Process and device for preparing a 3-dimensional body, in particular a green body

Abstract

The invention relates in a first aspect to a process for preparing a 3-dimensional body, in particular a vitreous or ceramic body, which comprises at least the following steps: a) providing an electrostatically stabilized suspension of particles; b) effecting a local destabilization of the suspension of particles by means of a localized electrical discharge between a charge injector and the suspension at a predetermined position and causing an aggregation and precipitation of the particles at said position; c) repeating step b) at different positions and causing the formation of larger aggregates until a final aggregate of particles representing a (porous) 3-dimensional body (green body) having predetermined dimensions has been formed; wherein the charge injector includes i) at least one discharge electrode which does not contact said suspension of particles or ii) a source of charged particles. A second aspect of the invention relates to a device, in particular for performing the above process, comprising at least the following components: —a vessel for receiving an electrostatically stabilized suspension of particles, —a charge injector, in particular including one or more electrodes or a source of high-energy charged particles, —means for moving the electrode and/or the vessel in the x, y and z directions, —a counter electrode arranged in the vessel for a contact with the suspension of particles, —one or more sensors for determining geometrical and physical parameters within said vessel. In one preferred embodiment, said device further comprises a means for directing a beam of gas-ionizing radiation, in particular a laser beam, to a predetermined position within the vessel.

Claims

1. A process for preparing a 3-dimensional vitreous or ceramic body, which comprises at least the following steps: a) providing an electrostatically stabilized suspension of particles; b) effecting a local destabilization of the suspension of particles by a localized electrical discharge between a charge injector and the suspension at a predetermined position and causing an aggregation and precipitation of the particles at said position; c) repeating step b) at different positions and causing a formation of larger aggregates until a final aggregate of particles representing the 3-dimensional body having predetermined dimensions has been formed; wherein the charge injector includes i) at least one discharge electrode which does not contact said suspension of particles or ii) a source of charged particles.

2. The process according to claim 1, further comprising at least the following step d) densifying said 3-dimensional body.

3. The process according to claim 2, wherein step d) is effected by sintering and/or atomic layer deposition (ALD) of a filling material in pores of said 3-dimensional body.

4. The process according to claim 1, wherein the electrical discharge takes place in a gaseous medium which is selected from the group consisting of air, Xe, Ar, O.sub.2, air containing CCl.sub.4, air containing CH.sub.3I, and mixtures thereof.

5. The process according to claim 1, wherein the charge injector includes a discharge electrode and the localized electrical discharge between the charge injector and the suspension is induced at a predetermined position and time by irradiation with air-ionizing radiation.

6. The process according to claim 5, wherein the irradiation is effected by laser light.

7. The process according to claim 5, wherein the irradiation is triggered in dependency on a distance between a tip of the discharge electrode and a surface of the suspension and/or an electric field strength between the electrode and surface of the suspension.

8. The process according to claim 6, wherein a laser irradiation is applied for a time period in a range from 5 fs to 100 ns and at a peak energy density of higher than 0.1 TW/cm.sup.2.

9. The process according to claim 1, wherein the charge injector includes a source of charged particles and step b) comprises directing the charged particles by a transportation and targeting means to a predetermined position on a surface of the stabilized suspension of particles.

10. The process according to claim 1, wherein the charged particles have energies in a range from 2 keV to 10 MeV.

11. The process according to claim 1, wherein initially a primary layer of aggregated particles having predetermined dimensions in x, y and z directions is formed after steps b) and c) and further layers with predetermined dimensions are deposited successively on each other in a layer-by-layer deposition by repetition of steps b) and c).

12. The process according to claim 1, wherein the particles are inorganic particles selected from the group consisting of SiO.sub.2, glasses, TiO.sub.2, Al.sub.2O.sub.3, MgAl.sub.2O.sub.4, ZrO.sub.2, PbZrO.sub.3, HfO.sub.2, carbides, nitrides and mixtures thereof.

13. The process according to claim 1, wherein the particles constituting the suspension have a mean diameter in a range from 4 nm to 200 μm.

14. The process according to claim 1, wherein the particles have a positive or negative surface charge imparted by a charged compound or material coupled to a surface of said particles via covalent or non-covalent interactions.

15. The process according to claim 14, wherein the charged compound or material is selected or derived from the group consisting of ammonia, ammonium compounds, an inorganic basic compound, an acidic compound, and ions thereof.

16. The process according to claim 1, wherein the particles and/or the suspension of particles comprise(s) dopants selected from the group consisting of metal oxides and metal ions.

17. The process according to claim 1, wherein the liquid forming the suspension of particles is an aqueous medium or any other medium which is removable from the 3-dimensional body preferentially by evaporation.

18. The process according to claim 1, wherein one or more of the following conditions are met: a position of the charge injector relative to the suspension is adjusted by mechanical motions in x, y and/or z directions, a position of a vessel comprising the suspension of particles is adjusted relative to the charge injector by mechanical motions in the x, y and/or z directions, a position of a surface of the suspension and/or the aggregate(s) of particles or 3-dimensional body relative to the charge injector is adjusted in the z direction by shifting a liquid level of the suspension in the vessel or by mechanically moving the formed aggregate(s) in the vessel in the z direction.

19. The process according to claim 18, wherein i) the motions of the charge injector, the vessel, the suspension and/or the forming 3-dimensional body, and/or ii) the localized electrical discharge between the suspension of particles and the charge injector, which is induced by either irradiation with air-ionizing radiation, are controlled by a software program.

20. A device for performing a process for preparing a 3-dimensional vitreous or ceramic body, said device comprising at least the following components: a vessel for receiving an electrostatically stabilized suspension of particles, a charge injector including one or more electrodes or a source of high-energy charged particles, means for moving the one or more electrodes and/or the vessel in x, y and z directions, a counter electrode arranged in the vessel for contact with the suspension of particles, and one or more sensors for determining geometrical and physical parameters within said vessel, wherein the process comprises at least the following steps: a) providing an electrostatically stabilized suspension of particles; b) effecting a local destabilization of the suspension of particles by a localized electrical discharge between a charge injector and the suspension at a predetermined position and causing an aggregation and precipitation of the particles at said position; c) repeating step b) at different positions and causing a formation of larger aggregates until a final aggregate of particles representing the 3-dimensional body having predetermined dimensions has been formed; wherein the charge injector includes i) at east one discharge electrode which does not contact said suspension of particles or ii) a source of charged particles.

21. The device according to claim 20, further comprising a means for directing a laser beam to a predetermined position within the vessel.

22. The device according to claim 20, further comprising means for adjusting a level of a suspension comprised in said vessel and/or means for moving a solid body within said vessel.

23. The device according to claim 20, wherein the geometrical and physical parameters within said vessel represent distances between different objects in said vessel, values of electrical field strength, charge, and optical properties of the different objects in said vessel.

24. The device according to claim 20, wherein the vessel contains an electrostatically stabilized suspension of particles, and the charge injector includes a plurality of discharge electrodes which is/are not in contact with the suspension of particles.

25. The process according to claim 3, wherein the deposited filling material is selected from the group consisting of glasses, ceramics and metals and wherein the atomic layer deposition comprises at least the following steps: covering an inner surface of the 3-dimensional with a precursor of the filling material from the gas phase; converting the precursor adsorbed at the inner surface into a non-volatile material by a chemical reaction with a component from the gas phase; and repeating these 2 steps until the pores of the 3-dimensional body are at least partially filled with the non-volatile material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 schematically illustrates two principal approaches for implementing the present invention: FIG. 1, left, shows a device including a contact-less discharge electrode and a laser for initiating a laser-guided electrical plasma discharge; FIG. 1, right, shows a device including a hollow capillary for directing a beam of charged particles to the suspension.

(2) FIG. 2 schematically illustrates a device of the prior art for electrophoretic deposition.

(3) FIG. 3 shows 2 different arrangements for the counter electrode in a device according to the present invention.

(4) FIG. 4 schematically illustrates a device comprising a discharge electrode and a laser, wherein the position of the surface of the suspension is adjusted in the z direction by shifting the liquid level of the suspension in the vessel using a pump.

(5) FIG. 5 schematically illustrates a device comprising a discharge electrode and a laser, wherein the formed green body is moved in the vessel in z direction.

(6) FIG. 6 schematically illustrates a device comprising a discharge electrode and a laser, wherein a suspension film is generated on a sloped substrate surface by a pump-driven suspension flow over said surface.

(7) FIG. 7 schematically illustrates an arrangement analogous to that of FIG. 6, with a device wherein the discharge electrode and a laser is replaced by an ion source.

(8) The following examples illustrate the invention in more detail.

Example 1

(9) An exemplary green body was prepared by the process of the present invention as follows:

(10) 1. Preparation of a SiO.sub.2 Slurry:

(11) 35 g of colloidal silica 50 nm from Evonic

(12) 1 g 35% tetramethylammoniumhydroxid solution in methanol

(13) 60 g distilled water

(14) Shaking for 1 h in a PE bottle.

(15) This procedure results long term stable slurry.

(16) 2. Preparation of a Green Body: A cylindrical glass vessel having a diameter of 60 mm and a height of 40 mm is filled with the slurry in that way that the bottom is covered fully. The vessel is placed on a rotation stage and is connected with a wire on from the top. A tungsten wire (discharge electrode) is placed with one end over the slurry, the distance is 3 mm. The rotation stage is switched on, the rotation velocity is 1 turn per 2 min. A high voltage is applied between the connection of the slurry (−) and the tungsten wire (+) until a discharge is visible and the noise of the spark can be observed. The needed voltage depends on several experimental parameters, 1 kV is a common value. The voltage remains switched on until a full turn of the rotation stage is done. The first layer of a cylinder is precipitated. The process is repeated as often as desired.

(17) Here no laser guidance of the precipitation was used.

Example 2

(18) An exemplary green body was densified by the ALD process of the present invention as follows: The green body from the precipitation is dried in air at room temperature for 3 days. The green body is heat-treated in a rapid annealing furnace under Ar at 850° C. for 20 min The sample is transferred into an ALD machine at 200° C. using a BENEQ TFS200 ALD, trimethyl aluminium (Al(CH.sub.3).sub.3) and DI H.sub.2O were used as precursors.

(19) A calibrated number of cycles were used to precipitate 100 nm Al.sub.2O.sub.3,

(20) The resulting was not shrunk to such an extent that it could be measured with a slide caliper.

(21) The mechanical properties was tested qualitatively. The pressure required to crack the body is considerably higher after the ALD.

Example 3

(22) Characterization of an exemplary green body obtained by the claimed process:

(23) Density Measurement:

(24) Geometrical measurement of the volume and measurement of the weight with a balance: 50-60% as compared to compact glassy SiO.sub.2

(25) Microporosity:

(26) Measured with an Xray microscope (Zeiss Versa)

(27) Results: self-produced slurry 2.8 vol % bubbles having a diameter larger than 2 μm commercial slurry: 0.5 vol % bubbles having a diameter larger than 2 μm

(28) Nanoporosity:

(29) Electron micrograph of a fresh crack: the spheres are dense packed, between 40 and 60 vol % spheres. Internal cracks in the material are very rare.