpm). The milky suspension was stirred for an hour. [0140] 4. The suspension was filtered (MD 615) and dried in an oven at 60 C. Depending on the time in the oven, two different crystal phases can result from this procedure (Hydrate phase: Brushite, or anhydrate phase: Monetite). [0141] 5. The dry powder was ground in a mortar and stored ready to use. [0142] 6. For a typical compression experiment, 1.22 g of powder was filled in the cavity of the mould (P/O/Weber, Germany). This was placed in a uniaxial press (200 kN capacity, P/O/Weber, Germany) and 160 KN of force was applied. Optionally additional liquid was added (up to a ratio powder to liquid of 0.3). The liquid used was a 0.9 wt % NaCl solution.

[0143] Typical results from a test with the anhydrate phase (Monetite) and no additional liquid: [0144] Density: 2.534 g/cm.sup.3 [0145] Strength at break as described in the mechanical test of example 1:22 MPa (1.5 MPa)

[0146] The modulus of rupture (MOR) was measured on a disk sample of at least 18 mm diameter and 2 mm thickness, with a setup called biaxial flexion, and the maximum value was 20 MPa.

Example 3: Application of Cold Sintering to Different Materials

[0147] Calcium Carbonate (Vaterite) [0148] Cold sintering achieved with any size of vaterite agglomerates which consist of small nanoparticles. Ethylene glycol may be employed to reduce the agglomerate size. [0149] The agglomerate size does not have an influence on the process and final maximal density. However the mechanical properties are worse (around ) with larger agglomerate sizes (which is probably related to the bigger pore size); nanoparticles are absolutely required as starting material.

[0150] Calcium Phosphate (Monetite): [0151] Platelet-like particles consisting of nanoparticles (produced by the co-precipitation route and under dried at 100 C. for several days). Pressed with water (1 g powder and 0.3 ml water) at 500 MPa for 1 hour. [0152] Product: Very dense material, insoluble in water

[0153] Calcium Phosphate (Hydroxyapatite) [0154] Monetite platelets, which undergo phase transformation when left in 0.1M NaOH solution overnight. The platelet shape remains after the phase transformation. Pressed with water (1 g powder and 0.3 ml water) at 500 MPa for 1 hour. [0155] Product: Imaging indicates very dense structure.

[0156] Magnesium Carbonate (Amorphous) [0157] Nano particles produced by the co-precipitation method. After pouring the two solution in one beaker, the particles were washed immediately to prevent crystallization. Pressed without water (1 g powder and 0.3 ml water) at 500 MPa for 1 hour. [0158] Product: Very dense structure.

[0159] Boehmite (Commercial Product) [0160] Disperal P2W (Sasol, Germany). Pressed with water (0.2 g powder and 0.08 ml water) pressed at 500 MPa for 1 hour. [0161] Product: Very dense structure.