TRANSPARENT FLUORIDE CERAMIC MATERIAL AND A METOD FOR ITS PREPARATION

Abstract

A method for preparing polycrystalline fluoride ceramics using powder of fluoride ceramics nanocrystallites as starting material, wherein the method includes: (a) Optionally, a pre-processing step at a temperature ranging from 100 C. to 300 C. at vacuum of 10-5 mbar (10-3 Pa) to 10-8 mbar (10-6 Pa) for 30 minutes to 10 hours, (b) Applying a uniaxial pressure in the range from 1 to 200 MPa, at or around ambient temperature, to obtain a pre-compacted sample, (c) Applying to the pre-compacted of step b) a hydrostatic pressure by Cold Isostatic Pressing, to obtain a pre-compacted sample, (d) Loading the pre-compacted sample from step (c) into a die and submitting the sample to a uniaxial compression in combination with electric field-assisted sintering, under vacuum equal to or higher than 5 Pa. Polycrystalline fluoride ceramics obtained by this method find use in IR devices.

Claims

1-15. (canceled)

16. A method for preparing polycrystalline fluoride ceramics using powder of fluoride ceramics nanocrystallites as starting material, wherein said method comprises: (a) A pre-processing step comprising subjecting the fluoride ceramics nanocrystallites to a temperature ranging from 100 C. to 300 C. at vacuum of 10.sup.3 Pa to 10.sup.6 Pa for 30 minutes to 10 hours, (b) Applying to the powder of fluoride ceramics nanocrystallites a uniaxial pressure in the range from 1 to 200 MPa but less than the level of pressure applied during step (c), during 0.5 to 30 minutes, at a temperature from 2 to 80 C., to obtain a pre-compacted sample, (c) Applying to the pre-compacted sample of step (b) a hydrostatic pressure by Cold Isostatic Pressing, in the range from 150 to 250 MPa, during 0.5 to 30 minutes, at a temperature from 2 to 80 C., to obtain a pre-compacted sample, (d) Loading the pre-compacted sample from step (c) into a die and submitting said sample to a uniaxial compression in combination with electric field-assisted sintering, under vacuum equal to or higher than 5 Pa.

17. The method according to claim 16 wherein the electric field-assisted sintering of step (d) is achieved in the following conditions: Pressure superior or equal to 10 MPa, Current is a pulsed DC electric current of from 1 A to 3000 A Pulsed DC current voltage from 1 V to 20 V Duration of the pulsed current during 0.5 minute to 30 minutes, Temperatures are from 250 C. to 800 C., The sample is in a vacuum equal to or higher than 5 Pa.

18. The method according to claim 16, wherein said method comprises before step (a) a step of ball-milling of the nanocrystallites.

19. The method according to claim 16, wherein at least 90% of the nanocrystallites have a grain size within a range of x10 nm, wherein x is the average or medium grain size, x is inferior or equal to 100 nm.

20. The method according to claim 16, wherein step (c) comprises application of a pressure in the range from 180 to 220 MPa during 1 to 15 minutes.

21. The method according to claim 16, wherein the fluoride ceramics nanocrystallites respond to one of the formulas (I) or (II) below:
XF.sub.(2-z)O.sub.z(I)
M: XF.sub.(2-z)O.sub.z(II) Wherein X represents an element selected from alcali earth metals, and M represents an element selected from lanthanides, z represents a number, 0z<2.

22. A method according to claim 21, wherein z=0.

23. A method according to claim 21, wherein X represents an element selected from: Ca, Mg, Ba.

24. A method according to claim 21, wherein M represents an element selected from: Yb, Dy, Er, Tm.

25. The method according to claim 21, wherein fluoride ceramics nanocrystallites is selected from CaF.sub.2 and doped CaF.sub.2, wherein the dopant is selected from lanthanides.

26. A method according to claim 16, wherein the polycrystalline fluoride ceramics is made of one material.

27. A method according to claim 16, wherein the polycrystalline fluoride ceramics is part of a multimaterial.

28. A method according to claim 27 wherein the multimaterial precursor materials, including the fluoride ceramics nanocrystallites, are arranged in a geometry corresponding to the multimaterial arrangement and steps (b), (c) and (d) are applied to the multimaterial precursor materials arrangement.

29. The method according to claim 16 for making polycrystalline fluoride ceramics, wherein a sample of this polycrystalline fluoride ceramics of 10 mm width and thickness of 2 mm presents light transmission in the wave lengths between 6 m and 11 m, superior or equal to 85%.

30. The method according to claim 29, wherein a sample of this polycrystalline fluoride ceramics of 10 mm width and thickness of 2 mm presents light transmission in at least part of the domain of wave lengths from 400 nm to 800 nm, superior or equal to 50%.

31. The method according to claim 29, wherein the fluoride ceramics respond to the formula (1) below:
XF.sub.(2-z)O.sub.z(I) Wherein X represents an element selected from alcali earth metals, and z represents a number, 0z<2.

32. A polycrystalline fluoride ceramics obtained by the method according to claim 16, wherein a sample of this polycrystalline fluoride ceramics of 10 mm width and thickness of 2 mm presents light transmission in the wave lengths between 6 m and 11 m, superior or equal to 85%.

33. The polycrystalline fluoride ceramics according to claim 32, wherein a sample of this polycrystalline fluoride ceramics of 10 mm width and thickness of 2 mm presents light transmission in at least part of the domain of wave lengths from 400 nm to 800 nm, superior or equal to 50%.

34. The polycrystalline fluoride ceramics according to claim 32, wherein the fluoride ceramics respond to the formula (I) below:
XF.sub.(2-z)O.sub.z(I) Wherein X represents an element selected from alcali earth metals, and z represents a number, 0z<2.

35. The polycrystalline fluoride ceramics according to claim 32, wherein it is an optical element of a laser window, a microscope, a spectrometer, a refractory telescope, a spectrograph for astronomy instrumentation, an instrument for space, thermal imaging and night vision, a photolithography equipment, a scintillator, a breath analyser.

Description

FIGURES

[0161] FIG. 1: Graphic representing CO2 transmission at 10.6 m for the following materials

[0162] CaF.sub.2: Single crystal obtained by the Bridgman technique (https://en.wikipedia.org/wiki/Bridgman % E2%80%93Stockbarger technique)

[0163] CF SORESingle crystal obtained by the Czochralski technique (https://en.wikipedia.org/wiki/Czochralski_process)

[0164] CPF 6Ceramic sintered by SPS with random sintering parameters

[0165] CPF 32Ceramic sintered by SPS as per the procedure given in the example A below.

[0166] FIG. 2: Graphic representing CO2 transmission at 1064 nm for the following materials

[0167] CaF.sub.2: Single crystal obtained by the Bridgman technique (https://en.wikipedia.org/wiki/Bridgman % E2%80%93Stockbarger technique)

[0168] CF SORESingle crystal obtained by the Czochralski technique (https://en.wikipedia.org/wiki/Czochralski_process)

[0169] CPF 6Ceramic sintered by SPS with random sintering parameters

[0170] CPF 32Ceramic sintered by SPS as per the procedure given in the example A below.

[0171] FIG. 3: Graphic representing CO2 transmission at wave lengths between 200 nm and 2000 nm for the CPF 32 material corresponding to a Ceramic sintered by SPS as per the procedure given in the example A below with 2 mm thickness.

EXPERIMENTAL PART

[0172] I Materials and Equipment: [0173] Precursor: Fluoride ceramics nanopowders: CaF.sub.2 commercialized by Fox Chemicals with purity 99. 9% (grain size 20-40 nm). [0174] Press for step 2: uniaxial hydraulic press (with single piston). Mould for pre-forming (step 2): One die and one punch in stainless steal for uniaxial pressing with inner diameter equal to the graphite mould of SPS. [0175] Pouch for compaction step: flexible themosealable polymer commercialized under the name Kangjie sterilization roll. [0176] Hydrostatic press: Conventional Cold isostatic pressing equipment with water as the pressure medium is used. The volume of the autoclave vessel is 1 liter. [0177] Sintering equipment: Sintering of the samples were done by Spark Plasma Sintering (SPS). SPS experiments were performed with Spark Plasma Sintering system, Model SPS-515S-FUJI equipped with secondary vacuum pump. The experiments were performed under a vacuum higher than 5 Pa (5. 10.sup.2 mbar) called in the following as high vacuum with the electric pulse (3.3 ms) sequence for the SPS applied a voltage of 12:2 (i.e., 12 ON/2 OFF). The experiment was carried out in a graphite mould with an inner diameter of 10 mm and an external diameter of 25 mm with internal diameter of the graphite die covered by carbon foil (Papyex). The mould was covered with carbon fiber felt to limit the loss of heat radiation. The preformed sample was inserted in the graphite mold between two punches.

[0178] II Methods:

[0179] Pre-Treatment Method:

[0180] Preferably, fluoride powders have increased reactivity after ball milling. Ball milling is achieved in a ZrO.sub.2 jar for 30 minutes in the dry state with a speed of 300 rpm for a volume of 20 ml containing 10 balls of ZrO.sub.2. The grain sizes after milling were in the order of 20 nm.

[0181] Method A: Preparation of a Monolith Transparent Calcium Fluoride Ceramics

[0182] Step 1: Pre-Processing of Fluoride Nanopowders

[0183] Heat treatment could be applied to calcium fluoride nanopowders at 200 C. and at high vacuum at least 10.sup.5 mbar (10.sup.3 Pa) for 5 h in a vacuum furnace.

[0184] Step 2: Pre-Forming of the Material

[0185] 0.5 g of powder from step 1 is weighed and compressed under uniaxial pressure between 1 and 200 MPa in the stainless steel mold of diameter 10 mm at room temperature. This value of pressure is less than the one applied in the Cold Isostatic Press but enough to handle easily the green body. Then the compressed sample is packaged in a flexible polymer pouch and is subjected to hydrostatic pressure of 200 MPa for 5 min. The final diameter of the pre-compacted sample is approximately 8 mm. It is reduced by the effect of high compaction of the powder.

[0186] Step 3: Sintering by Spark Plasma Sintering

[0187] The precompacted sample of 8 mm diameter from step 2 is loaded in a graphite mold of 10 mm diameter and applied the heating rate and cooling rate of 100 C./min at 70 MPa for a dwell time 5 min. A pre-sintering is carried out in the SPS processing equipment with a sintering step at around 400 C. under vacuum for 5 min. Then the sample was sintered at 500 C. by vacuum higher than 5 Pa (5 10.sup.2 mbar). The effect of texturing (change of diameter from 8 to 10 mm) happens during the processing of SPS and a final sintered sample of fluoride optical ceramic of diameter 10 mm is obtained. This effect increases the final transparency of the ceramic.

[0188] Method B: Preparation of Fluoride Transparent Ceramics with Core/Crown Structure

[0189] Step 1: Pre-Processing of Fluoride Nanopowders

[0190] Heat treatment can be applied separately to fluoride nanopowders of different starting powders: CaF2 and Yb:CaF2, at 200 C. and at high vacuum at least 10.sup.5 mbar (10.sup.3 Pa) for 5 h.

[0191] Step 2: Pre-Forming of the Multimaterial

[0192] Core/Crown Structure

[0193] 20.25 g of powders of two different compositions from step 1 are weighed and arranged as Core=CaF.sub.2 and Crown=Yb:CaF.sub.2, in a stainless steel mold of diameter 10 mm. They are compressed under uniaxial pressure between 1 and 200 MPa at room temperature. Here the mould is designed in such a way to fill the powders in the form of core and crown to accommodate two different types of compositions.

[0194] The compressed sample is packed in a flexible package and subjected to hydrostatic pressure of 200 MPa for 5 min. The final diameter of the pre-compacted sample is around 8 mm, reduced by the effect of high compaction of the powder

[0195] Parallel Multilayer (Planar) Structure

[0196] In the case of parallel multilayer or gradient variation, the powders of different compositions CaF.sub.2 and Yb:CaF.sub.2 are stacked together as parallel layers in a stainless steel mold of diameter 10 mm at room temperature. Then the compressed sample is packed in a flexible package and subjected to hydrostatic pressure of 200 MPa for 5 min. The final diameter of the pre-compacted sample is 8 mm.

[0197] Step 3: Sintering by Spark Plasma Sintering

[0198] Core/Crown Structure

[0199] The pre-compacted sample with diameter 8 mm from step 2 is loaded in a stainless steel mold of diameter 10 mm and we have employed the heating rate and cooling rate of 100 C./min at 70 MPa for a dwell time 5 min. Then the sample was sintered at 500 C. under vacuum higher than 5 Pa (5. 10.sup.2 mbar) The effect of texturing happens during the processing of SPS and the final sintered sample of fluoride optical ceramic of diameter 10 mm is obtained.

[0200] Parallel Multilayer (Planar) Structure

[0201] The pre-compacted/stacked sample from step 2 of 8 mm diameter containing different compositions is loaded in a mold of diameter 10 mm and we have employed the heating rate and cooling rate of 100 C./min at 70 MPa for a dwell time 5 min. Then the sample was sintered at 500 C. under high vacuum higher than 5. 10.sup.2 mbar (5 Pa) The effect of texturing happens during the processing of SPS and the final sintered sample of fluoride optical ceramic of diameter 10 mm is obtained.

[0202] III Results:

[0203] The ceramics obtained by Method A were ground and polished to a thickness of 2 mm with optical finishing. Powder X-Ray Diffraction (XRD) analysis was performed with a PANalytical X'pert MDP diffractometer with - Bragg-Brentano configuration with a backscattering graphite monochromator for K.sub. Cu radiation working at 40 kV and 40 mA. The density was measured by the Archimedes method in distilled water. The microstructure was observed by a scanning electron microscope (Joel 840 SEM) on the fractured surface without polishing. The optical transmittance spectrum was measured by using a double beam spectrophotometer (Varian Cary 5000) at a range of between 200 nm and 7000 nm for a sample thickness of 2 mm. Further the optical ceramic was tested with CO2 laser wavelength of 10. 6 m to study the light transmittance of this wavelength. Hardness measurements of the transparent ceramics were analyzed with Shimadzu DUH-211S Vickers hardness indenter with 500 g of the load.

[0204] It was inferred from XRD that no additional phases/decomposition are formed in the sintered samples. We have demonstrated the possibility to increase the optical and mechanical properties of the sintered samples of CaF.sub.2 in the present work.

[0205] We have studied a procedure by spark plasma sintering with optimized sintering parameters at low temperature and the procedure is a single step process and does not involve presintering steps. This is a rapid process and is feasible to fabricate fluoride optical ceramics for CO2 laser applications as lens, windows or other complex shaped optical components. Fluoride optical ceramics can be considered as a validate candidate to be used without AR coatings.